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L. B. Schwellenbach, Secretary
Ewan Clague, Commissioner

Labor Requirements to Produce
Home Insulation

B ulletin J\[o. 919

W A S H IN G T O N : 1947

For sale by the Superintendent o f Documents, U . S. Governm ent Printing Office
Washington 25, D . C. - Price 10 cents

Letter of Transmittal

U nited States D epartment of L abor,
B ureau of L abor Statistics,
Washington, D. C.9July 28, 19Jfl.

The Secretary of L abor :
I have the honor to transmit herewith a study on labor requirements in the
production of certain types of nonrigid home insulation. This study is the
fifth in a series conducted by the Bureau’s Construction Statistics Division to
determine the amount of labor needed to produce the more important construc­
tion materials; it was prepared by Carl R. Taylor and Benjamin Levine, under
the direction of Thomas M. Flanagan. The Bureau wishes to acknowledge the
cooperation of the companies which provided data for the survey, and of their
trade associations.
Hon. L. B.

E w a n C la q u e ,



Secretary of Labor.

The photographs reproduced in this bulletin are by courtesy of Johns-Manville
Corp., the Vermiculite Research Institute, and the U. S. Department of




Characteristics of industry’s products_________________________________
Scope and method of survey_________________________________________
Summary of man-hour requirements__________________________________
Mineral wool:
History and annual production---------------------------------------------------5
Production process,_,___________________________________________
Labor requirements in manufacturing____________________________
Outside labor requirements______________________________________
Cotton insulation:
Production process______________________________________________ 12
Labor requirements_____________________________________________
Expanded vermiculite:
Process of mining and exfoliating---------------------------------------------------15
Labor requirements_____________________________



Bulletin A[o. 919 o / the
U nited States Bureau o f Labor Statistics

Labor Requirements to Produce Home
Mineral Wool, Cotton and Expanded Vermiculite
Characteristics of Industry’s Products

This study is the fifth in a series1 which the Bureau of Labor Sta­
tistics has been conducting to determine the amount of labor needed
to produce the more important construction materials for the present
housing program or for other construction activity.
The present survey analyzes the amount of labor required to produce
3 important types of nonrigid home insulation material: mineral
wool, cotton, and expanded vermiculite. It describes the processes in­
volved in the manufacture of these materials, and analyzes the man­
hours required per unit of product at each production level, from
mining or growing the raw material to transporting the finished
The use of insulation in new residential construction has increased
rapidly in recent years and, during the war, insulation of existing
buildings became especially important as a fuel-conservation measure.
Savings in fuel costs are estimated at around 30 percent for insulated,
as compared with uninsulated homes.2
1 This survey, covering three important nonrigid insulation materials, is the first of two
studies of the labor required to produce home insulation. The second will cover labor
requirements for fiber insulating board, a rigid type of insulation material. Building
materials for which results will be reported in future articles include hardwood and
hardwood flooring, and plumbing and heating supplies. Previous studies covered labor
requirements in cement production, summarized in the September 1946 Monthly Labor
Review (p. 355) ; labor requirements in the production and distribution of concrete
masonry products and concrete pipe, summarized in the November 1946 Monthly Labor
Review (p. 681) ; in southern pine lumber production, summarized in the December 1946
Monthly Labor Review (p. 941) ; and in softwood plywood production, summarized in
the January 1947 Monthly Labor Review (p. 67).
2 Bowles, Oliver; Home Insulation With Mineral Products, in Information Circular
Number 7388 (U. S. Department of Interior), October 1946, p. 7.


Nonrigid insulating materials usually consist of interlaced wool­
like fibers or hollow or stratified granules, providing millions of
minute spaces in which dead air is enclosed and, thus, having especially
low heat conductivity. Their function is to reduce the transfer of
heat from one side of a wall, ceiling, or roof to the other, irrespective
of the side on which the heat is generated. In winter, the use of in­
sulation reduces the escape of heat from the inside of a dwelling,
maintaining a higher indoor temperature with decreased fuel con­
sumption. In summer, the reverse occurs, and insulation reduces the
penetration of the heat of the sun through walls and ceilings to the
interior of the house.
There are many types of insulating materials, but in general they
fall into two categories: the rigid, such as fiber board, used in new
construction or in remodeling in place of lathing or wall sheathing;
and the nonrigid, varieties of which are discussed in this report. Nonrigid insulation has no structural value, but is placed in the hollow
spaces of walls, between floor joists, or between roof and rafters. It
is used effectively to insulate existing buildings and is also installed
in new construction. Most types are fire resistant or flameproofed, and
it has been found that even in frame dwellings the spread of fires is
hindered by the use of insulation in ceiling and roof, as well as by
filling the hollow spaces of walls and partitions.
Of the three types of nonrigid insulation covered in this survey,
mineral wool is the most important in terms of tons produced and
sold each year. (See table 1.) The processed mineral wool is a fibrous,
intermeshing material, resembling natural wool. It is composed prin­
cipally of silicates of calcium and aluminum and is produced from
three principal types of raw material: natural rock or combinations
of various natural minerals or rocks, such as limestone and shale; iron,
copper, or lead blast-furnace slag; and such glass materials as silica
sand, soda ash, and limestone. The mineral wool is named rock wool,
slag wool, or glass wool, according to the raw materials from which
it is made. Mineral wool is sold commercially for home insulation
in three forms: loose, granulated, or as batts and blankets.
Loose wool is the initial product after the first processing operation
and it is from loose wool that the other mineral wool products are pre­
pared. Loose wool is sold by the bag and installed by hand.
Granulated wool is by far the most widely used of the three forms.
It is the only product from which the shot, or grains of slag have been
removed. Because the granules are formed into small pellets and
can be blown between the framing of buildings with an air hose,
granulated wool is used chiefly for the insulation of existing buildings.
Like loose wool, it is usually sold by the bag.
Blankets and batts of mineral wool have been fashioned into soft
flexible quilts, usually surfaced on one or both sides by specially


treated vapor-proof paper. Batts are similar to blankets except that
they are usually smaller in size. Batt and blanket wool are sold by
the roll or carton and installed by hand.
Processed flameproofed cotton is a comparatively new product, dat­
ing from 1940. It is sold only in the batt or blanket form, by the roll
or carton. Expanded vermiculite, granular in form, is marketed by
the bag, as a fill type of home insulation material. It is prepared
from vermiculite ore, which closely resembles mica in appearance and
is characterized by its extraordinary quality of expansion when heated.
T able 1.— Estimated

production of mineral wool, cotton, and expanded vermiculite
home insulation, 1942 and 1946

Type of insulation
Total (all types)___________________________
Mineral wool1_________ _____________
Loose wool.
_ _ _________ ________
Granulated wool_______________________
Blanket or batt wool___________________
Cotton 2__ __ __ _________ ___
__ _ _ __
Expanded vermiculite 3___ .

Tons produced


377, 603
58, 586
32, 974

681, 295
56, 213
408, 021
167, 377
9, 050
40, 634

1 For 1942 production figures, see Minerals Yearbook, 1943, p. 1583. Production figures for 1946 for all
mineral wool of the types shown are based on the production estimate for 1945 furnished the U. S. Bureau
of Mines by the Mineral Wool Association, and increased by 12 percent—the Bureau of Mines estimate
of the rise in mineral wool production in 1946 compared with 1945. Production in 1946 was distributed by
type of mineral wool in the same ratio as found in the production of the plants covered in the Bureau of
Labor Statistics’ survey during 1946.
2 For 1942 production figures, see E. H. Omohundro and N. B. Salant, Cotton Insulation. Washington,
D. C., U. S. Department of Agriculture, et al., December 1944, p. 2. The 1946 figure is an estimate fur­
nished by the U. S. Department of Agriculture. Production of cotton insulation not covered in the De­
partment of Agriculture incentive program is excluded, but this is believed to be very small.
3 For 1942 production figure, see Minerals Yearbook, 1943 p. 1588. It is estimated that 60 percent of
the total production is used in home insulation. The 1846 figure is an estimate provided by the U. S.
Bureau of Mines.
Scope and Method of Survey

The mineral wool survey covers 10 plants representing about onethird of 1946 production. The studies of labor requirements to pro­
duce flameproofed cotton and expanded vermiculite were made by
analyzing the records of four and six plants, respectively, accounting
for something over 50 percent of 1946 production in each of these
The companies and plants to be included in the survey were selected
after consultation with appropriate trade associations, government
officials, and industry members. Small, medium, and large plants in
each branch of the insulation field, were selected in approximately the
same ratio in which they are distributed in the industry. Figures on
unit man-hours for each plant were weighted by the plant’s estimated
1946 production in order to arrive at estimated average man-hours re­
quired per unit of output in each of the three insulation industries


Data were collected in the latter part of 1946 and, in each case, a
period was selected for survey that plant officials considered typical
of their 1946 operations. Field representatives of the Bureau of
Labor Statistics, working with plant officials, derived the informa­
tion on total man-hours spent on each manufacturing process and in
administration, from plant records. Man-hour requirements were
estimated for extracting and transporting the raw materials, for
power, and for transporting the finished product to local dealers and
users on the basis of the following: (1) plant information on quan­
tities copsumed and produced; (2) labor requirements information
obtained from various sources, including the extraction &nd utility
industries represented; and (3) earlier surveys of the Bureau of Labor
Statistics. No attempt has been made in this study to include the man­
hours expended by distributors, jobbers, or retailers in selling the
finished product.
Summary of Man-Hour Requirements

An average of 9.1 man-hours was required in 1946 to manufacture
a ton of mineral wool home insulation, according to the Bureau’s
survey. Loose wool, because of the simplicity of its manufacture,
required only 6.7 man-hours per ton to produce in the factory; granu­
lated wool required somewhat more labor (8.2 man-hours) as a result
of an added processing step, and batt or blanket wool still more (12.2
man-hours) reflecting primarily the process of fashioning the wool
into the blanket-type product and affixing the vapor-proof backing.
About 5.6 man-hours may be added on the average for producing and
transporting the raw materials, fuel, and other products used in
making mineral wool, and for transporting the finished wool to dealers.
Flameproofed cotton insulation is measured by the thousand square
feet. The Bureau’s survey shows that while only 4.2 man-hours were
required to produce a thousand square feet of cotton insulation in the
plant in 1946, 85.8 per thousand square feet were needed for the out­
side activities of making the cotton and other materials available, and
shipping the finished product. Of the 85.8 man-hours, 84 were re­
quired just for growing and preparing the cotton for delivery to the
insulation plant, according to estimates of the Department of Agri­
In the case of expanded vermiculite, also, man-hour requirements
for processing are not as high as those for producing and transporting
the raw materials, fuel, and other products used and for shipping
the finished insulation. Expanded vermiculite is measured by the
thousand cubic feet. An average of 25.2 man-hours was found to be
needed in 1946 to manufacture a thousand cubic feet of expanded
vermiculite, compared with 35.9 man-hours for the out-of-plant activi­


ties, making a total of 61.1 man-hours per thousand cubic feet from
mine to dealer. A large part of the man-hour requirements for pro­
ducing vermiculite insulation (19.4 man-hours) was expended in
transporting the vermiculite ore from the mines, which are situated
in a very few States, to the scattered processing plants. Since the
cubic footage of vermiculite expands considerably during processing,
the manufacturing plants are located near distributing centers and
often quite distant from the source of raw materials, to save trans­
portation costs.
It is clear from the above discussion that the production man-hour
requirements vary considerably per unit of product between the three
home-insulation materials discussed in this report, when each indus­
try’s customary unit of measurement is used. Using a common unit
of measurement with reference to the use to which the material is
put (i. e., the amount of insulation material 3 inches thick required to
cover a thousand square feet of space), the Bureau has computed
roughly that labor requirements in the factory vary little by type of
product, ranging from a little over 4 to something over 6 man-hours
per unit. Variations are much greater when nonmanufacturing
labor needs are considered, because of the differences in raw materials
used and in transportation problems, as described briefly above.
Mineral W ool


Production of mineral or rock wool was first reported in 1840 in
Wales.3 Commercial production in this country is believed to have
started at Alexandria, Ind., in 1897 when C. C. Hall established a
plant for the production of rock wool to utilize the slag produced as
waste in the operation of a steel plant. Annual production in 1900
was estimated to be 6,002 short tons, increasing to 11,626 tons in 1909
and dropping to 7,514 tons in 1911. Production in 1928 was estimated
at around 50,000 tons 4 increasing to 500,000 tons in 1936 and decreas­
ing to approximately 400,000 tons in 1938.5
The centennial celebration of the mineral wool industry was held in
Chicago in 1940.6*8 Only estimates rather than actual production
figures are available for most other years prior to 1942, although the
Bureau of Census has released some data on dollar value of mineral
wool produced for certain earlier years. Recent data on mineral wool
production are shown in table 2.
3 The Origin of Rock Wool, in Stone, vol. 57. No. 12, December 1936, p. 448.
Wool, by J. R. Thoenen, Bureau of Mines Information Circular 6142, June
1929, pp. 2 and 13.
* Mineral Wool, by J. R. Thoenen, Bureau of Mines Information Circular 6948R, June
1939, p. 4.
8 Minerals Yearbook, 1940, p. 1420.
4 Mineral

748937°— 47------2


T able 2.— Annual

'production of loose, granulated, and batt or blanket mineral
wool used in home insulation, 1942-^d

1942 i___________
1943 i___________
1944 2___________
1945 3___________
1946 4___________

Short tons of mineral wool produced


377, 603
426, 600

58, 586
63, 978
59, 787
56, 213

Granulated Batt or blanket
308, 411

135, 910
67, 256

1 Minerals Yearbook 1943, p. 1583.
2 Op. cit. 1944, p. 1541.
3 Estimate for the total was provided to the U. S. Bureau of Mines by the Mineral Wool Association.
Production by type of mineral wool was estimated by the Bureau of Labor Statistics, using the same ratio
to the total as in 1944.
4 Figure for the total was based on the production estimate for 1945, increased by 12 percent—the Bureau
of Mines estimate of the rise in mineral wool production in 1946 compared with 1945. Production by type of
mineral wool was distributed in the same ratio as found in the production of the plants covered in the Bureau
of Labor Statistics’ survey during 1946.


Though variations in the technique of manufacturing mineral wool
occur from plant to plant, the process may be described generally as
follows: Different combinations of coke with slag, rock, shale, dolo­
mite or limestone, among others, are fed into a cupola—a furnace
used in melting minerals. Automatic scales are usually used to weigh
the amount of each raw material to be put into each charge. The
charge is heated to a melting temperature of from 2,500° F. to around
3,500° F. and is then drawn off from the cupola in a thin stream. The
stream of melted slag, about the size of a lead pencil, drops from a few
inches to something over a foot. It is then intercepted by a blast of
steam which catches it and blows it into the adjacent blowing or wool
room. The blowing temperature is usually 300 to 500° F. lower than
the melting temperature.
As the slag droplets are traveling at high speed through the air,
each tails out into fibers or strands. The minute amount of each
droplet of slag that does not strand out or form fibers, but remains
as a tiny ball or knot of slag, is called “shot.” A single piece of shot
is about the size of a grain of sand. As shot has no insulation value, its
elimination in the production process is highly desirable, and the
degree of efficiency of the blowing process is in part measured by the
small shot content of the loose wool. To reduce the dust, an oil spray
is mixed with the steam, or strikes the wool after it has been blown a
few inches.
The blowing rooms are enclosed and are narrow and high and
usually from 20 to 50 feet in length. A conveyer belt ordinarily forms
the floor and removes the blown wool from the wool room through a
small opening. In some plants the wool is removed by hand. At this
point the processing of loose wool is completed and it may be bagged
and shipped.


In the case of granulated wool, the conveyer may move the wool
out of the wool room onto a belt through a series of grinders that break
up the fibers. The wool then passes into a rotary screen which causes
the formation of small round pellets or nodules, producing granulated,
often called nodulated wool. The shot drops through the screening to
be returned to the furnace and remelted. There is about a 17-percent
loss in pounds between the raw slag used and the loose wool produced.
If the loose wool is further processed into granulated wool, an addi­
tional loss of 25 percent is incurred, making an average loss of about
42 percent in processing granulated wool.


1. Blowing mineral wool : As the molten slag leaves the cupola in a thin stream,
it is intercepted by a blast of steam and blown into the wool room, thus forming loose
mineral wool.


To make batt or blanket wool, the loose wool is removed from the
wool room, usually on a conveyer belt, and sprayed with resin or some
other binder. It then passes through rollers which compress the wool
with the binder and set it into a continuous mineral wool blanket.
Following this, the blanket passes through ovens which complete the
process of binding and setting the blanket in permanent form.
Blankets are made from 1 to 4 inches thick to suit various insulation
purposes. The conveyer moves the newly formed blanket along to
the point where the moisture-proof vapor-barrier paper is added as
backing, though batt or blanket wool is sometimes made without paper
backing. The mineral blanket is then cut and trimmed automatically
to batt or blanket sizes, automatically rolled, and then hand packed for


shipment. The wool clippings from the trimming operation are
salvaged and made into granulated wool.
In some of the larger plants, there is a separate production line for
each type of wool, but in smaller plants one cupola produces the
loose wool used to make granulated and batt or blanket wool.

F igure 2. Loose mineral wool : An endless belt conveys the blown wool from the wool
room for bagging or further processing into granulated or batt and blanket wool.

The labor requirements to manufacture a ton of mineral wool for
all processes combined have already been quoted (p. 6). Figures
showing detail by department or process are presented in table 3.
It will be noted that the processing of granulated wool requires
more labor than loose wool chiefly in the maintenance department, as
a result of the additional machinery required for nodulating or gran­
ulating the wool, removing the shot, and then bagging the wool
Also, more labor is required to make granulated wool than loose
wool, because of the loss or waste in raw materials caused by removal


of the shot. This necessitates handling and heating more raw ma­
terial to produce a ton of the granulated mineral-wool product.
While granulated wool is bagged automatically, loose wool must be
bagged by hand, as the fibers tend to cling together to form large
masses or lumps. Man-hours required for packing loose wool, in
fact, exceed those for processing. In contrast, the pellets of granu­
lated wool are easily bagged by machine.
Most of the additional labor required to produce batt or blanket
wool as compared with granulated wool is employed in the manufac­
turing and packing department where additional machine operations
require a larger number of machine tenders. In addition, batt or
blanket wool is hand packed in contrast with the automatic bagging of
granulated wool. Machine maintenance and labor supervision also
are more extensive in making batts or blankets.
T able 3.— Man-hour

requirements per ton to manufacture mineral wool home
insulation in 1946, by type of mineral wool product

Average man-hours required per ton of mineral
wool for—
Major department or process
All departments___________________________________
Handling raw materials______________ ____________
Manufacturing and packing____ ___________________
Loading and shipping,. ___________________________
Administration, clerical, and sales_____ _ . ... __.
Maintenance . _ __ __ __
__ .. ____
Superintendent and foreman_______________________

All types



Batt or
Granulated blanket






A relationship can be seen between production level and average
man-hour requirements per ton in the mineral wool plants surveyed.
The greater the plant’s production, the lower the average man-hour
requirements to produce each ton. The relationship appears to affect
practically every processing department, as shown in table 4.
T able 4.— Average labor requirements per ton of mineral wool produced in

10 plants
in 1946, by estimated monthly production and by major department

Major department
All departments___________________________________
Handling raw materials_____________________________
Cupola_____ __________________________ _ .
Manufacturing and packing_________________________
Loading and shipping______________________________
Administration, clerical, and sales___________________
Superintendent and foreman________________________

Man-hours required per ton in plants with
monthly production of—
Less than 1,000 1,000 to 2,000 More than 2,000





To follow the employment needs for producing mineral wool from
mine to dealer, it was necessary to study how much labor was required
to produce and transport the raw materials, fuel, and other major
products needed to make this type of home insulation and it was neces­
sary to compute, also, the labor required to transport the finished
product from the plant. Estimates of these labor needs are, at most,
approximate and could be prepared only for loose wool.
Labor needs for all these activities averaged 5.6 man-hours
per ton of loose wool produced. When added to manufactur­
ing man-hours (6.7 per ton), a total of 12.3 man-hours per ton
is derived for the labor needs from extraction of the raw materials to
delivery of the finished product. The nonmanufacturing labor re­
quirements undoubtedly vary somewhat for the granulated and batt
or blanket types as compared with loose wool. On the average, how­
ever, it is safe to say that about 15 man-hours (9.1 for manufactur­
ing and 5.6 to produce the material used, etc.) are required to
produce one ton of all type mineral wool home insulation, from mine
or slag pile to dealer.
The method by which these totals were derived may be described
briefly as follows: An average cupola charge contains about 2,400
pounds of blast furnace slag, 40 pounds of gravel, and 400 pounds of
coke. It is estimated from information of the National Slag Asso­
ciation and previous studies of the Bureau of Labor Statistics,7
that an average of 1.3 man-hours is needed to produce these
raw materials at their source, distributed as follows: 0.24 man­
hour for the slag, 0.01 for the gravel, and 1.07 for the coke. Using
earlier Bureau figures on labor needs in transporting raw materials
to the site of manufacture,8 it is estimated that a total of 0.5 man­
hour was required to deliver the raw materials to the mineral wool
plants—0.26 man-hour for the slag, 0.01 for the gravel, and 0.20 for
the coke.
The fuel used at the plant—about 280 lbs. of coal and 39 kw.-hr.
of electricity per ton of loose wool—is estimated to require 0.18 9 and
0.1010 man-hour, respectively, and other materials (principally the
7 For studies covering man-hour requirements to produce coke and gravel, see Monthly
Labor Review, May 1935, Man-Hours of Labor per Unit of Output in Steel Manufacture
(also reprinted as Serial No. R. 240), and Monthly Labor Review, July 1939, Labor
Requirements in Production and Distribution of Sand and Gravel (also reprinted as
Serial No. R. 944).
8 See Monthly Labor Review, October 1937, Labor Requirements in Rail Transportation
of Construction Materials (also reprinted as Serial No. R. 637), and see also Monthly
Labor Review, June 1938 (p. 11), Labor Requirements in Production and Distribution
of Plumbing and Heating Supplies (also reprinted as Serial No. R. 733).
9 See U. S. Bureau of Census, Mineral Industries, Vol. 1, 1939, p. 224.
10 See Bureau of Labor Statistics Bulletin No. 888-1, Labor Requirements in Cement
Production, pp. 16-18.


57 bags required for a ton of loose wool) about 0.5 man-hour. The
transportation of the coal requires 0.21 man-hour, making an ap­
proximate total of 1 man-hour for fuel and materials used.
The rail haul of the finished wool is estimated to average 375 miles,
requiring 2.8 man-hours per ton of wool transported.11 An estimated
average of 13 tons of mineral wool is carried per car.
Cotton Insulation

Cotton insulation is made from sound, staple, well-cleaned, and
firmly matted material. The matting of the individual hollow tubu­
lar fibers, each of which contains tiny dead air spaces, forms additional
air spaces, thus providing an exceedingly effective natural barrier to
the passage of heat.12
Cotton insulation was first produced commercially in 1940. It was
developed under a United States Department of Agriculture program
to create a new outlet for the grades of cotton which are usually sur­
plus. This surplus cotton is made available under incentive payments
enabling the producers of cotton insulation to compete successfully
with other types of nonrigid home insulation. The acceptance of
cotton insulation and the commercial development of the industry
since 1940 has been rapid. (See table 5.)
In general, the machinery used to make cotton insulation was
adapted from existing types of textile machinery. With the assist­
ance of the Department of Agriculture, methods of flameproofing and
mildew-proofing the raw cotton were devised. The firms in the indus­
try now have individual formulas for flameproofing, which they re­
gard as trade secrets.
T able 5.— Production of cotton insulation under the

U. S. Department of Agriculture
incentive program 1940-46



Annual pro­
duction in
thousands of

1940__________ _________ ________________
1942_______________________________ ...

1, 746
7, 447
8, 544

1 U. S. Department of Agriculture. Excludes small amount of production outside the Department of
Agriculture program for which data are not available.
n See Monthly Labor Review, October 1937 (pp. 846-853), Labor Requirements in
Rail Transportation of Construction Materials.
12 Cotton Insulation by E. H. Omohundro and N. B. Salant, Washington (U. S. Depart­
ment of Agriculture), December 1944, p. 5.



Although the methods and machinery for making cotton insulation
vary somewhat from plant to plant, the operations are basically the
same in all plants.
The baled raw cotton is unloaded from the freight cars or trucks
and stored in a warehouse. As the cotton is needed it is moved by con­
veyor or hand cart to the beginning of the production line.
The raw cotton is fed into the opener or willower machine which
loosens and fluffs it up. The fluffed cotton is then blown through tubes
or conveyed on belts to the dipping or soaking vats, which are fed
automatically by means of weighing machines. The vats are filled
with a combination flameproofing and mildew-proofing chemical.
The cotton moves slowly from one end of the vat to the other, being
mechanically agitated all the time so that it is thoroughly soaked with
the chemicals. In some cases, the soaking vats are variations of wool
washing machines. The soaking process may require as long as 1 hour
to complete.
Dry cotton is being continuously fed into one end of the vat while
soaked cotton is removed from the other end by a conveyor belt. The
belt carries the wet cotton through large rubber squeeze rolls which
operate under hydraulic pressure and remove most of the moisture.
The conveyor belt continues into a battery of drying ovens where a
combination of heat and circulating air thoroughly dries the cotton.
The processed cotton is then fed by blower or conveyor into the
blanket-forming machine, sometimes called a batting or spider web
machine. Its function is to process the loose cotton into a continuous
thin blanket. This is done by a series of carders or combs moving at
high speed.
As the cotton leaves the batting machine, it is in the form of a thin
blanket. It is then fed layer on layer to the final conveyor belt until the
desired thickness of blanket is formed. The blankets are produced in
rolls up to 50 feet in length, in widths of 12,16, 20, and 24 inches, and
in thickness from %-inch to 4 inches.
The final processes of cutting the cotton blanket to proper width,
applying moisture-proof paper backing, and rolling for packing are
usually accomplished in one automatic operation. At the end of the
batting machine the newly formed blanket is moving along a conveyor
belt about 4 feet wide. Slitting knives at the end of the conveyor cut
the blanket to the desired 12,16, 20 or 24-inch width. From under the
conveyor belt the special kraft paper, wax coated on one side and
asphalt coated on the other, is heated on the asphalt-coated side and
fed to the underside of the blanket roll. The entire conveyor moves
through a roller which laminates the paper to the blanket and com­


presses the blanket for packing. At the end of the line the completed
blanket is formed automatically into rolls of predetermined size. The
rolls are cut automatically to the proper length and are then removed

F igure 3. Installing cotton insulation : The cotton blankets are installed between the

framing structure of the w alls in the new house. Other types of batt and blanket
insulation are installed in the same manner.

by hand and hand-packed into corrugated boxes. A sample from at
least every 2,000 square yards of finished insulation is subjected to cer­
tain minimum specification tests as outlined by the Department of



Most of the operations performed in processing cotton insulation
are automatic, so that manufacturing labor requirements per unit of
product are at a minimum. Of the 4.2 man-hours required per 1,000
square feet to process cotton insulation in 1946, half were in the flame­
proofing and drying, and blanket-forming and cutting departments
where most of the processing takes place under the supervision of
machine tenders (table 6).
In contrast, the growing, picking, and ginning of the 210 pounds of
cotton used per 1,000 square feet is estimated to require 84 man-hours.13
The material used in producing cotton insulation is lint (virgin) cot­
ton not less than %-inch staple and of at least certain minimum qual­
ity as required by the Department of Agriculture for participation in
the cotton-insulation incentive program. It is shipped to the manu­
facturers of cotton insulation from various points in the southern cot­
ton belt in 500-pound bales.
T able 6.— Man-hours

required to produce and distribute 1,000 square feet of
8-inch-thick cotton insulation in 1946

Major department or operation

Man-hours per
1,000 square
feet of 3" cot­
ton insulation

Total, all operations.. __ _____________
Production of raw materials.. _ _________ .
Transportation of raw materials _______
_ __ ... _
Handling raw materials_____________________
Flameproofing and drying
Blanket forming and cutting _ _ _ ________
Packing _ . _______
. _ _____ _
___ _
Administrative, clerical, and sales ______ _
Maintenance.. _ _____ ________ ____
Superintendent and foreman _ _______ .
Transportation of finished product______ ... ...

4. 2

The rail haul from cotton gin to insulation plant is estimated from
factory records to average 900 miles for which 0.5 man-hours would be
required for the 210 pounds of cotton used per 1,000 square feet of
finished product.14 Another 0.5 man-hours are needed, according to
industry sources, to produce the 30.3 pounds of paper used in making
1,000 square feet of insulation. The average rail haul of 300 miles to
the plant, as indicated in plant records, is estimated to require only
the fractional sum of less than 0.1 man-hours.
Since the amounts and kinds of chemicals used in flameproofing
and mildewproofing the cotton are trade secrets, no information on
13 Based on estimates for 1946 furnished by the Bureau of Agricultural Economics of
the U. S. Department of Agriculture.
14 Based on an average haul of 25 tons of cotton per car. See Monthly Labor Review,
October 1937, Labor Requirements in Rail Transportation of Construction Materials.


labor requirements could be obtained for these materials, but the labor
needed per unit of product is believed to be very small.
The average rail haul of the finished insulation was estimated at 500
miles in 1946, requiring 0.8 man-hours per 1,000 feet of insulation, com­
pared with 0.5 man-hours for the average of 900 miles that the raw
baled cotton was hauled. The difference is in the fact that 25 tons of
baled cotton can be carried per car, as compared with but 5 tons of the
finished cotton insulation.
Expanded Vermiculite

When vermiculite ore is subjected to heat of 1,400° F. to 2,000° F., it
expands 8 to 15 times its original size and the resulting porous substance
has especially high insulating properties. Exfoliation is the term used
in the industry to describe the expansion process. Dead air cells within
the granules, as well as between the granules of the expanded ore, act
as nonconductors of heat, thus giving vermiculite its insulating value.
Vermiculite is also incombustible.
It is estimated that about 60 percent of all vermiculite ore mined is
used for home insulation. In 1946, this resulted in the production of
something over 13 million cubic feet of expanded vermiculite with an
estimated value of over 3 million dollars.15 The exfoliated ore leaves
the furnace in various sizes of feather light, resilient pellets ranging
from powder or dust to granules about three-quarters of an inch square.
The medium granular sizes are used as a loose fill type of home insula­
tion. Other sizes of expanded vermiculite are used to make precast
concrete slabs for roof, floor, and wall construction. Because of the
light weight of vermiculite, these slabs weigh as little as 25 pounds
per cubic foot. Compressed vermiculite wall board also is being manu­
factured, and in increasing quantities. The powder and smallest
granule sizes of the expanded vermiculite are used as concrete or plaster
aggregate in place of sand. They are also added to garden soil to make
the soil more porous and easy to work.

Vermiculite ore was originally discovered in this country in 1824
near Worcester, Mass., by Thomas H. Webb.16 At present most of
the ore comes from Montana, with a few small mines operating in
Colorado, North Carolina, South Carolina, and Wyoming. At least
one company has also begun to import ore from South Africa. Raw
unexpanded vermiculite ore is composed of minute laminations of
15 Expanded vermiculite was priced at about $75 per ton, f. o. b. plant, in 1946. See
U. S. Bureau of Mines Information Circular 7388, October 1946, p. 11.
lfl G. R. Gwinn, Marketing Vermiculite, Bureau of Mines Information Circular 7270,
January 1944, p. 1.


mineral ore which have a glossy surface similar to isinglass; as
mentioned earlier, the ore is like mica in appearance.
Most vermiculite ore comes from open pit mines. A small percentage
of gangue17 or mineral rock containing the ore is mined along with
the vermiculite. The raw ore is first crushed into smaller flakes.
The oversize gangue is eliminated by a screening operation and the
partly crushed vermiculite goes into a rotary or conical type drier.
Here it is subjected for a short time to a heat of 200° F. to 300° F.
to eliminate part of the free moisture content of the ore. The vermi­
culite is then graded by screening it into four sizes. Each size is
shipped separately in freight cars of 42 to 50 tons capacity. Exfolia­
ting plants order the raw ore by size depending on the ultimate use to
which the expanded ore is to be put. This screening, grading, and
drying operation is usually done at the mine site, but may be per­
formed at the exfoliation plant when it is located near the mine.
While expanded vermiculite has been in continuous commercial use
since the early 1920’s, production has increased considerably in recent
years, and more than tripled between 1940 and 1946. (See table 7.)
T able 7.— Amount

and value of screened and cleaned raw vermiculite ore sold or
used in the United States, 1935-46


Short tons
7, 068
16, 933
26, 556
20, 700
22, 299

185, 787
260, 664
192, 000
174, 587
137, 698


Short tons
57, 848
46, 645
64, 808
71, 289

319, 931
541, 744
712, 890

1 Data for 1935-42 from TT. S. Bureau of Mines Circular 7270, January 1944, p. 6; data for 1943-45 are from
U. S. Bureau of Mines Circular 7388, October 1946, p. 11. Figures for 1946 are estimated on the basis of
information from the Bureau of Mines that production increased 10 percent over 1945 with the price per ton
of vermiculite remaining about the same. (All data are for unexpanded ore.)

When the freight cars of raw ore arrive at the exfoliation plant
siding, they are usually unloaded by hand. The ore is hauled and
dumped or shovelled into a bucket-type conveyor which lifts it over­
head into storage hoppers, from which it moves automatically by
conveyor belt to the top of the furnace.
The various plants use different types of furnaces for the exfolia­
tion process. Each is designed to produce the expanded ore by sub­
jecting it to a heat of 1400° F. to 2000° F. for a few seconds. The
furnaces are usually heated by forced-air oil burners. Some plants
use baffles to delay the fall of the ore through the furnace, while others
have the furnace set at an angle so that the ore flows rather than
falls through the furnace. As the expanded flakes come out of the
17 The mineral or rock material accompanying the ore in a vein.


bottom of the furnace they are again picked up by conveyor belt or
bucket elevator. The flakes, now expanded some 8 to 15 times, are
tough and pliable.
The expansion process occurs as the result of a minute amount of
water between the laminations in the granules of the raw ore, which,
when subjected to the heat of the exfoliation furnace, expands and
turns to gas, forcing the laminations apart to form expanded vermiculite. Excessive heating drives out all the moisture content and
renders the flakes permanently brittle.
When the expanded ore is picked up from the furnace it is ready
for grading and bagging. The plants have different methods of
grading, depending on the uses to which the product is to be put.

Figure 4. Processing and bagging vermiculite : On the left is the furnace used to expand
verm iculite ore. The expanded ore is then elevated overhead into the loading hopper
(center) below which the bags are filled and sealed.

One method is to lift the expanded ore to a long mesh screen set over
the bagging hoppers. As the ore slides down the tilted screen, it
passes over meshes of various sizes. The powder or very fine flakes
drop through the fine mesh into the first hopper, larger flakes through
the second size of mesh into another hopper, etc. Thus the expanded
flakes can be graded into several sizes.
A second method of grading is by the use of forced air. As the
granules leave the furnace and are elevated overhead, a fan drives the


lighter and finer material across an opening into a hopper while the
unwanted heavier granules and unexpanded gangue drop through
the opening. This method is used when very fine ore is being fed
into the furnace, thus producing little of the larger and heavier
material after exfoliation. The average loss by this method is esti­
mated to be about 5 percent.
Until the flakes reach the bagging hoppers the process is entirely
automatic, except for furnace and machine tenders. Bagging, how­
ever, requires some hand labor. A 4-cubic-foot bag is placed under
the spout extending from the bagging hopper. When the bag is
filled it is removed by hand and sealed with gum tape or tie wires.
One 4-cubic-foot bag of home insulation weighs approximately 25
Since the exfoliating of vermiculite is a relatively simple operation,
one or two foremen generally supervise the entire plant.

Three-fifths of the 19.2 man-hours required to process 1,000 cubic
feet of expanded vermiculite in 1946 were needed for the most im­
portant operations of heating, grading, and bagging. An average
of 25.2 man-hours was needed for all production operations, including
the 6 man-hours for plant administration, clerical, and selling func­
tions. Between individual plants, man-hour requirements for proc­
essing and administration ranged from 20.5 to 29.4 man-hours. This
range was found to depend in large part on the degree to which the
plants approached capacity operations. For example, plants operat­
ing on a three-shift basis required an average of 24 man-hours per
1,000 cubic feet of production, compared with the average of 29 man­
hours required by plants operating fewer than three shifts.
It will be noted from table 8, that as much labor was needed to
transport from mine to plant the 3.3 tons of ore used per 1,000 cubic
feet of expanded vermiculite (19.4 man-hours), as was needed to
process the ore (19.2 man-hours) when it arrived. The reason for
this is the long haul, averaging 1,500 miles, from mine to the widely scat­
tered plants, which are situated near their markets rather than near
the few mine locations. Because of its light weight and consequent
bulk, it is impracticable to ship the finished product any great dis­
tance. An average of only 13 tons, in fact, constitutes a fully loaded
car. The average rail haul for the finished product is 200 miles, for
which an estimated 4.7 man-hours18 are required per 1,000 cubic feet
of insulation.
18 See footnote 11.


T able 8.— Labor requirements for all operations in production and distribution of

1,000 cubic feet of vermiculite home insulation in 1946
Total—all operations __
._ _______ _ __
Extraction of ore 1_____________________________
Transportation of ore__________________________
Unloading raw materials___ ____
... _
Heating, grading, and bagging____________ .
_________ . . . .
Administration, clerical and sales________________
Production of fuel used 2________________________
Production of paper bags used 3_________________
Transportation of final product__________________

Man-hours per
1,000 cubic feet
of the final
19. 2
5. 5
2 .2
4 .7

1 About 3.29 tons of ore are needed to produce 1,000 cubic feet of vermiculite home insulation.
2 About 8.9 gallons of fuel oil at mine, 16.3 gallons of fuel oil at plant, and 26 kilowatt-hours of electricpower were required per 1,000 cubic feet of expanded vermiculite, taking, respectively, an estimated 0.2,
0.4, and 0.1 man-hours to produce.
3 An estimated 250 paper bags were used per 1,000 cubic feet of expanded vermiculite produced. The
estimate shown does not include the labor required to produce the paper used in the bags.

Nearly 9 man-hours are expended in producing 3.3 tons of ore in
the mine, covering administration requirements as well as the mining,
cleaning, screening, grading, and loading operations. Over a third
of the mining man-hours are needed in mining the ore, and in pre­
liminary crushing and sorting. Cleaning, screening, and grading the
ore require nearly half the total man-hours. The remaining labor is
needed for maintenance, clerical, and supervisory labor.