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and ferric-oxide which we interpreted to be contamination from crucibles, ball mills, etc. To verify our analyses we began syntheses of these powders beginning with the two that appeared to contain zinc oxide only.

From the literature on the subject we had constant reference to zinc oxide being highly calcined for cement powders, so after trying both Justi's and Fellowship original liquids, and our synthesized liquid from the analyses of these originals, with Kahlbamn's, and Schuchardt's, and Baker's zinc oxide with a guaranteed analysis for impurities we began to heat these different samples of zinc oxide in both the electric and gas furnaces.

These different powders when uncalcined and mixed with the liquids suggested made cements not suitable for dental purposes. The finer and lighter the zinc oxide the more rapidly they appeared to set forming little nodules apparently due to the rapidity of the reaction.

We began the calcination of the Baker's product because we were not certain that if the supply of Kahlbamn's and Schuchardt's zinc oxide should become exhausted during the experiments that we could replenish it on account of the conditions existing abroad at the time.

Our first sample was placed in a plati. num crucible and covered with the platinum cover that accompanied the crucible, and placed in the electric furnace and the temperature raised to 1000°C from a cold furnace inside of 30 minutes, and then this temperature held for 30 minutes, after which the crucible was removed and allowed to cool. The zinc oxide showed a considerable loss from the ignition, probably mostly from dehydration. It is possible, however, that even this short time at this temperature would cause some of the zinc oxide to volatilize. Schnabel quotes Stahlschmidt as saying that zinc oxide is volatile at 970°C, and that at 1054°C it is about 15%. A further possibility of

some of the loss on ignition being due to other things than dehydration is shown in our experiments, that will be mentioned later, that show that zinc oxide is reduced to metallic zinc to some extent while firing, and metallic zinc is known to be volatile at a point somewhat lower than the zinc oxide is. The working properties of the zinc oxide from this amount of calcination were changed but little. Another sample was given the same treatment except that the temperature was maintained at 1000°C for an hour instead of thirty minutes as before. Little change in the working properties of this zinc oxide was noted. A third sample was given the same treatment except that the temperature was maintained at 1000°C for two hours. This zinc oxide was little different than the others.

As we were about to clean the crucible for the fourth sample a hole appeared in the side of it and upon close inspection it was found that a defective ring extended nearly all the way around it. We then went to the Department of Chemistry and purchased some pure Magnesium Oxide crucibles that we knew were made for some similar work some years before, and continued to maintain the temperature of the furnace an hour longer each time we fired the zinc oxide. On the fourth firing our pyrometer failed to register the temperature. Inspection revealed that the thermo-couple which was being held near to the crucible in which the zinc oxide was, had fallen to the floor of the furnace in several pieces, which appeared like a dark colored powder. This convinced us that platinum or its alloys with iridium or rhodium could not be used either for crucibles or thermo-couples unless some form of protection could be devised which would keep them from what appears to us as either volatilized zinc, or zinc oxide. This proved somewhat of an obstacle because we had but one pyrometer available at the time. We purchased another Siemen's & Halkse and calibrated it with the one used as a standard in the Department of Chemistry and used our original one to experiment with. We purchased a thermo-couple from the Pelton & Crane of Detroit covered with a metal cover that was said to be one used in connection with the tempering of tools in hot lead baths and asked Mr. Crane of the Pelton & Crane Co. to come to Ann Arbor and witness the making of a scale and the calibration of the pyrometer that we were to use for experimentation. This done and a new Siemens & Halske instrument already connected to check the one with the covered couple we thought we might go on with our study of the amount of calcination to be given zinc oxide to make a good cement. At the end of the six or seven firings the metal cover (said to be iron) fell to the floor of the furnace in small pieces. This furnished another source of trouble. We had a standard pyrometer already attached this time, however, with which to construct a working instrument. This time we went to the Eberbach & Son Co. of this city and purchased a piece of Royal Berlin porcelain tubing with a 36 inch hole in it and arranged the carbons in the reflectoscope in the lecture room so that they made a good sized arc and fused one end of this tube together. We then had made up a thermo-couple about ten inches long and placed inside this tube, and made another scale for the instrument. Using this instrument for the work and the newer instrument for a check has been very satisfactory and will in our opinion prove the most satisfactory way for future workers on the subject of zinc or zinc oxide, or similar substances to determine the temperature. One thing seems certain, viz., that either zinc oxide or zinc volatilize sufficiently during the calcination process to destroy metal thermo-couples and crucibles. One thing that leads us to believe that it is not zinc oxide alone that makes the puble is that often tim the

zinc oxide in the crucible takes on a greenish color around the outside of the mass next to the crucible and some distance beneath the surface showing apparently that there has not been a free enough passage of oxygen in the crucible in the furnace to keep the zinc in oxide form. This is especially noticeable when the gas furnace is used and a poor adjustment of the air and gas exists.

It was soon found that the electric furnace was not going to stand the temperatures that appeared necessary to calcine the zinc oxide to a point where it would make a good cement so we continued the work in the Meeker gas furnace. This furnace had a capacity of slightly over 1400°C tho it was supposed to be capable of 1800°C. In it we made up samples of calcined zinc oxide from Baker's dry process product with an analysis as follows for impurities:

Fe..002
Pb..05
Cd. trace
C1 .01
S03.05

These samples were all made in the magnesium oxide crucibles referred to at 1400°C for from one hour to fifty hours in the furnace. In each case the zinc oxide came out of the crucible in one solid piece shrunken to a small sized piece and quite hard with the longer firings. We then took up the question of grinding these apparently fused cakes of zinc oxide and settled upon five by seven inch Royal Berlin porcelain ball mills containing not less than 25 Royal Berlin porcelain balls and revolv

68 revolutions per minute the equipment. As

observed the effect of these mills in grinding and the appearance of the powder portion of Justi's and Fellowship cements under the microscope and by measuring the particles with the micrometer on one of the microscopes we found that it took from fifteen to twenty hours to get some of

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the samples that had been fired for a long time to have the appearance of Justi's powder. Then came the question of getting a powder that had particles of somewhere the same size. For this purpose we purchased several grades of bolting cloth some running as fine as 200 mesh to the inch. We soon arrived at the size of the powder particles that resembled Justi's and Fellowship powders but to determine which of the various grades of firing from one to fifty hours in the furnace at 1400°C, would react with the liquid for these cements caused us to do some careful work in mixing under conditions that could be duplicated. Finally, we settled upon the powder that had been fired at 1400°C for 14 hours at the one having the properties nearest to Justi's. It proved to be so close that we could substitute our synthesized product for the Justi powder in skilled operators hands without its being detected. Our work this far seemed very satisfactory as far as the results were concerned, tho we had encountered some obstacles.

We then went over the Fellowship powder in the same manner but could not select a single powder from the lot we had made that seemed to come near duplicating it. We then began mixing various grades of the zinc oxide and finally decided that a mixture of 15 grams of zinc oxide that had been fired 14 hours at 1400°C, and 8 grams that had been fired 8 hours when the two had been mixed in the ball mill for about ten hours was very near to the original powder. We had observed that the longer zinc oxide was fired the slower it reacted with the cement liquids, that they were very adhesive and expanded slightly, that the zinc-oxide took on a yellow color at about six hours at 1400°C which became deeper as we approached the samples fired at 50 hours, that at about eight hours the zinc oxide in the crucible began to clinker into one solid cake, that the broken and ground mass had a

heavy glassy appearance, and that the reaction between zinc oxide and cement liquids decreased with the calcination, tho not very marked after passing that which had been fired for 15 hours at 1400°C. It appeared to us at this time that a cement composed of zinc oxide alone would be limited in color to the slightly yellowish white of the zinc oxide that had been fired at 1400°C for four hours or the deeper shades of yellow that followed the higher calcinations, the unfired white zinc oxide appearing to be too rapid in setting for use even in small quantities. We had observed that the Ames' and Caulk and some other products had a variety of colors, principally yellows and whites. The brown and yellow crown and bridge powders we found to be tinted with a small percentage of Ferric Oxide. There

some question whether even the yellowish white and yellow colors could be produced from a straight zinc oxide powder since we had statements in the literature that we interpreted to be conflicting in regard to zinc oxide turning yellow when it was heated. An authority like Ostwald says:

“Zinc oxide is white in the cold, but appears yellow when hot; on cooling it again acquires a white color. This color change must not be regarded as a sign of the conversion of the zinc oxide into another, perhaps allotropic, condition, for it does not take place suddenly, as in such a case it would do, but gradually. It is solely due to the fact that the region in which zinc oxide absorbs rays moves, on heating, from the ultra-violet portion of the spectrum, in which it is situated at the ordinary temperature, towards the visible violet portion. This is a very general phenomenon, viz., that the region of absorption of rays changes in the above sense with the temperature. White substances become yellow on being heated, yellow ones red, and red ones brown; blue and green substances, on the other hand, generally undergo no marked change of color on heating."

There is some question whether Ostwald intended that in heating these substances the temperature should be as high as we had to use. Practically the same statement, however, is made by Walter Renton Ingalls, (Production and Properties of Zinc-by. Walter Renton Ingalls), page 156, which is “Zinc oxide is normally a white powder of 5.5 to 5.7 sp. gr., which upon heating acquires a canary yellow color and upon cooling again regains its former appearance." Schnabel says, “Zinc oxide is a white or pale yellow powder, which becomes lemon yellow on heating, but regains its white color on cooling,” (Handbook of Metallurgy, Schnabel, Page 7).

On the other hand we find such dental authorities as Hodgen-Millberry, (page 148), stating that “After the zinc oxide has been properly calcined, which requires the highest forge heat for several hours, it is found to be greatly contracted in mass, semi-vitreous, and light yellow in color." Also the following from “Chemistry for Dental Students” by H. Carlton Smith, (page 121), “A pure Zno may be made by calcining the precipitated carbonate of zinc, Zn5(OH)6(CO3)2 +heat=5Zn(+2C02+3H20. The heat should be below 500°F, because, if too strongly heated, the color suffers, becoming yellowish." Bennett, (Science and practice of Dental Surgery, page 397), states, “The zinc oxide is placed in a perfectly clean and previously annealed clay crucible, and calcined at a bright red heat in a muffle furnace for several hours. During the process the oxide shrinks very considerably, whilst its color is changed to a pale yeilow; with a very high temperature a deeper yellow is produced, and the powder is converted into a semi-vitrified mass."

The strictly dental literature seems to favor the opinion that zinc oxide turns yellow when heated to the point where the mass becomes partially or wholly

fused together, while the more strictly chemical and metall

literature seems unanimous that its color turns from white to yellow on heating and then turns back to white on cooling. While it is not definitely stated, it appears that the zinc oxide that is referred to by the dental authorities is one, that has a higher specific gravity as a result of the apparently partial fusion and volatilization, than the one referred to by the more strictly chemical and metallurgical authorities. In order that we might determine that straight zinc oxide pow. ders could be consistently prepared into yellow colors from zinc oxide we then took samples of Baker's dry process zinc oxide with the analysis mentioned for impurities, also some of Kahlbamn's and Schuchardt's marked chemically pure, and made careful tests for ferric oxide before firing with negative results, and made repeated firings in both the electric and gas furnaces with both covered and uncovered pure magnesium oxide crucibles, and in the same crucibles lined and then highly fired with a mixture of zinc oxide and zinc chloride, a type of crucible adopted later when our supply of magnesium oxide crucibles became exhausted. In all the firings an oxidizing atmosphere was kept as near as possible to avoid our former experience of having the zinc oxide reduced. In all of these tests we found the color of the zinc oxide to become a very faint yellow as soon as they were held at 1000°C for an hour, and at 1400°C for six hours the yellow color became very easily detected and as this temperature was maintained for a greater number of hours the yellow became deeper. As the zinc oxide began to appear fused when examined with the microscope it also began to show the yellow color. Various authorities claim that allotropic forms of zinc oxide exist, while others say that it is possible that they do. Schnabel and Ingalls state that it is not fusible, while in our tests all that had

no

been subjected to 1400°C for six hours or longer appeared like small fused, glassy particles of the zinc oxide. While there were several authorities that claimed zinc oxide infusible, it appeared to us that at extremely high temperatures it was, and that as it became fused it turned yellow. It also appeared to us that the reason the strictly chemical authorities and strictly dental ones differed on the question whether zinc oxide turns yellow on heating was due to the extreme temperatures employed by dental investigators causing it to fuse. That is fused zinc oxides appeared yellow and precipitated, or other finally divided zinc oxides, appeared white. We then took up the question of the relation between the preparation of zinc oxide and its properties with a view of determining whether there existed a relation between density and color. It was found that zinc oxide might be prepared from oxidizing metallic zinc; by heating the carbonates, nitrates, hydrates and various other salts; and by roasting zinc sulphide, zinc sulphite, and the various sulphates.

From our experience and from some preliminary tests, we assumed that zinc oxide prepared from the nitrate would give the heaviest product and began the preparation of a quantity that would allow us to do some experimenting.

This zinc oxide prepared from the nitrate gave a product that resembled very closely the crystal form shown by Ames as the kind used for his crown and bridge cement but about which he says nothing regarding the preparation. It proved to be very dense, yellow in color, glassy in appearance, and was composed of definitely shaped crystals that were seen very easily with powers from twenty to forty magnifications.

After we had subjected a sample of this nitrated zinc oxide to the grinding of a ball mill for about fifteen hours we again heated it to remove more of the nitrous oxide and found it to be

lighter in color. Then we searched the chemical literature for references to the question of preparing zinc oxide from the nitrate. We found very little that had a direct bearing on the subject in question, except an article by Thomas W. Richards, in the American Chemical Journal for 1908. A statement from the article bearing on the subject was "When zinc oxide is prepared by heating the pure nitrate during a long period of time at 280°C, is ignited at temperatures between 500°C and 800°C, the amount of gas retained diminishing as the time is lengthened and the temperature raised, other conditions being the same. Continued heating at one temperature causes a slow evolution of gas. The total amount occluded carries as the physicial condition, the finer the powder the less gas occluded." With repeated grindings and heatings we were able to make the color lighter but in no case did get the color as light as it was before we converted the oxide to the ni trate. This furnished additional evidence that there was a relation between the density and the color, because we could not get as fine a state of division of the nitrated product with a ball mill as the original zinc oxide was.

In most respects this zinc oxide prepared from the nitrate was similar to that which we had fused for from ten to fourteen hours at 1400°C. To one who was not a very close observer we could substitute it for the fused product when mixing it with one of the cement liquids. When it was examined with the microscope, however, it could always be detected by the crystals that appeared so near alike that one could not be told from another.

About the time that we had begun a study of the effect of oxide of bismuth in combination with zinc oxide for cement powders, lack of ball mill capacity caused us to abandon one line of work or the other so we took up the question of the use of oxide of bismuth, We regretted

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