From Industrial Hygiene Bulletin.

CARROLL M. SALLS, PH.D., Chemical Engineer.


Factories in which ther was believed to be a possible hazard due to exposure to dust containing silica were visited, the conditions of exposure noted, and samples of silica and siliceous materials were taken for laboratory examination.

A representative group of these samples were analyzed to determine the percentage of total silica and the percentage of very fine dust below ten microns diameter; the former being an index of the degree of hazard due to the chemical composition of the dust; and the latter being an index of the degree of hazard due to the physical form of the dust.

The analytical results are presented in Table 1. The percentage of total silica was determined as follows: Fuse one gram of material with potassium hydroxide to a quiet fusion. Dissolve in distilled water in a beaker. Acidify with hydrochloric acid. Boil a few minutes and then evaporate on a sand bath. Take up with water and a little hydrochloric acid and boil. Let settle and filter through an ashless filter. Dry filter after washing with water acidified with a little hydrochloric acid to remove the last trace of iron, and burn in a tared platinum dish. After weighing the dish and precipitate add a little hydrofluoric acid and a few drops of sulfuric acid and evaporate to dryness on a sand bath, then finally ignite over a Bunsen burner. The difference between the weight of the material now and the first weight will give the amount of the pure silica present in the original material.

The percentage of dust under ten microns in size was determined by sedimentation in water as follows: Two grams of the material are thoroughly stirred with water in a small beaker and allowed to stand at about 20 degrees C. for three minutes per centimeter depth of water. The liquid carrying the fine dust in suspension is poured off; and the sediment again stirred with the same quantity of water, settled for three minutes per centimeter depth, and poured off. The sediment consisting almost entirely of particles above ten microns in size is then filtered in a tared gooch crucible, dried, and weighed.

The difference between the weight of the dry sediment in the crucible and the original weight of the sample is the weight of the very fine dust which the sample contained. Examination of combined portions of the liquid under a microscope shows that it contains very few particles above ten microns in diameter, the specific gravity of the particle having much less effect upon the rate of settling than the size of the particle. Less than ten per cent. of this very fine dust is lost by this procedure, an amount which is of no consequence.

The fineness determinations were made in the laboratory of the Department of Labor, but due to inadequate ventilation and to the lack of necessary platinum apparatus in that laboratory it was necessary to call upon Dr. F. B. Flinn, of the Columbia University Institute of Public Health, for the silica analyses.


Per Cent. Per Cent.

Sample Character of Sample Total Very Fine

number Silica Dust

2 Granite dust – end of exhaust system 60.44 25.5

3 Granite dust – Cyclone separator 60.12 0.1

4 Granite dust – dust room 72.99 4.3

6 Granite dust – bag separator 76.94 49.9

8 Sand blast (foundry) – refuse 91.14 9.3

9 Sand blast (foundry) – fresh sand 91.28 0.1

9 Sand blast (foundry) refuse 78.16 16.1

9 Sand blast (enamelling) – bag separator 90.65 4.4

13 Sand blast (granite etching) – Shelf 91.69 5.0

14 Sand blast (glassware) – cabinet 97.99 0.4

15 Construction sand 86.19 0.5

17 China clay for elec. porcelain 45.51 0.7

18 Powdered quartzite (enamels) 88.15 16.6

19 Powdered flint (elec. porcelain) 97.12 22.6

20 Florida clay (elec. porcelain) 44.48 6.8

22 Glass sand 96.27 0.0

23 Pumice, near rolls 65.25 9.0

24 Pumice, near pulverizer 68.86 4.8

29 Granulated quartz 95.81 0.1

30 Slate powder 54.28 19.0

31 Raw brick clay 67.52 13.1

33 Sand paper flint 97.64 0.8

34 Artificial cryolite (glass mfg.) 0.30 —

35 Powdered glass (filtration) 67.73 8.6

36 Kieselguhr 77.08 —

37 Lithographers flint 88.15 1.6

40 Glass grinding – emery, pumice 61.51 7.7

42 Glass grinding – carborundum 33.55 1.9

46 Synthetic gypsum – not calcined 0.60 —

47 Synthetic gypsum – Calcined 1.50 —.

Upon examination of the results presented in Table 1 it will be noted that many of these materials contain a high percentage of silica. all of the samples of granite dust and sand blast dust contain more than 60 per cent. of silica. Other materials containing more than 60 per cent. silica include construction sand, powdered quartzite, powdered flint, glass sand, pumice, granulated quartz, raw brick clay, sand paper flint, powdered glass, kieselguhr, lithographers flint, and refuse emery and pumice abrasive used for grinding glass.

Not all these materials involve a silicosis hazard, however. It has been conclusively proven that there must be an exposure to dust containing particles less than 10 microns (1/2500 inch) ink diameter. McCrae found that the dust extracted from the lungs of deceased miners, by a process of acid oxidation, consisted of extremely minute particles. On measurement under microscope it was ascertained that one of the particles were larger than 12 microns in diameter, and that the great majority of them were less than one or two microns, or very much smaller in size than a red blood corpuscle, which is about 8 microns in diameter.

As a result of a careful laboratory investigation, Watkins-Pitchford, working by a different procedure, came to the same conclusion as regards size of particles, but in addition to particles of free crystalline silica they found particles of silicates and other materials. They conclude, “Little, if any physiological selection takes place, either in respect of the chemical nature or shape of the mineral particles which are deposited in the lung tissues; the only physiological selection which appears to occur is in the matter of size”.

Upon examination of Table 1 it will be noted that some of the materials having a high percentage of silica are practically free from very fine dust. Although one sample of granite dust contains nearly 50 per cent. of dust below 10 microns in diameter, another sample has only one-tenth of a per cent.; showing that a cyclone separator will not separate the injurious dust from the air. the results on the sand blast samples show that fresh unused sand is not injurious, but in use the particles are broken down to form particles of injurious size. Every sand blast outfit, therefore, should have some means to separate very fine dust from the larger particles before the sand is used over again. The low percentage of very fine dust for other silica materials shown in the table explains why there is no silicosis hazard in he use of these materials.

Still another characteristic of silica dust is emphasized by some investigators. That is the percentage of free, crystalline silica. Even though particles of combined silica (silicates) have been found in silicotic lungs there is no assurance that these particles are injurious. In fact Professor Beattie could find no evidence of injury when he exposed animals to siliceous dust which contained no free crystalline silica. Quoting Professor Beattie, “Certain mineral dusts, such as coal, shale, slate, clay, cement, were not shown by experiment to be injurious. Other dusts, e.g., silica, quartz, flint, sandstone, are injurious, as are also carborundum and emery. After careful consideration, therefore, we feel justified in concluding that, even although further investigation should disclose other dusts as injurious, the dust of fine crystalline silica is specially injurious and is the most potent cause of fibrosis”.

The difficulty in the free, crystalline silica theory from the legal point of view is the uncertain nature of chemical methods which might be used to establish the presence of free, crystalline silica in a given sample of dust. Upon the basis of a chemical analysis, a chemist would hesitate to say whether or not a dust contains free, crystalline silica.

The following statement of J.W. Mellor, an internationally known chemist, having extensive experience in the analysis of silicates, testifying before a British commission, illustrates this difficulty: “The term free silica in technical literature applies to the silica which is not removed form the clay by treatment with sulfuric acid, and that free silica may be really combined as in felspar and mica, and it is reported in the older analyses as free silica.” In other words, free silica is sometimes combined silica.

That is like saying black is sometimes white. One chemist may say that a given dust contains free silica; and another, that it contains combined silica, and both chemists may be right depending upon what he means by free silica. that, obviously, is a very unsatisfactory legal situation. The way to clear up this situation is to go back to the physiological investigations where the idea of free, crystalline silica originated; and then if it is found that acid insoluble silica is the real meaning of free silica it would be much clearer to use the phrase “acid insoluble, crystalline silica,” instead of “free, crystalline silica”.

Of course, if the chemical analysis shows 90 per cent. or more of silica it is reasonably safe to assume that the material contains free silica. That is the case with most of the silica sand that is used in sand blast machines, also quartz, flint, quartzite, and glass sand. But granite frequently contains less than 70 per cent. although it has been proven by means of a complicated and delicate petrographical procedure that about one- third of the silica in granite is present as quartz which is undoubtedly free, crystalline silica.

In view of these facts generally known to mineralogists and in view of the chemical results for percentage total silica it seems reasonably certain that the following materials named in Table I contain free, crystalline silica, using the term “free” in its strict interpretation, namely – all granite dust samples, all the sand blast samples, construction sand, powdered quartzite, all the flint samples, quartz, and glass sand.

On the other hand the following samples probably do 3not contain free, crystalline silica, although the writer would hesitate to say with certainty until the term free silica is defined, namely – all of the clay samples, pumice, slate, kieselguhr, glass abrasives.

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