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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=5ZnO+2C02+3H2O. 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 yellow; 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 metallurgical 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 powders 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 no 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
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. 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
it, however, for the similarity of the zinc oxide that has been highly calcined and that prepared from the nitrate caused us to believe that many, if not all, the more modern cement powders, such as Caulk's, Ames', and possibly Smith's
were made from the nitrated rather than the calcined product.
Our work on bismuth was the last that we did and consists of but a comparatively limited amount of work to that which appears may be done on this material. The salt of bismuth that we had assumed was used by the manufacturers was the tri-oxide (Bi203), which is a yellow powder. With cement liquids and phosphoric acid it appears not to set. At least it did not set in the time that we watched it. It mixes with cement liquid into a perfectly smooth, oily appearing mass. On heating it melts, turns reddish brown and becomes crystalling when cooled down. A series of experiments to determine what temperature would volatilize it showed that above 500°C there was considerable loss.
A series to determine what temperature was necessary to make it combine with zinc oxide and magnesia showed that between 600°C and 700°C it began to impart its yellow color to the zinc oxide and magnesium oxide, and at 800°C no further change in color could be made by the heating. From this we concluded that there was likely to be a variation in the quantity of bismuth tri-oxide in our synthesized cement powders unless we determined the extent of the volatilization at the temperatures found necessary to combine the bismuth tri-oxide, magnesium oxide and zinc oxide in such powders as Petroid, and made allowance for it under well controlled temperatures necessary to combine the three. This, however, we did not get time to do.
We did try one or two syntheses of Petroid light yellow powder using 84% of nitrated zinc oxide, 10% of magnesium oxide, and 6% bismuth tri-oxide. The three constituents were placed in a ball
mill containing one dozen 4 inch porcelain balls, and revolving at 68 revolutions a minute and mixed for six hours. They were then taken out and placed in an unglazed Royal Berlin crucible, that had a lining of a mixture of zinc oxide and zinc chloride and had been fired for 10 hours at 1400°C. The crucible containing the three constituents was then placed in the electric furnace which was running at 770°C and left for 30 minutes. At the end of this time the three constituents were fused into a clinker which had to be broken up into pieces to get it out of the crucible. These pieces were then placed in the ball mill containing one dozen 4 inch porcelain balls, one dozen balls 1⁄2 inch in diameter, and revolving at 68 revolutions a minute. This powder was then removed and placed upon a bolting cloth of 180 mesh to the inch and that which passed thru the cloth tested with the liquid which accompanied this powder.
While we are not certain that with more fully developed technic we could improve this product for dental purposes, we feel certain that it is along these lines that this powder is made because of its similarity to the original powder. The thing of interest at this time was not SO much the technic employed by the makers of this product as to determine something of the effect of the bismuth. In this case the result was similar to the one reached when we fired zinc oxide and bismuth tri-oxide without the magnesia, viz., it gave a product much smoother in working properties.
While these tests are but a few suggestions for future workers we feel safe in saying that the use of bismuth trioxide and probably other bismuth salts is the principal distinguishing feature between the more modern, smooth working, quick setting, hydraulic cements, and the older varieties containing zinc oxide only in the powder portion. Our reason for saying that probably other
bismuth salts are used in these powders is that when Petroid Light Yellow powder was heated to about 800°C it turned a deeper yellow, while the same treatment of Ames' Pearl White would remain white, both powders showing bismuth on analysis.
While much of this work needs to be checked over, and other parts carried to completion before definite conclusions are drawn we believe the main part of it will serve as a basis for future workers on the subject that will enable them to go over our work in much less time than we have consumed. It appears to us that enough has been done to show the necessity for more work on the subject for the sake of the literature which seems to be several years behind the The more progressive manufacturers. lack of papers on the subject of cements before our societies, in fact, the lack of
By Donald Mackay Gallie, D. D. S., Chicago, Ill.
To the Members of the House of Delegates of the National Dental Association.
MONG the many recommendations made by my distinguished predecessor, Dr. Homer C. Brown, was one to the effect that the President of the National Dental Association should prepare, deliver and have distributed, a president's address. Like many of his recommendations this one was concurred in by the committee appointed to report on his message. The Constitution, however, comes to your rescue by stipulating that the address shall not exceed fortyfive minutes in length. A strict interpretation of these recommendations hardly applies to this year's officer, for, technically speaking, we are holding not a meeting of the National Dental Association, but a meeting of the House of Delegates, for the purpose of considering such business as an association of our size and many interests require. However, your President feels that a brief resume, of the many departments and interests should be presented at this time.
When we were invited three years ago at Washington to join with our Pacific Coast friends for the purpose of holding a Dental Congress, we were assured by the emissaries sent by the Panama-Pacific authorities that it was their idea to have this congress mark an epoch in dental affairs; that their hopes are to be realized, I am sure is apparent to all. In behalf of the National Dental Association I desire to thank our colleagues in this great western section for the opportunity of joining with them in this ten day session to discuss and consider the many
questions so vital to us as a profession. I also desire to express in behalf of the dental profession of America our thanks to the Panama-Pacific officials for the opportunity of taking part in this great National, yes, International celebration of the completion of one of the greatest achievements of all times; the joining of the mighty waters of the Atlantic and Pacific; the surmounting of obstacles that for years have baffled the greatest engineers. The engineering success of this great waterway is not the only thing the world should consider. There is still greater achievement in connection with this wonderful undertaking and that is the triumph of sanitary science and medical skill, making it possible for human beings to live in the Canal Zone. The complete transforming of this zone from one of pestilence and disease to one of health, comfort, and in many ways beauty, is the greatest triumph of all, and the man who deserves all the credit is Surgeon General W. C. Gorgas. In planning and carrying into effect this wonderful exposition, it was not the idea of those responsible for its conception, to celebrate this national achievement simply by building wonderful structures to house exhibits of all the arts and crafts-not merely to show the products of the work shop, the soil, forest, mines and seas, but also to show the advancement of the world in affairs not industrial or commercial, and to this end a department of Congresses is one of the great features of this wonderful exposition. No fewer than eight hundred and fifty congresses, conventions and confer