John Ripley Freeman.

On the safeguarding of life in theaters; being a study from the standpoint of an engineer online

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stitution which go under the name of asbestos:



42 ON THE SAFEGUARDING OF LIFE IN THEATERS.

1. Chrysotile, which contains about fifteen per cent, of
water, twelve and nine-tenths per cent, chemically combined,
and about two per cent, hygroscopic. This is essentially a
silicate of magnesia.

2. Tremolite, which is anhydrous and is a silicate of lime
and magnesia, with sometimes a little iron.

3. There is a mineral which is asbestiform in character,
a silicate of iron and magnesia, known as anthophyllite.

The first named loses its strength at about six hundred and sixty
degrees Centigrade, or just below redness, on the driving off of
its water, but the last two, containing no combined water, stand
more heat and are said not to fuse until about thirteen hundred
degrees Centigrade, equal to twenty -four hundred degrees Fah-
renheit, is reached. We did not measure this high fusing point.
The behavior of some filaments in a blast lamp indicates a lower
fusing point for the tremolite asbestos than as just stated on
text-book authority.

The Canadian fiber is Chrysotile. This is now the common
asbestos of commerce, and possessing in greater degree than the
others the properties required for spinning and weaving, has come
to be the only kind used in the manufacture of asbestos, canvas.

The Georgia asbestos, although free from water in its chemical
combination, and therefore not decomposing at low red heat, has
for the most part a fiber too brittle for spinning, and is used for
purposes not requiring strength of fiber.

The anhydrous Tremolite and amphibole asbestos are also found
in Siberia and in South Africa; but all the anhydrous asbestos
mined or quarried makes up an insignificant part of the asbestos
of commerce, and although some of the cabinet specimens of
anhydrous asbestos have long, silky, pliable fiber, I was unable
to anywhere obtain cloth made from anhydrous asbestos.

Several kinds of asbestos canvas can be procured in the market.
There is a distinction sometimes made in the trade between " ab-
solutely pure" asbestos canvas, which contains no cotton, being
made for filter cloth or other industrial purposes, and " com-
mercially pure" asbestos canvas, which may contain from
five per cent, to fifteen per cent, of cotton carded in with the
asbestos fiber. These can be distinguished by picking a piece of
the yarn into fine feathery condition and touching a match to the
ends of the fiber and noting the flash and smell of burned cotton.




ON THE SAFEGUARDING OF LIFE IN THEATERS. 43

Certain manufacturers claim that a small percentage of cotton,
besides facilitating the spinning and weaving into a strong, pli-
able canvas, improves the cloth for the purpose of painting a
picture upon it, as for a drop curtain, and claim that this small
amount of cotton does not impair its fire resistance. Asbestos
fibers are very slippery and difficult to card and spin, and by
taking advantage of the spiral structure of the relatively few
cotton fibers to bind the asbestos fibers together, the process of
manufacturing a smooth canvas is greatly facilitated.

A third variety of asbestos cloth which has been highly
recommended (on a priori grounds rather than from tests), by
fire chiefs and architects for a theater curtain, contains very
fine brass wires, of No. 33 and No. 34, standard gauge, or only
about the T F part of an inch in diameter, woven in with the
asbestos yarn. My tests proved that these fine wires add nothing
to the strength of the heated canvas. The wire used was found by
analysis two-thirds copper, one-third zinc, with a trace of lead,
perhaps two per cent. ; this analysis proving it to be an ordi-
nary brass wire. Probably the extreme fineness of the wire used
and the quick oxidation or volatilization of the zinc is a cause of
its weakness when heated.

All of the alleged asbestos curtains that I have seen have really
been of the ordinary commercial asbestos, and I regard the stories
of painted burlap masquerading as asbestos in theater curtains as
mostly idle talk.

Test of Asbestos Canvas.

Since my experiments on the effect of heat upon the tensile
strength of asbestos cloth and asbestos fiber quickly disclosed that
the ordinary commercial asbestos lost its strength at a heat just
below redness, sufficient to drive off the combined water, in order
to be sure of our ground, I had three independent series of tests
upon asbestos made by three different experts, and by very dif-
ferent methods, myself laying out only the outlines of the test
desired and leaving the observations and reports to the respec-
tive experts. The results of all three tests proved independently
that the character of asbestos cloth as to resisting a high degree
of heat is utterly different from what is popularly supposed.

\st Series.

The first series of tests were made by Prof. Charles E. Ful-
ler, in the Mechanical Engineering Laboratory of the Massa-



44



ON THE SAFEGUARDING OF LIFE IN THEATERS.




FIG. 9. ARRANGEMENT FOR TESTS OF STRENGTH OF ASBESTOS CANVAS IN
MECHANICAL ENGINEERING LABORATORY, MASSACHUSETTS INSTITUTE OF
TECHNOLOGY. TENSILE STRENGTH MEASURED EITHER COLD OR WHILE
HEATED BY COMMON GAS FLAME OR BY BLAST LAMP FLAME TO
VARIOUS DEGREES.



ON THE SAFEGUARDING OF LIFE IN THEATERS. 45

chusetts Institute of Technology, on a special testing machine
designed for measuring the strength of sailcloth, and which had
been previously used in a series of tests for the United States
Government. We had this newly fitted with double Bunsen gas
burners, as shown in Fig. 8, so arranged that a specimen could
be either tested cold or tested for strength while heated to any
desirable degree in either an ordinary gas flame or heated up to
moderate redness in the flame of the blast lamps. These tests
were made with great care, repeating the test on two or more
specimens in almost every case and with a degree of attention
to detail which I have not space here to set forth. I believe the
results absolutely reliable.

In brief, we found that every one of these specimens of asbestos
canvas, English, French and American alike, when heated for
from two to five minutes to a little Mow redness in a common gas
flame, or barely to redness in the Bunsen flame, lost from sixty per
cent, to ninety per cent, of its strength, and that the fiber became
very brittle.

We were surprised to find that the samples with the wire in-
sertion, when tested hot, were no stronger than the samples
without wire. On cooling, they regained a little of the strength
due to the wire.

I have condensed the results of these tests into the tables
which follow on pages 110-113.

At time of weighing and measuring thickness of American
samples, relative humidity of air was 80 per cent., temperature,
68 degrees F. When weighing and measuring foreign samples,
relative humidity, 25 per cent., temperature, 71 degrees F.
Thickness measured between glass plates, 2 inches square, under
8 Ibs. pressure. Specimens prepared by cutting strips 2|- inches
wide and ravelling out threads carefully from edge until width
of 2 inches to the nearest thread was left.

All specimens held with 12 inches between the grips.

Load applied, in nearly all tests, by stretching the cloth at rate
of -J- inch per minute. A few tests were run at speed of stretch-
ing at rate of | inch per minute for comparison, and no differ-
ence found in strength between these rates of speed.

The results of test of strength while hot, given in the table, are
averages from three specimens. These three different trials gave
substantially uniform results.



46



ON THE SAFEGUARDING OF LIFE IN THEATERS.



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samples tested lengthwise (warp).
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[Total strength of wire per inch in


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Per cent, of elongation lengthwise (mean
" " tk " crosswise
Strength of Cloth Tested Cold (in Ibs. per inch
Crosswise Filling
Lengthwise Warp (mean of 2)



ON THE SAFEGUARDING OF LIFE IN THEATERS.



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ON THE SAFEGUARDING OF LIFE IN THEATERS.





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CONSTRUCTION.


ber marked on sample
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cent, loss of weight found after heating bright r
>able per^ cent, or cotton


f No. of warp threads per inch
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[ Total strength of wire per inch in widt


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" strands asbestos yarn
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g j Diameter of brass wire in inches
| Breaking load 1 wire cold in Ibs
t Total strength of wire per inch in widt


tch of Cloth at Maximum Load:
Per cent, of elongation lengthwise (mean of 2).
; ' " u " crosswise (mean of 2) ...
ngth of Cloth Tested Cold (in Ibs. per inch in w
Crosswise filling


Lengthwise warp

nqlh, of Cloth While Heated (in Ibs. per inch
1 ' of following samples tested lengthwise (wan
Heated 1 side in common gas flame 5 in In. (did
Heated 1 side in common gas flame 10 min. (die
Heated both sides in common gas flame 5 min.


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ON THE SAFEGUARDING OF LIFE IN THEATERS.



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50



ON THE SAFEGUARDING OF LIFE IN THEATERS.



The following tests were made on strength of samples of
asbestos twine, cord, and rope, manufactured by the H.W. Johns-
Man ville Co. :

TABLE 3.



Material.


Description.


No. of
Specimen.


Breaking Load.


ELONGATION
AT MAXIMUM LOAD.


Measured
Length.


Elongation.


Three-
eighth-
rope.


3 strands, each
made up of 8 threads
of plain asbestos
with a manila core.
Each thread made
up of 2 strands of
yarn, of which 15.5
ft.in length weighed
1 pound.


11-1
11-2

11-3
11-4


588 Ibs. broken cold.
600 Ibs. broken cold.

( broken hot after
108 Ibs. 4 heating dull red 5
( minutes.

( broken hot after
81 Ibs. < heating bright red
f 5 minutes.


3 inches.


18.7*












One-
quarter-
inch rope.


3 strands, each
strand made up of 4
threads of plain as-
bestos.
Each thread made
up of 2 strands of
yarn, 67.5 feet per
pound.


12-1
12-2

12-3


330 Ibs. broken cold.

( broken hot after
45 Ibs. { heating to dull red
( 5 minutes.

( broken hot after
37 Ibs. < heating to bright
( red for 5 minutes.


3 inches.


14.6










One-
eighth-
inch cord.
No. 804.


4 threads of plain
asbestos, each
thread consisting of
2 strands of yarn,
of which 130.5 feet
weighed 1 pound.


13-1

13-2

is-;

13 <
13-5

13-6


143 Ibs. broken cold.
132 Ibs. broken cold.
124 Ibs. broken cold.
( broken hot after
14 Ibs. < heating to dull red
1 5 minutes.
( broken hot after
18 Ibs. < heating to bright
( red for 5 minutes,
f broken hot after
Q- lhs J heating in com-
1DS - 1 mon gas flame 5
[minutes.


12 inches.


7.3*






















Plain
asbestos
sewing
twine.


2 strands plain
asbestos yarn, of
which 1,364 feet
weighed 1 pound.


14-1
14-2
14-3

14-4
14-5

14-6


12.8 Ibs. broken cold.
11.3 Ibs. broken cold.
1 .Olbs. broken cold.
( broken hot after
1.1 Ibs. -j heating bright red
f 2 minutes.
( broken hot after
1.0 Ib. -< heating bright red
f 5 minutes.
i broken hot after
heating in com-
mon gas flame 5
minutes.






























Sewing
twine
with wire
insertion.


2 strands of plain
asbestos yarn, 2
strands of brass
wire; diam. .0070
inch; 833.3 feet per
pound.

of 1 wire cold, 2.12
pounds.


15-1
15-2
15-3

15-4
15-5

15-6


17.0 Ibs. broken cold.
19.5 Ibs. broken cold.
16.5 Ibs. broken cold.
I broken hot after
2.1 Ibs. -< heating bright red
( 2 minutes.
i broken hot after
1.9 Ibs. < heating bright red
{ 5 minutes,
f broken hot after

o ih e j heating in com-
~.6 IDS. mon gas flame 5

t minutes.





























It will be seen from the above that the asbestos cord was
affected by heat very much as the canvas was, and lost nearly all
of its strength after brief exposure to dull red heat, or even to a
heat a little below.



ON THE SAFEGUARDING OF LIFE IN THEATERS. 51

%d Series.

I desired tests on larger sheets of the canvas, more nearly
reproducing the conditions of use, and so the tests of our second
series were made at the Underwriters' laboratory, in Chicago, by
constructing asbestos curtains about six feet square and testing
them \vith the same furnace and apparatus that had been provided
in the yard attached to this laboratory for testing fire shutters
and fire doors. Unfortunately, we found the furnace in poor
working condition because of a temporary defect in the gas
supply, such that we could not regulate the temperature evenly
over the entire curtain or measure it precisely by the pyrometer.
After testing several curtains we therefore suspended these tests.

The large sheets under these conditions made a much better
showing than the small samples had made in the laboratory.
Nevertheless these Chicago tests fully confirmed the conclusion,
derived from our Boston tests, that asbestos cloth is rapidly
weakened by the heat of an ordinary fire to an extent that
makes a curtain composed wholly of asbestos cloth an unreliable
fire screen for the proscenium arch of a theater, if expected to
endure more than a few minutes; and it was proved that the
asbestos canvas was so weakened that it would be ruptured easily
by a blow from any falling material or by a strong current of air.
It was noted during these tests that the Johns curtains were
found particularly weak in their horizontal threads,, permitting
vertical rents to be easily made. That the canvas of the other
makers is similarly much less strong horizontally than it is
vertically, or in the warp threads, may be seen from the Boston
tests set forth in the previous table. The seams sewed with as-
bestos thread showed no special weakness more than the canvas.

A notable feature in these furnace tests with those curtains
that contained from five per cent, to eight per cent, of cotton
was the flame that played all over the outer face of the cloth
when the furnace was lighted, and which might be disquieting
to an audience in giving for a moment an impression that the
asbestos curtain was burning up.

3d Series.

For the third series of tests, the friendly services of Prof .AVilliam
Otis Crosby, of the Massachusetts InstituteJ of Technology,
in charge of the Department of Economic Geology, and Dr. C.
H. Warren, Professor of Mineralogy in the same institution, were



52 ON THE SAFEGUARDING OF LIFE IN THEATERS.

enlisted to examine all of the varieties of asbestiform mineral
found in the extensive cabinets of the Institute of Technology and
the Boston Society of Natural History, in the hope of finding
specimens from some locality that possessed all the qualities prop-
erly attributed to asbestos. The result of this search, in brief,
was that nothing was found possessing characteristics materially
different from the hydrous Canadian fiber on the one hand, and
the anhydrous fiber from Georgia on the other. Specimens of
asbestos of the first class lost their strength at a heat which drove
the water off; the specimens of the second class had fibers that
were too stiff or too brittle for spinning and weaving, or they
were reported as occurring in quantity too small for commercial
purposes.

Tests were next made by Professor Warren to learn the precise
degree of heat required to injure the strength of asbestos fiber.

Upon testing specimens of the Canadian fiber by heating it in
a platinum crucible within a clay cup and within a coil heated
electrically, raising the temperature slowly, weighing the spec-
imen repeatedly, and all the time measuring the temperature in
the crucible by electrical methods, it was found that a tem-
perature up to 250 Centigrade, equivalent to 482 Fahrenheit,
caused no driving off of the water chemically combined, and no
apparent change in the pliability or strength of the fiber.

A heat just below dull redness proved to be the critical point.
One-half hour at from MO C. to 480 C., equivalent to 850
F., drove off about three per cent, of the combined water and
made the fiber slightly more brittle than at first, with some loss
of natural luster.

Heated to from 630 to 650 C. for five minutes, averaging
1,152 F., eleven per cent, of the water is driven off, and the
fiber becomes slightly brown and very brittle and crumbly.

Heated for only thirty seconds to 750 C., the fibers of
Canadian asbestos lost their cohesion.

A faint red heat corresponds to about 1,100 F. (The
Austrian engineers, from their experiments on a theater model
in 1885 already referred to, concluded that a temperature of
800 C., equal to 1,472 F., would be reached in a fire on a stage
crowded with scenery.)

In other trials Professor Watson found that a piece of asbestos
canvas a foot square lost its strength badly, so that it could be
torn between the fingers, after it had been held five minutes



ON THE SAFEGUARDING OF LIFE IN THEATERS. 53

against a moderate wood fire that did not heat it to visible red-
ness, and therefore probably not to 650 C.

From a series of such tests Professor Warren concluded that a
theater curtain made of the Canadian or chrysotile asbestos fiber
alone could not be expected to hold together for more than a few
moments if a temperature of 650 0., equivalent to about 1,200
F., was reached, and calls attention to the fact that, being a
non-conductor, the asbestos canvas would arrest and absorb the
radiant heat from the burning scenery and have its own tem-
perature rapidly raised.

In the course of sundry other tests he found independently that
the fine brass wire inserted in certain samples of the canvas added
practically nothing to their strength while hot. Wires pulled
out from the canvas and held in an open Bunsen flame lost
their strength instantly. This led Professor Warren to suggest
that iron wire of, say, twenty-five or twenty-eight gauge, would
prove a much greater addition to the strength of curtain cloth,
but in the Chicago furnace tests I noted that iron wire, about
No. 18, was rapidly oxidized in the gas flame.

The asbestos residue left after driving off the water is practi-
cally infusible and would doubtless adhere together, and being
supported by the iron wire would form, an effective screen long
enough to permit escape and to restrain the progress of the flames
while the firemen were coming. Wrought iron is less readily
fusible than steel, but the steel wire would probably hold up to,
say, 900 C., or 1,650 F. Steel melts at 1,200 to 1,300 C. ;
pure iron about 1,600 C.

I found the small iron stairway over the Iroquois stage showed
effects such as are produced by red heat. Glass in the skylights
over the Iroquois stage was fused, which indicates about 875
to 900 C., or 1,650 F.

Steel Plate Protected by Asbestos Material.

In our Chicago furnace tests we also experimented upon sun-
dry combinations of asbestos, .asbestos felt and asbestic cement,
with thin steel plate and combined with wire netting, the asbestos
being placed on the stage side in the hope that it might shield the
steel from the full heat and thus prevent it showing red hot on
the auditorium side, while the steel would give strength. We
had to suspend these tests because of some temporary trouble with



54 ON THE SAFEGUARDING OF LIFE IN THEATERS.


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Online LibraryJohn Ripley FreemanOn the safeguarding of life in theaters; being a study from the standpoint of an engineer → online text (page 4 of 8)