This report bar been prepared *;

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NATIONAL BUREAU OF STANDARDS REPORT

v.

8398

WALL CLADDING MATERIALS;

SMOKE PRODUCTION MEASUREMENTS UNDER SIMULATED FIRE CONDITIONS

First Progress Report

by

D, Gross

J. J* Loftus

and

R. Harris

¥ x 1, 0

U. S. DEPARTMENT OF COMMERCE

NATIONAL OUREAO OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

42102-12-4210405

42102-12-4210120

July 14, 1964

NBS REPORT

8398

WALL CLADDING MATERIALS:

SMOKE PRODUCTION MEASUREMENTS UNDER SIMULATED FIRE CONDITIONS

First Progress Report

by

D. Gross

J, J, Loftus

R. Harris

Fire Research Section

Building Research Division

Sponsored by;

Federal Housing Administration

Under Agreement No. IAA-35

and

National Bureau of Standards

IMPORTANT NOTICE NATIONAL BUREAU OF f

for use within the Governmenl

and review. For this reason, whole or in part, is not auth Bureau of Standards, Washinj

the Report has been specifical

Approved for public release by the

Director of the National Institute of

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ress accounting documents intended

is subjected to additional evaluation

re listing of this Report, either in the Office of the Director, National

Dy the Government agency for which

copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Table of Contents

Page

1. Introduction 1

2„ Physical Optics of Light Transmission 3

3. Method of Test 6

4. Results 7

A. Dilution Tests 7

B. Gas Flame Exposure Tests 7 C. Electric Furnace Exposure Tests 8

D. Smoke Data Correlation 9

5* Conclusions 9

6„ Future Work 10

7. References 11

v

WALL GLADDING MATERIALS:

SMOKE PRODUCTION MEASUREMENTS UNDER SIMULATED

FIRE CONDITIONS

by

D. Gross, J. J . Loftus and R. Harris

ABSTRACT

Preliminary test results are presented of the smoke

production characteristics of several materials within the

Rohm and Haas XP2 Srnoke Density Test Chamber. A variety

of specimen exposure conditions were examined and the

results were compared for reproducibility.

The Beer~Bougu,er law appears to be applicable to

the attenuation of light by smoke, and quantity of smoke

is therefore reported in terms of optical density rather

than percent light absorption.

Preliminary results are presented which indicate a fair degree of correlation between the optical density of

smoke produced by a material in the test chamber and the

gravimetric smoke deposit from a standardized flame- spread

test (ASTM E162-62T)

1. Introduction

When accidental fires occur, the smoke generated often represents the

major life danger to occupants. Although lethal amounts of carbon monoxide

are invariably •‘present , the obscuration of vision by dense smoke all too

often prevents the direct and logical escape by occupants, or rescue by

firemen, during the few minutes available prior to the onset of unrespirable

conditions, monoxide unconsciousness, etc.

Smoke hazard is appreciated only in a qualitative way, and uniformity

in its description and quantitative measurement are seriously lacking.

The ’"smoke generation®* limitations on structural or decorative finish

materials imposed by some building codes are almost entirely empirical and

very little is known about the actual hazard under the wide variety of real

fire situations. Many localities and jurisdictions have no smoke requirements for decorative finish materials; and ordinary furnishings are almost

entirely free of restrictions.

The test methods presently used for establishing smoke requirements

were devised principally for comparative surface flammability measurements,

and the relationship between the results of such tests and the visual obscuring qualities of smoke are. not well established. It is the immediate

objective of this study to investigate the suitability of a laboratory

uietbod. If ar—evaluafeliigltlie appropriate optical properties of smokes which

obstruct human vision in bui lding fires.
2

Some people consider smoke to be the gaseous products of burning organic

materials in which small solid and liquid particles are also dispersed.

Others limit smoke to the unburned solids, such as carbon and ash, suspended

in air. However, under typical conditions of non-flaming thermal decomposition, it has been fairly well established that wood smoke consists for the

main part not of carbon particles, but primarily of homogeneous spherical

tarry droplets [1], and of liquid droplets of organic substances of fairly

high boiling, points, approximately 100 °C 1 2] . In addition to its effect

upon visibility, smoke is usually extremely irritating both to the eyes and

to the mucous membranes of the respiratory tract.

The earliest smoke measuring methods appear to have been developed in

attempts to control stack effluents and the terminology used in describing

the smoke quantitatively is unfortunately lacking in clarity and uniformity.

"Smoke density" and "opacity," terms normally appropriate for generic descriptions have been associated with a variety of unassociated test methods.

For example, the "total obscuring power" (T.O.P.) of a smoke is considered

to be the product of (1) the volume produced per unit weight of material

used, and (2) the "density of concentration*' [3],

Considering only the limited problem of visibility through smoke, a criterion sometimes used is based on the assumption that when the "visibility"

(preferably "visual range") of a light source or an illuminated sign drops

to four feet, a room is smoke-logged to a degree that would seriously impede

the escape of occupants [4]It was inferred that this limit was reached

when the light transmitted was reduced to 10 percent of the value in the

absence of smoke.

Test methods for measuring smoke are generally of two typesj (1) those

in which light transmission measurements are made on the smoke aerosol

directly, and (2) those in which the smoke particles are collected on a suitable filter paper which can be either weighed or measured for light

transmission.

Of the first group, those employing unaided visual methods, such as the use of the Ringelmann chart, are generally not considered adequate for

laboratory measurements , Instrumental visual methods include the Smokescope,

Umbrascope, etc., in which visual comparisons of the unknown smoke are made

relative to reference shades or gray glasses. The most common method for

smoke suspension measurements today employs a light source', and a photoelectric

cell, arranged so that the electrical output of the cell may be used as a measure of the attenuation of light by the smoke. The readings are commonly

expressed as percent absorption, or in terms of the Ringelmann scale

(0 to 5). Attempts have been made to relate the percent light transmission

to an optical density scale and to a quantitative smoke concentration (i.e,,

mass per unit volume) [1, 4] . Such methods are considered generally valid

and will be discussed in more detail later. There does not seem to be any

satisfactory justification for integrating the area under a percent light

absorption versus time curve [5] or for making allowance for residue deposits

on lenses at the end of a test by simple subtraction of light absorption

readings.
- 3 -

In the second type of smoke measurement, a known volume of gas is

filtered through a known area of filter paper and the resultant spot is

classified according to its degree of blackness, and commonly referred to as "smoke shade." It has been assumed that the Beer-Bouguer law could be

applied to smoke shade spots, and that optical density ( = log 100

10 XT ) based on light transmission is an appropriate measure [6]

.

Others have

shown a preference for light: reflectance measurements on smoke shade [7].

2„ Physical Optics of Light Transmission

In formulating the requirements for a suitable laboratory smoke meter,

primary consideration should be given to the optical properties of the smoke,

but it is necessary also to understand the influence of the target, the

amount and distribution of light and certain psychophysical properties

related to human vision (e.g. contrast level, response time and adaptation

to levels of illumination). The problem is a very complex one lying as it

does somewhere between physics and psychology. Considerable aids to understanding various aspects of the problem are the very complete study of

particulate clouds in a book by Green and Lane [8] and the comprehensive

survey of available knowledge on vision through the atmosphere given in a book by Middleton [9]. To reduce the problem to a level of possible solution,

certain simplifications and assumptions must be made and it is necessary

at the outset to ignore complications due to eye irritations or respiratory

effects, hysteria and associated physiological and psychological factors.

To relate the visibility of an object to the obscuration caused by the

presence of a "quantity of smoke" -(or other particuldte’ cloud) , it is

necessary .to apply the appropriate laws of extinction and of reduction in

contrast. A "quantity of smoke" can only be defined when the total weight

of material in suspension, its physical properties and the state of its

dispersion are completely known. Since it is impracticable to determine

all this information even in the laboratory, it becomes necessary to substitute for it a single measurement which is as characteristic of the

smoke as possible. The property most frequently selected for this purpose

is its optical density, the attenuation of a beam of light passed through

the smoke [10].

If we consider a volume of smoke through which a parallel

light is passed, then the law of extinction, commonly known as Bouguer law (sometimes also as the Lambert-Beer Law, or simple

is given by

-Kcl

e

beam of

the BeerBeer’s Law)

where F is the transmitted flux

F is the incident flux

o K is the attenuation coefficient

c is the concentration, and

1 is the path length.
- 4 -

Optical density is defined as

2.303

Kcl

( 2 )

This relation and the Beer-Bouguer law are strictly true only for

monochromatic light for which the optical density is independent of the

light intensity and the receiver sensitivity. However, it has been shown

that the relation can be applied to light from a tungsten lamp passing

through wood smoke by the use of an effective extinction coefficient [1],

When two smoke clouds are compared by optical density measurements

(reduced to a common path length), a comparison is thus being made of the

product Kc. Since the attenuation of light is the very property required

to determine visibility through smoke, it may be unnecessary to relate

particle size distribution, refractive index, coagulation, etc. to particle

concentration. It has been shown, however, that the particle size distribution in a smoke is a major factor in determining its optical density, and

that in many cases, an increase in the size range of a given smoke leads

to a decrease in optical density [10], When coagulation of heterogeneous

smoke is considered, the variation in optical density with time is expected

to be less pronounced.

The contrast between an object, of brightness B and the background of

brightness B' (e.g. a small luminous object seen against an extensive darker

background) is given by

As G decreases, a point is reached where the object can no longer be

distinguished from its background, and the value of C is then known as the

"threshold of brightness-contrast" or simply "threshold contrast." An

unpleasant aspect, of the present problem is the question of the effectiveness

of a person’s vision when he is aware that failure to use his eyes sufficiently

well may result in his sudden death. We will assume that this is the situation

which confronts a person trapped in a smoke-filled room or building.

The presence of a particulate cloud between the object and the observer

serves to attenuate the light from the object and from the background, and

also to scatter external light coming from all directions. The attenuation

of light directly transmitted through a particulate cloud is expressed in

terms of the exponential extinction relation, so that, for the apparent

brightness of the object

t B e -2.303D

(4)
5

and for the apparent brightness of the background

K

= B

-Kcl

B'

-2.303D

e (5)

External light scattered from the cloud gives rise to glare, G, which is

superimposed on the brightness of both object and background.

Hence the new contrast, which is in effect an "obscuration threshold,"

will be

C

t

»n.jcrTrl«wv.i»ni (B Mi B') n i nr n nfc--i» „ -%cl

-Kcl

&

+ G

(B - B

1 ) e 2.303D

B e2 - 303D

+ G

( 6 )

If the value of falls to the threshold value for given conditions,

the object will be indistinguishable from its background, i. e„, it will be

obscured.

Elaborate research on the threshold of brighness-contrast was carried

on during World War II and reported by Blackwell [11]. In general, G was

found to be a function of the angle subtended by the stimulus, the adaptation

brightness, the sharpness of the boundary, and the presence of disturbing

stimuli in the field of view.

Using this data, it may be instructive to estimate the optical density

of smoke through which a 60 watt incandescent lamp (brightness ~1000 footlamberts) may be distinguished against a dark background and in the absence

of glare. As an extreme, it is assumed that adaptation from a normal

brightness level of 10 foot-lamberts occurs with sufficient rapidity down

to a practical lower limit of 10”'“’ foot-lamberts. At a distance of 6 feet, a typical incandescent lamp subtends an arc of 2 degrees, and from the data

of Blackwell (for a 6-second viewing exposure and for 50 percent probability),

the threshold of brightness contrast is 0.346. This fields an optical density

of 7. 85, or 1.31 per foot, and suggests that, for a one -foot. smoke path, it

is probably desirable to make laboratory measurements of transmittances of

1 percent or less.

It should be borne in. mind that these results apply to a 50 percent

probability of detection by trained observers who were seeking a contrast

difference in a known location. To allow for 98 percent probability of an average observer detecting a contrast difference in any (unknown) direction,

a "field factor" of 10 or so should probably be applied to the contrast

threshold value . Furthermore, adaptation to such levels of brightness may

not be sufficiently rapid or even attainable due to interference in sensory

adaptation by the smoke. Finally, in atmospheres of dense, heterogeneous

smoke, and in the presence of external light, the effect of scattering (glare)

would be expected to affect the contrast threshold.
- 6 -

It may be useful to note that the apparent color of light seen through

a haze or smoke is a function of the color temperature of the lamp, the

thickness of the smoke layer and the number and size of the particles. For

example, an ordinary lamp operating at 2850°K may appear to be a fairly

saturated orange at a distance of 1/2 mile through a common industrial haze.

The apparent color of red signal lights is not appreciably changed by the

atmosphere,

3, Method of Test

The basic test apparatus used in the initial phase was developed by

the Physics Laboratory of the Rohm and Haas Company [12] and was made

available by them for our use. As shown in Fig, 1, the Smoke Density Test

Chamber consists of a sheet metal chamber 12 by 12 by 30 inches, instrumented

to permit the measurement of the obscuring qualities of the smoke with

respect to light and visibility produced during the burning of a small

specimen. No account is taken of the irritating or toxic effects of the

combustion products.

A 3-watt light source, a barrier-layer photoelectric cell with visual

correction filter and a meter are used to measure the transmission of light across the chamber. The 12-in, light beam path is located approximately

11 inches from the top of the chamber. A white-on-red '’Exit*' sign, backlighted by two 6-watt fluorescent lamps, is mounted on the back panel and

may be viewed through a transparent glass door. However, no visual observations with respect to obscuration of this sign have been made.

The chamber is closed and unventilated during test except for 1-inchhigh ventilation openings around the bottom. Although with high smokeproducing materials there is some spillover of smoke through these openings

this may be considered negligible. As indicated in the Introduction, all

light transmission values were converted to the logarithmic optical density

scale, the instantaneous values of which are a direct measure of the total

or integrated quantity of smoke produced up to that time. After the chamber

is cleared of smoke at the end of a test, a final optical density value is

obtained which may be subtracted from the test values to allow for residue

deposits on the lenses.

The standard exposure, according to Rohm and Haas Physics Laboratory

Method P-148B, consists of a flame from a propane burner operated at a pressure of 40 psi. The test specimen is 1 by 1 by 1/4 inches, and is preconditioned prior to being placed on a prescribed supporting screen for

test, the duration of which is 4 minutes. However, because of large variations in test results caused by random variations in ignition behavior

during test, modifications from "standard conditions" were freely made and

are discussed under "Results." One of the principal advantages of this

test method is that procedural changes and the exploration of variables can

be made quickly and economically.
7

In order to provide a continuous record of light transmission values

during each test, the meter output was fed into a 100 millivolt full-scale

recorder. Since the Test Chamber was provided with a high sensitivity switch

(XI 0) , and since the recorder sensitivity could be readily switched to 10 mv

full scale, a total of three ranges of light transmission were available,

0 to 100 percent, 0 to 10 percent, and 0 to 1 percent.

4. Results

A, Dilution Tests

A series of tests were performed to demonstrate, the applicability of the

exponential extinction law for a range of smoke types and levels produced with

a propane burner exposure. For this purpose, a large cylindrical carton was placed close to the Smoke Density Test Chamber and connected to it at top and

bottom with 3 in. dia. tubes (See Fig. 2). By attaching the upper tube to

the Chamber's blower exhaust and closing the ventilation openings (except

for the lower tube), a closed system of greater volume resulted through

which a given quantity of smoke could be circulated. Using a variety of

test conditions (propane pressure, screen type, specimen size, etc.) and

materials (cellulose-base, rubber, magnesium), a wide range of light transmission (maximum smoke absorption) readings were obtained. Upon reaching an established value, which was noted, the exhaust blower was turned on and the

smoke circulated and diluted into the larger volume. A second light transmission value was then obtained. The ratio of the optical density for the

initial condition (test chamber volume only = 2.58 cu ft) to the optical

density for the final condition (total volume of test chamber, carton and

interconnecting tubes, 8.49 cu ft) is plotted in Fig. 3 as a function of

the. optical. . density of the smoke prior to dilution. Exact conformance to

the Beer-Boriguer law, for which optical density is directly proportional to

concentration or inversely proportional to volume, would yield a value of

8.49

~ 3.29 for this ratio. It might be noted that deposition, condensation

or other loss of suspended smoke particles upon circulation would result in

high values for the optical density ratio. Considering the wide range in

the level of smoke investigated, the general validity of the exponential law

appears fairly wTell established. The applicability of the Beer-Bouguer law

to the attenuation of light by wood smoke over a mass concentration range

of 0 to 2 mg/1 was also verified, by Foster [l] on the basis of optical and

gravimetric density measurements.

B. Gas Flame Exposure Tests

Using the "'standard11 exposure, Rdb.m and Haas Method P-148B, considerable

variation in test results were obtained from duplicate tests on a standard

hardboard. These differences were mainly associated with whether ignition

occurred slowly or rapidly, whether one comer was exposed to a greater

extent than another, and whether flame extinction occurred during a fiveminute exposure. More reproducible results were obtained at 20 psi than
« 8 ”

at 40 psi and this pressure was used for subsequent tests. When the optical density reading for one test is plotted against the optical density

reading for a repeat test, the results for a variety of wall finish materials

plot as shown in Fig. 4A. Good reproducibility of results would yield

points close to the 45® line.

Several alternate exposure conditions were investigated in an attempt

to Improve reproducibility of data. One involved placing the 1 by 1 in.

specimen over a metal shield containing a 7/8 by 7/8 in. opening (to re- duce edge smoking effects) and employing a horizontal multi-holed burner

of larger area (1.5 in. dia.) in order to provide a more uniform flame

exposure. However, due mainly to the metal shield, increased smoking and

generally higher values of optical density were obtained. Results for

duplicate tests under these conditions are shown in in Fig. 4B.

Another alternate exposure condition involved mounting the 1 by 1 in.

specimen vertically in a holder, so that the standard burner flame impinged

at a 45 degree angle on the surface and with all other surfaces (except the

top> edge) covered. This permitted a greater access of air to the flameexposed surface and resulted in generally lower optical density readings,

the results for duplicate tests being shown in Fig. 4G.

G. Electric Furnace Exposure Tests

In an effort to obtain more reproducible results, a small electricallyheated furnace was constructed, and placed on the floor of the Test Chamber.

The 1 by 1 in. specimen was placed in a shallow porcelain combustion boat

and inserted into the pre-heated furnace at zero time. When operated at

80 volts, the air temperature at the specimen location was approximately

750°G and active flaming resulted in every case. The use of a small pilot

burner was explored but was not considered necessary at this time. Results

of duplicate tests under these conditions are shown in Fig. 4D.

A series of tests were also performed on a standard hardboard material

using a range of voltage settings (and corresponding temperatures). The

results of this series is shown in Fig. 5. It may be noted that at a voltage

setting of 40 volts, corresponding to an air temperature of about 360°G, a slow non flaming pyrolysis was obtained with considerable smoke production.

This smoke appeared to consist of small, particles of light grey or blue

color, the smoke produced during flaming under 750°G exposure consisted of

large black particles. These exposure conditions are considered representative of those encountered in the early and advanced stages of building fires,

respectively, and may represent two possible test conditions for the evaluation of the, smoke generation characteristics of a material.

For these test conditions, replicate tests were performed on the following materials* hardboard, red oak, and acrylic and polystyrene plastics.

Typical results are shown, in Figures 6A and 6B. Whereas the higher
9

temperature exposure produced rapid flaming and peak optical density values

in 1 or 2 minutes (Fig. 6A) , the lower temperature exposure under nonflaming

conditions resulted in much slower smoke production rates. (Fig. 6B) . It may

be noted that the peak optical density values for hardboard and red oak were higher for the lower temperature exposure whereas those for the two

plastics were lower.

As shown in Fig. 4 and Table 1, the electric furnace exposure provided fairly consistent results, and generally low coefficients of variation compared to the other test conditions explored. However, it is

possible that some variability may be introduced due to the existence of

temperature gradients along the furnace axis. A second electric furnace

is being planned in which the electric resistance winding will be arranged

to provide a more nearly uniform temperature in the central portion of the

furnace.

D. Smoke Data Correlation

A comparison was made between the optical density values from the

Smoke Density Test Chamber (electric furnace exposure, 80V) and the smoke

deposit values from a standard radiant panel flame spread test (ASTM E 162-

62T) on a wide variety of materials. These results are shown in Fig. 7.

Although there is considerable scatter for several heavy smoke producing

materials, a definite and reasonable correlation is seen to exist.

5. Conclusions

A small-scale laboratory test chamber appears to be useful for the

evaluation of the smoke generation characteristics of materials under

simulated building fire conditions. The use of a controlled exposure

within an electrically-heated furnace appears to be capable of yielding

more reproducible results in the Rohm and Haas XP2 Smoke Density Test

Chamber than any of several exposures using an open propane burner flame.

On the basis of the dilution experiments described in this report

and the results of Foster [l], the Reer-Bouguer law for the attenuation

of light by smoke over a wide range in concentration appears to be valid.

The entire theoretical background of the subject leads naturally and

logically to a logarithmic rather than a linear scale, and it is appropriate, therefore, to use optical density rather than a linear light ab.- sorption scale to characterize "smoke density".

Preliminary results indicate that a fair correlation exists between

the optical density of smoke produced by a material in the electricallyheated Test Chamber and the gravimetric smoke deposit from a standardized

flame-spread test (ASTM. E 162-62T).
- 10 -

6* Future Work

Work will continue in an effort to establish suitable and reproducible

test conditions and procedures with the Rohm and Haas Smoke Density Test

Chamber. Tests will be performed on a wide variety of typical wall cladding

materials,. Tests results on typical materials will be analyzed and com- pared with similar results obtained by other smoke test methods. In addition, the applicability of the test results to conditions in full-size

rooms will be considered.
- 11

7. References

[1] Foster, W. W. , "Attenuation of Light by Wood Smoke", Brit. J. Appl.

Physics, JLO, 416-20, 1959.

[2] Akita, K. , "Studies on the Mechanism of Ignition of Wood", Report

of the Fire Research Institute of Japan, 9 , March 1959, p. 43.

[3] Prentiss, A. M., "Chemicals in War", McGraw-Hill Book Co., 1937.

4 [ 4] Kingman, F. E. T„, Coleman, E. H. and Rasbash, D. J., "The Products

of Combustion in Burning Buildings", Brit. J. Appl. Chem. , J3 , 463-8,

1953

[5] "Standard Method of Test for Surface Burning Characteristics of

Building Materials", ASI'M Designation E84-61, 1961.

[6] Hemeon,W.G.L, et al, Air Repair, pp. 22-28, August 1953.

[7] "Tentative Method of Test for Smoke Density in the Flue Gases from

Distillate Fuels", ASTM Designation D2156-63T, 1963.

[8] Green, H. L. and Lane, W. R., "Particulate Clouds: Dusts, Smokes

and Mists", Van Nostrand Co., Inc., 1957.

[9] Middleton, W. E. K. , "Vision Through the Atmosphere", U. of Toronto

Press, 1952.

[10] Axford, D. W. E„, Sawyer, K. F., and Sugden, T. M. , "The Physical

Investigation of Certain Hygroscopic Aerosols", Proc, Roy. Soc.,

Al 95 , 13-33, 1948.

[11] Blackwell, H. R. , "Contrast Thresholds of the Human Eye", J. Opt.

Soc. Amer., 36 , 624-43, 1946.

[12] Anon., "A Method of Measuring Smoke Density", N.F.P.A. Quarterly,

57, 276-87, 1964.

USCOMM-NBS-DC
Table 1

Reproducibility of Test Results for Several Exposure Conditions

Method

No.

Materials Tested

Avg. Coefficient

of Variation

percent

A. Rohm & Haas 18 .52 35

Method P-148R

B. Horizontal 25 .91 11

Multi-Holed Burner

with Metal Shield

G. Vertical

Burner

Mount Gas 40 .11 74

B. Electric Furnace 26 .39 15

(750°G. Exposure)
Fig „ 1 Smoke Density Test Chamber

(Courtesy Rohm & Hass Co.)

PIG. 2 — SKETCH OF SMOKE DILUTION EXPERIMENT

Light

Transmission,

Ftrcenf

10

>*t

to2! yjQ

-J<UH

a,o<w>OH<

a:>

CD2

UJ

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0101 roo

u.

COH

CO

yjH

z:oH3

-JQ

UJoz

co;

*0j4®y Xiisusq joDjidQ

Optical Density, ( Per Foot)

FIG.4” COMPARfSQM OF DU P LiCATE TEST RESULTS

A Standard Burner Exposure, 20 p$i, Rohm And Haas Method P148B

B Multi- Holed Burner Exposure With Metal Mask

C Standard Burner Exposure, Specimen Vertical

D Electric Furnace Exposure, 80 Volts ( 750* C)

o»©

©

>

* 2

O

HO

3O

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ELECTRIC

FURNACE

EXPOSURE

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o

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FIG.

6A

-TYPICAL

TEST

RESULTS,

ELECTRIC

FURNACE

EXPOSURE,

80

VOLTS

(750°C)

|udDidd*uo|4djosqv

Time,

(In

Minutes)

FIG.6B

—

TYPICAL

TEST

RESULTS,

ELECTRIC

FURNACE

EXPOSURE,

40

VOLTS

(360°C)

Optical

Density

(Per

Foot)

|750®C

Electric

Furnace

Exposure

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