T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY ON THE RELATION BETWEEN THE PHYSICAL PROPERTIES AND CHEMICAL COMPOSITION OF GLASS VI1 - ETCH FIGURES By EDWINW A R D TILLOTSON, JR. Received June 21. 1917
When glass is exposed t o t h e vapor of hydrofluoric acid, or dipped in a solution of hydrofluoric acid, with or without t h e addition of soluble fluorides, t h e surface is etched unevenly, there being produced “etch figures” of varying forms, and more or less crystalline in appearance. An explanation of t h e formation of these figures was first offered b y LeYdolt,’ who supposed t h e m t o be due t o “preexisting crystals” in t h e glass, for t h e figures obtained b y etching with hydrofluoric acid a n d also with steam possessed a remarkable similarity t o crystal forms. This work was repeated in I857 b y Daubree,’ who found no evidence for pre-existing crysta1s, and explained t h e figures as being produced b y a protective coating of insoluble substances, such as KzSiF6, which might be deposited locally On the surface Of t h e glass. In 1866, an exhaustive study of carried Out by Weatherill,’ who this phenomenon observed t h a t , in general, these etch figures were in relief a n d therefore supported t h e protective-layer theory; b u t , in one instance, a star-shaped acicular and this was refigure was found to be etched garded as disproving t h e view set f o r t h b y Daubree, Reinitzer4 noticed t h a t t h e crystal form of t h e etch figures corresponded t o t h a t of t h e insoluble substances which might be formed, such as potassium or calcium fluosilicates, a n d therefore supposed t h e m t o be due t o t h e formation of these substances on t h e surface of t h e glass. H e also observed, as further evidence of this protective action, t h a t concentrated HF,a n d HF a n d &So4 did not f o r m etch figures, since these reagents were sufficiently active t o dissolve both glass and t h e insoluble substances. Jackson a n d Rich6 attempted t o correlate t h e etch figures with lack of homogeneity of t h e glass. They demonstrated b y differential flotation of glass powders t h a t ordinary glass is not homogeneous; b u t were unable, b y t h e method employed, t o detect any lack of homogeneity in optical glass. Nevertheless etch figures were obtained with optical glass as well as with t h e more common kinds. Lately Prink6 has announced t h a t i t is possible t o differentiate between good a n d faulty glass b y reason of differences in t h e etched surfaces produced by hydrofluoric acid, a n d t o determine whether t h e fault is due t o chemical inhomogeneity or t o mechanical strains. Frink also inclines t o t h e view t h a t these figures are caused by preexisting crystals. There are, therefore, two well defined opinions regarding t h e cause of t h e mat-etch: t h e one supposes “prekxisting” crystals within t h e glass, while t h e W k n . A k a d . Ber , 8, 261; Compt. tend.. 84 (1852). 565; Pogg. A n n . , 86, 104; 186, 494.
Comfit. rend.. 46 (1856). 792. 8 A m . J . S c i , [2] 41 (1866), 16. Dingin’s polytech. J . . 262 (1866). 322. 6 J . SOC.Chem. I n d . . 20 (1901). 555. a Trans. A m . Ceram. Soc., 11 (1909), 299; 14 (1912). 662, 16 (1913), 7 1 5 ; Trans. 8th Inl. Cong. A f i p l . Chcm., 6 (1912), 57
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other is based on t h e protecting action of insoluble fluorides a n d fluosilicates formed during t h e etching process. It seemed distinctly worth while t h a t this question should be settled, not only because of its importance in t h e Practical etching of glass, b u t also for whatever bearing i t might have on t h e theory of t h e molecular structure of glass. A series of experiments was accordingly carried Out in t h e endeavor t o throw more light On this subject. MATERIALS
The materials used in this investigation were c. p. hydrofluoric acid, containing about 45 per cent H F , a n d C. p. ammonium fluoride. The etching solutions were prepared in platinum dishes, a n d t h e ingredients were weighed with a n accuracy of about I per cent. The following glasses were available a n d were used: Window glass Plateglass
potash-lead glass for cutting Borosilicate laboratory glass
Fused silica Microscope slides
The microscope slides were from an unopened box and were therefore presumably uniform in composition. These slides also approached more nearly t o optical glass in homogeneity t h a n t h e other glasses which were used. EXPERIMENTAL
The first series of experiments was made with t h e microscope slides, using etching solutions varying in composition from pure hydrofluoric acid, t o a solution’ of hydrofluoric acid saturated with ammonium fluoride. The etching solution Was applied by immersing t h e in the solution. Figs. I t o 6 show Photomicrographs Of this series enlarged 60 diameters. It should be mentioned t h a t a Very troublesome optical illusion is associated with these etch figures both under t h e microscoge a n d i n t h e Photographs. At one instant they appear as “Pits” in t h e glass, and at another instant they appear t o be Protuberances O r Pyramidal figures Projecting outward f r o m t h e surface of t h e Plate. The figures under t h e writer’s observation have been without exception “in relief,” as noted by Weatherill; t h a t is, they are Protuberances a n d not Pits. I n examining Figs. I t o 6, i t is apparent t h a t Pure hydrofluoric acid dissolves t h e glass, in general, evenly, a n d with t h e production of only occasional etch figures. When, however, t h e amount of ammonium fluoride is increased, t h e etch figures become more a n d more definite in shape. For low concentrations of m ~ m o n i u m fluoride, as shown in Fig. 2 , t h e etch figures are suggestive of acicular crystals, b u t , with higher concentrations, t h e figures become more nearly equilateral a n d “granular” in appearance. It is of interest t o note t h a t t h e shape of t h e etch figures is not materially altered by increasing t h e amount of ~ n ~ m o nium fluoride beyond t h e molecular ratio, “IF. 2HF. are made up into a paste with When these a n inert filler, t h e character of t h e etch is changed in t h e direction of producing a finer grain, t h a t is, t h e 1 About two parts by weight of 45 per cent hydrofluoric acid are necessary to dissolre one part of ammonium fluoride. When the quantity of ammonium fluoride exceeded this proportion, the mixture was used without separating the liquid from the undissolved salt.
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etch figures are smaller a n d more numerous. This is illustrated in Fig. 7 . I n general, too, t h e more concentrated t h e etching solution t h e smaller will be t h e etch figures. Fig. 8 shows the result obtained by diluting a concentrated solution (Fig. 4) with a n equal weight of water. T h e result is to t h e eye a coarse a n d sandy surface, which, under t h e microscope, shows widely scattered etch figures possessing striking geometrical shapes. It has been pointed ou ) t h a t a plane glass surface dissolves substa nly a n d uniformly
in pure hydrofluoric acid. Whe etched surface, a s shown in Fig. j, is immersed in a solution of hydrofluoric acid, a similar action takes place. Figs. 9 t o I I illustrate such action for one-quarter, one-hali and one minute immersion in a solution of equnl parts of concentraterl hydrofluoric acid and water. T h a t this result is a necessary consequence of uniform solution b y t h e acid may he ilcmonstrated geometrically. The first effect a s shown in Fig. 9, is a n enlarging a n d rounding out of t h e “valleys,” finally resulting, a s shown in Fig. 1 1 , in a series of smooth “saucer-like” depressions. This operation, which changes in so profound a w w t h e
character of t h e glass surface, is made evident to t h e naked eye only a s a slight decrease in t h e opacity of the glass. It should also he mentioned t h a t similar depressions are occ,asionally observed when certain unetched glasses are immersed in pure hydrofluoric acid. This phenomenon is characteristic of mechanically polished glass surfaces, and of t h e resistant borosilicate glasses; and, because in such cases these depressions appear in lines or rows. it scems probable t h a t they result from minute scratches in t h e glass surface. They are therefore not true etch figures since they
result from a secondary n a non-uniform surface. It was observed in general t h a t all solutions which produced a mal-etch, as in Figs. 4, 5 and 6 , when diluted, produced etch figures resembling true crystals, a s showii in Fig. 8. Sucli a diiutcd solution a’as therefore employed with t h e scvcral varieties of glass mentioned above. The etching solution was made u p as follows: Ammonium fiuoride 5 grams, hydrofluoric acid (42 per cent) i o grams, water 10 grams. Figs. 1 2 t o 18 are photonlicrojiraphs, el~larged zoo a n d zoo diameters, of window glass, piate glass,
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potash-lead glass, laboratory glass and fused silica glass, etched in t h e solution given abovc It is evident t h a t t h e eteh figures of all of these gla.;res are surprisingly similar in appearance. This makes i t difficult t o explain these results on t h e theory of “preexiiting” crystals, especially when account 1.; take t h e wide differences in t h e compositions of t h e gin.:
I k Ib-LmORATOBY GLASS(ford& Pm S 9 - A l ~ O N I V I PnioezLrcArE CBYSTILS
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Certain differences are noticeable, however, which, i n t h e light of later knowledge, support t h e generalization given above The most noticeable difference with t h e borosilicate laboratory glasses (Figs I i ) and with t h e potash-lead glass (Fig. 15) The final explanation for t h e formation o figures was secured as a result of watching, wi
Pro 1 7 - L ~ ~ o ~ b ‘ lGLASS o l ~ ~ (domesite) Fro ZO-WINWW GLASS(6 a
The facts brought out b y these ex s appear h 6pres to be, broadly, as follows: The for i s indspendent of the chemical compositzoii of the glass, aicd as delermzifcd h y the composition of the elciiing S d U taon This generalization speaks for t h e protective-c theory, and against the preexisting crystal hypotliesis.
micro e, t h e etching process. The solution was applied t o t h e glass plate in the form of a and was viewed through t h e plate from a t f a r as possible a n y corrosion es. Within a few seconds after t h e solution contact with t h e glass, a great many small t crystals appeared floating in t h e solution.
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These were in general of t h e same shape as t h e etch figures a n d increased rapidly in size as t h e etching proceeded. A certain percentage of these crystals was stationary a n d occasionally a moving crystal was seen t o stop suddenly, as though i t had become attached t o t h e glass surface. Both t h e floating and stationary crystals grew rapidly in size as t h e etching proceeded. When t h e plate was then cleaned, etch figures, of t h e size a n d shape of these stationary crystals, were observed in their places. From this i t appears t h a t t h e etch figure is produced by t h e protective action of crystals which are formed as t h e etching proceeds. The composition of t h e deposited crystals was t h e n investigated. From t h e fact t h a t etch figures were formed on fused silica (Fig. IS), i t was suspected t h a t t h e crystals consisted of ammonium fluosilicate. Ammonium fluosilicate was therefore prepared by partially neutralizing fluosilicic acid with ammonium hydroxide. A finely crystalline precipitate was obtained, which, on examination under t h e microscope, appeared identical with t h e crystals formed during t h e etching process. These are illustrated in Fig. 19. ,4nalysis showed t h e m t o be ammonium fluosilicate: Calculated for (NH4)BiFs Per cent “8.. . 19.09
. . .. .
FOUND 18.76 and 19.04
This substance crystallizes in t h e regular system, and, in general, t h e individual crystals are cubes modified b y octahedral a n d dodecahedral faces. As has been pointed out, t h e etch figures result from two simultaneous actions: t h e solution of t h e glass, a n d the growth of t h e crystal which is in mechanical contact with t h e glass surface. The etch figures are therefore neither pseudomorphs nor casts; and, consequently, can not be expected t o be exact duplicates of t h e crystals. T h e y are essentially silhouettes of crystals which are constantly growing. T h e figures often show striations normal t o t h e direction of growth of t h e crystal (Fig. 13), which suggests t h a t possibly t h e crystal growth or perhaps t h e solvent action is subject t o rhythmic disturbances. It has been observed t h a t t h e etch figures are commonly hexagonal in appearance and i t might be expected t h a t other shapes should predominate if they were t h e result of t h e protecting action of cubic crystals. It is evident t h a t , if t h e octahedron were in contact with t h e glass surface, a n d if t h e cubical a n d dodecahedral faces were t o develop simultaneously, a hexagonal figure should result. This is apparently what occurs in t h e majority of cases, and no explanation, except t h a t of habit, can be offered t o account for it. It is t o be expected t h a t etch figures of triangular, square a n d octagonal outline should also be formed, a n d these figures are nearly always observed in limited numbers (Figs. 1 2 . 13, 14, 18 a n d 2 0 ) . If this theory is correct, i t is t o be expected t h a t t h e presence of other metals which form insoluble fluosilicates, will produce characteristic etch figures in accordance with their crystalline habit. This is illustrated in t h e case of t h e potash-lead glass (Fig. 15); t h e etch figures are decidedly characteristic of potassium; and, so far as t h e writer has observed, they are
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a l w a y s produced b y a n etching solution w h i c h contains p o t a s s i u m , irrespective of the composition of the glass. The laboratory glasses (Figs. 16 and 17) also illustrate some characteristic figures. These glasses were free from potash, b u t contained considerable percentages of boric acid, alumina and zinc oxide, a n d are extremely resistant t o t h e action of t h e etching solution. AS will be seen from t h e photomicrographs, t h e habit of these figures is “tabular” with square and triangular forms predominating, b u t six- a n d eight-sided figures are not uncommon. This theory also indicates t h a t , if t h e etching solution contains no substance which produces insoluble fluosilicates or fluorides, t h e glass should be dissolved evenly, as with hydrofluoric acid alone, and without characteristic etch figures. This was found t o be t h e case when t h e solution consisted of t h e acid fluorides of sodium, lithium, aniline, pyridine, zinc and urea. I t is therefore apparent t h a t t h e glass, per se, is dissolved evenly a n d as a whole b y t h e etching solution, whatever the composition of the solution m a y be; and t h a t t h e matness and etch figures result from a secondary protective action of whatever insoluble materials are formed in contact with t h e glass surface. The character of t h e etch figures will be determined b y t h e most insoluble substance deposited; a n d this perhaps explains t h e results obtained with acid ammonium fluoride a n d t h e potash glass, a n d also with t h e borosilicate laboratory glass. SUMMARY
From t h e foregoing i t is concluded t h a t : I-All glasses dissolve evenly in acid fluoride solutions. 11-Matt etches a n d etch figures are obtained only when insoluble substances are formed in contact with t h e glass surface, and when t h e solution of t h e glass in such local areas is thereby prevented. 111-When t h e etching solution is acid ammonium fluoride, crystals of ammonium fluosilicate form t h e protecting material. IV-The etch figures result from a solution of t h e glass and t h e growth of crystals, deposited from t h e solution, in contact with t h e glass surface. Both reactions take place simultaneously. THEMELLON INSTITUTE
OF INDUSTRIAL PITTSBURGH. P A .
RESEARCH
THE DETERMINATION OF IRON IN GLASS SAND By JOHNB. FERGUSON Received July 26, 1917
One of t h e absolute essentials in t h e manufacture of optical glass is a supply of good glass sand, a n d one of t h e indispensable attributes of such a sand is a minimum iron content. I n searching for such a sand one is compelled t o rely on chemical analysis, a n d now t h a t t h e problem of optical glass has become a question of national importance it is essential t h a t no errors be made in t h e seemingly simple determination of t h e iron in a sand. I n t h e past, sand analyses generally have shown too low a n iron content a n d since t h e Geophysical Laboratory has taken up t h e problem of optical glass, this has’ been t h e source of some rather vexatious
