Feedback to Patricia Anderson, pfa@umich.edu
Date last edited: June 13, 2006.
Authors | Topics | Forthcoming
AT the ending of my paper on silver-tin amalgams in the August number of the Cosmos, I895, I made the statement that several examinations contemplated had not been made for lack of time. Among these was mentioned the ageing of alloys. At the time I did not know whether or not I should be able to take up the subject again. But after the series of articles were completed and I had studied the results more leisurely, and again reviewed my notes of experimental work, I was more than ever impressed with the idea that there was some influence operative in causing shrinkage and in increasing the percentage of flow that I had failed to discover. Certain formulae had behaved badly both in flow and in shrinkage, which, from their composition, should have given better results. Also, in many cases the shrinkage was unaccountably erratic. This impressed me so strongly that after a few weeks' rest I took up the study of ageing of the cut alloys, to see what its effects might be upon the mass. It has long been known that the cut alloy changes somewhat in its working properties as it becomes older. It comes to set slower, and loses that crisp brittleness coming on almost immediately after wringing out the excess of mercury. In a word, the mass becomes mellow and works softer, is more pliant. This has generally been regarded as desirable. Indeed, J. Foster Flagg states that "no amalgam is fit for use until it has been cut at least two months,"* and enumerates the advantages accruing from ageing.
This ageing is nothing more nor less than a slight oxidation of the cut alloy brought about slowly at ordinary temperature by more or
* Plastics and Plastic Filling, page 65.[p. 44]
less contact with the air, and I shall drop the term ageing and use the better term oxidation.
I know of no hint in the literature that oxidation of the cut alloy caused it to shrink. Indeed, so desirable has the effect of slight oxidation of the cut alloy been regarded that many manufacturers have sought and found means of producing it immediately in any degree desired. Any means that will rapidly induce such a degree of oxidation as will occur normally at common temperatures within two to four months produces like results. It may be done by subjecting the cut alloy to boiling water for a few moments, or, for a greater effect, from ten to fifteen minutes, and then drying at room temperature. Or it may be brought about by exposing the cut alloy to certain elevations of temperature dry. It may be done by shaking for some time a comparatively small portion of the cut alloy in a large glass jar, so that the air contained is repeatedly brought in intimate association with its particles. It may be done by running the machinery for cutting so rapidly as to develop considerable heat. Indeed, any means that will induce comparatively rapid oxidation will produce this result.
In order to study the subject completely I erected machinery for cutting alloys in the several favorite forms for use, and asked a number of manufacturers to make for me ingots of certain formulae. While these preparations were in progress, a number of manufacturers sent me new formulae for tests of their qualities. Among these were several of the same formula. In testing these for shrinkage and expansion, I found they were not at all alike. The reputation of the makers eliminated all idea of any intended deception, and convinced me that something had happened to the material. Very soon I had ingots of this alloy from several makers. It was cut with the utmost precautions as to the development of heat, and test fillings made from one to six hours after cutting. Treated int this way I found the alloy from the several makers substantially alike.
Now taking the formula, silver 65, tin 35 per cent., which I had found in previous experiments to neither shrink nor expand, and cutting it with precautions as to the development of heat, it was found upon test to expand from one to three ten-thousandths of an inch. Within two or three days after cutting there was neither shrinkage nor expansion. A trial eleven days after cutting, the alloy having been kept in a well-corked (common cork) bottle, but opened once per day and turned up as in taking material for use, gave three ten-thousandths of an inch shrinkage. In the mean time the fresh-cut alloy was oxidized artificially. The alloy was shaken together after cutting, and one half placed in a test tube and distilled water poured over it. The tube was then placed in a vessel of boiling water for ten minutes, after which its contents were thrown on the filter to drain off the water, and the alloy spread to dry at room temperature. Of the remaining half, part was used at once for test fillings, and part retained in a tightly-corked bottle to be used at the same sitting in comparison with the oxidized alloy. In the comparative tests the fresh alloy (unoxidized) did not shrink, but the oxidized alloy shrunk nine ten-thousandths of an inch at each test. These tests were repeated a number of times without variation in the results.
Then another sample of the fresh-cut alloy was shaken together so
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that the different portions of the ingot should be well mixed, and one half placed in a dry eight-ounce glass flask, the neck of which was packed with dry absorbent cotton, and a piece of rubber-dam tied securely over it to prevent the entrance of moisture. This was placed in a vessel on boiling water, and the body of the flask covered over with several thicknesses of cotton cloth so that it should be enveloped in live steam. It was allowed to remain in this position for thirty minutes, except that it was lifted and well shaken every five minutes. The results of tests with this were found to be the same as with that oxidized wet. The alloy not oxidized made an amalgam that did not shrink, but the amalgam made from the oxidized alloy shrunk nine ten-thousandths at every test. These tests were repeated a number of times with similar results. A shorter time given to the oxidation produced less shrinkage. Exposing the alloy spread upon an iron plate to a higher temperature, that which gave a slight tinge of brown to white paper, produced a condition in which the alloy did not set for about sixteen hours, and forty-eight hours afterward I could crumble the mass between my thumb and finger. The shrinkage of this was six ten-thousandths of an inch.
A portion of an ingot of the alloy was annealed, and after it was fully cold, cuttings were made from it and test fillings made to see if heating had any influence upon the behavior of the metal. This took a little more mercury than the alloy unannealed, and expansion occurred a little quicker, but it was not greater. There was no shrinkage. Therefore, the conclusion seems inevitable that oxidation of the cut alloy is a direct cause of shrinkage.
Now here was shrinkage from oxidation that covered all but a few of the worst tests which I had published in the Cosmos in August last, and at once it became a question as to whether or not I had been entirely wrong in the conclusions drawn as to the value of the various formulae. Indeed, it furnished strong reasons for the supposition that the shrinkage I had found was from oxidation, and did not necessarily appertain to the formulae. I had not inquired as to the ageing of alloys by this or that manufacturer, nor as to the length of time they had been cut. With this shadow upon my published records and conclusions, I lost no time in studying other formulae containing less silver and more tin, varying in grade down to forty-four per cent. of silver. These alloys were made especially for these tests by Dr. P. J. Kester, The S. S. White Dental Mfg. Co., and Dearborn Refining Co., as an assistance to me in the prosecution of this work, and other manufacturers are making alloys for its further prosecution. I have now studied nine formulae, freshly cut and oxidized artificially in varying degrees. In every case the effect of oxidation has been practically the same, and in proportion apparently to the extent of the oxidation. If the alloy used fresh shrunk, that which was oxidized shrunk more.
In following out this study thus far with the artificially oxidized alloys in comparison with the fresh-cut alloys, I have been fully reassured as to the conclusions I had drawn from previous work. All formulae containing less than sixty per cent. of silver have shoAwn contraction, both when measured by the special micrometer and by the opening of the margins as seen with a half-inch objective. But in every formula in which I have had the opportunity of comparing,
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the shrinkage has been very much less with the freshly-cut alloy than it was with the alloy used last summer. The greatest shrinkage found in fresh-cut alloys thus far is five ten-thousandths of an inch, in the alloy containing but forty-four per cent. of silver. The shrinkage gradually disappears as the silver is increased to sixty or sixtyfive per cent., the exact dividing line appearing to be between these two.
Several of the alloys studied have had mixtures of other metals. From the study of these, which has been very limited, it would appear that the conclusions drawn in my papers last summer as to the effect of the addition of other metals to the silver-tin alloys are but partially sustained. Indeed, from what study I have been able to give this point, it would appear that good non-shrinking alloys containing other metals, as gold, zinc, etc., may be made, though some changes in formulae will be required.
In this series of studies great care has been taken in noting the working qualities of the amalgams, the amount of mercury required by the alloys, fresh or oxidized, and every other peculiarity noticed that could be expected to be of value, all of which have been made a matter of record at the time the filling was made.
The oxidation of the cut alloy produces marked changes in the working qualities of the mass, which are best appreciated when the fresh and the oxidized material are mixed and used at the same sitting. The profession seems to have been captivated by alloys that work smoothly and softly without setting too quickly, and manufacturers have so tempered their alloys as to meet the demand, neither party knowing that the resulting amalgam was caused to shrink. Freshly-cut alloys do not make smooth and soft-working masses, but, on the other hand, they are crisp and harsh, and set quickly. This is especially true if the alloy contains a sufficient percentage of silver to prevent shrinkage, or if small quantities of other metals are added to the silver and tin. From all that I have thus far seen it appears that we must give up soft, smooth-working alloys if we are to have amalgams that do not shrink.
Oxidation of the cut alloy produces marked changes in the percentage of mercury required to make a good working mass. Careful trial has shown that if an alloy be mixed with an excess of mercury and kneaded into a pliant mass, and then wrung out in a certain way, always as nearly as possible with the same force, a certain percentage of mercury will be retained; and that different formulae will differ in the percentage of mercury retained against the wringing. In applying this test there must be a sufficient excess of mercury to make the mass fairly pliant, and it must be wrung out before setting begins. If there is too little mercury in the mix, an excess will be retained. Mixing considerably more than the required amount of mercury does not affect the result. By working in this way and gaining experience in the manipulation, it is found that a freshly-cut alloy holds a certain per cent. of mercury against the wringing process, and that each formula holds a different amount. If, however, the cut alloy is oxidized, either naturally or artificially, there is a marked change in. the percentage of mercury retained. For instance, the alloy silver 65, tin 35 per cent. held against the wringing process fifty-one to fiftytwo per cent. of mercury when used within one to six hours after
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cutting, and gave an expansion of two, and three, ten-thousandths of an inch. The same alloy kept in a tightly-corked bottle, but opened daily, the alloy being turned up as in taking material for use, for eleven days held then forty-three per cent. of mercury, and the amalgam shrunk three ten-thousandths of an inch. The same alloy again, when oxidized by exposure to boiling water for ten minutes, held thirty-five to thirty-seven per cent. of mercury, and shrunk nine tenthousandths of an inch. Similar results have been noted with each of the alloys studied.
Again, in packing the freshly-cut alloys after thorough wringing out, little or no free mercury is brought to the surface. The softening under the instrument is but little more than sufficient for good cohesion, and no free mercury can be expressed with the rubber point. On the other hand, the oxidized alloys, wrung with the same force and containing a much smaller percentage of mercury, soften greatly under the instrument in packing, and yield a considerable amount of free mercury to the pressure of the rubber point.
Retaining more mercury in the mass, or forcing the mass to extreme dryness by any effort that would be compatible with fillingoperations, does not seem to affect the shrinkage in marked degree. If, however, the mass is forced so dry as to work dead hard, or is fairly set before packing, the shrinkage will be modified or partially prevented. Mercury is not the element that causes the shrinkage of amalgams.
In freshly-cut alloys the expansion curve is more marked, and comes on sooner than in oxidized or slightly oxidized alloys. With the alloy (silver 65, tin 35), used immediately after cutting, expansion begins within six hours, running to two or three ten-thousandths of an inch. Twenty-four to forty-eight hours later, the alloy having been kept in a stoppered bottle, this expansion disappears almost entirely. A silver 60, tin 40 alloy usually showed a very slight contraction within the first three hours, rarely passing one ten-thousandth of an inch, the margins as seen with a half-inch lens remaining perfect. Within the next twelve hours an expansion of two (one above the starting-point) ten-thousandths occurred. Silver 55, tin 45, gave pretty regularly two ten-thousandths of an inch shrinkage, the margins opening so as to be plainly visible with a half-inch lens, and expanded about the same within the next twelve hours. This expansion does not close the margins opened by the shrinkage. With whatever alloy, so far as I have seen, an expansion following a shrinkage that has opened the margins fails to mend the breaks. It is evident that the contraction is mostly in the upper half of the filling, while the expansion is mostly in the lower half. An expansion of less than five ten-thousandths of an inch rarely disturbs the margins of a filling, the rise being in its center, and is probably no injury whatever. But a contraction of two ten-thousandths of an inch opens the margins visibly (with the microscope), and undoubtedly will cause the filling to leak.
Washing processes, with the view of restoring the alloy by removing the oxid, have as yet been but partially tried. The usual methods of washing with ether or alcohol proved to be of no value, though apparently much oxid was removed.
This covers in brief the main features developed since the publica-
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tion of the articles on amalgams in the July and August numbers of the Cosmos. The studies are yet far from complete, and the nature of my engagements is such that the work must be suspended until the spring or early summer. I thought, therefore, that this brief report ought to be made now, leaving a more detailed report to follow when more complete data have been obtained. Preparations for further work have been in progress. Cut alloys have been laid aside to test plans of keeping them in good condition,-in bottles with common corks, in sealed bottles, and in hermetically sealed glass tubes. It is evident, from the results of these tests, that changes must be made in the commercial handling of this material.
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URL of this page: http://www.umich.edu/~pfa/denthist/articles/Black1896.html Feedback to Patricia Anderson, pfa@umich.edu Date last edited: June 13, 2006. |