In some mouths tin does not discolor, but retains a clean, unpolished tin color, yet when there is a sesquioxid of the metal formed, fillings present a grayish appearance. In the same mouth some fillings will be discolored, while others are not. As a general rule, proximal fillings are most liable to show discoloration. Perhaps one reason is that on occlusal and buccal surfaces they are subject to more friction from mastication, movements of the cheeks, and the use of the brush.

We have seen a large number of fillings which were not blackened, yet were saving the teeth perfectly, thus proving to a certainty that blackening of tin in the tooth-cavity is not absolutely essential in order to obtain its salvatory effects as a filling-material.

Where there is considerable decomposition of food which produces sulfuretted hydrogen, the sulfid of tin may be formed on and around the fillings; it is of a yellowish or brownish color, and as an antiseptic is in such cases desirable. To offset the discoloration, we find that the sulfid is insoluble, and fills the ends of the tubuli, thus lending its aid in preventing further caries. A sulfid is a combination of sulfur with a metal or other body. A tin solution acted on by sulfuretted hydrogen (H₂S) produces a dark-brown precipitate (SnS), stannous salt, which is soluble in ammonium sulfid (NH₄)₂S₂; this being precipitated, gives (SnS₂) stannic salt, which is yellow. Brown precipitates are formed by both hydrogen sulfid and ammonium sulfid, in stannous solutions. Yellow precipitates are formed by hydrogen sulfid and ammonium sulfid in stannic solutions. The yellow shade is very seldom seen on tin fillings; the dark brown is more common.

An oxid is a combination of oxygen with a metal or base destitute of an acid. In oxidation the oxygen that enters into combination is not sufficient to form an acid. The protoxid of tin (SnO) is black, and can be obtained from chlorid of tin, or by long exposure of tin to the atmosphere. The oxygen in the saliva helps to blacken the tin, and the metallic oxid penetrates the dentin more or less, acting as a protection, because it is insoluble. Oxygen is the only element which forms compounds with all others, and is the type of electro-negative bodies; it combines with all metals, therefore with tin, and in many cases only the metal is discolored, and not the tooth. Steam boilers are made tight by oxidation.

Where there is complete oxidation, the tooth is blackened to but a very slight depth, and the oxid fills the ends of the tubuli, thus affording an additional barrier to the entrance of caries. The filling itself will prevent caries, but oxidation acts as an assistant.

"In the mouth, a suboxid is more likely to be formed than a protoxid, but both are black; sulfur and oxygen are capable of acting on tin under favorable circumstances, such as warmth, moisture, full contact, condensation of elements, and their nascent conditions; the first three are always present in the mouth. The protosulfuret of tin is black." (Dr. George Watt.) Others give the color as bluish-gray, nearly black.

Experiments show that slight galvanic currents exist between fillings of dissimilar metals in the mouth, and practical experience demonstrates that these currents occasionally produce serious results.

Direct galvanic currents do not decompose normal teeth by true electrolysis, but acids resulting from decomposition of food and fluids react upon the lime constituents of the teeth and promote secondary caries.

When two metals are so situated in the mouth that the mucous membrane forms a connecting conductor and the fluids are capable of acting on one metal, galvanic action is established sufficient to decompose any of the binary compounds contained in these fluids; the liberated nitrogen and hydrogen form ammonia, which being exposed to the action of oxygen is decomposed and nitric oxid formed, resulting in nitric acid. We also have in the mouth air, moisture, and decomposing nitrogenous food to assist in the production of nitric acid.

"Galvanic action is more likely to develop hydrochloric acid, for the chlorids of sodium and potassium are present in the normal saliva and mucus, and when decomposed their chlorin unites with the hydrogen derived from the water of the saliva." (Dr. George Watt.)

The fact should also be noted that both nitric and hydrochloric acids are administered as medicine, and often assist in producing decay.

When there is a battery formed in a mouth containing tin fillings and gold fillings, and the fluids of the mouth are the exciting media, tin will be the positive element and gold the negative element; thus when they form the voltaic pair, the tin becomes coated or oxidized and the current practically ceases.

There is more or less therapeutical and chemical action in cavities filled with tin, and its compatibility and prophylactic behavior as a filling-material depends partly upon the chemical action which occurs.

Some dentists fill sensitive cavities with tin, in order to secure gentle galvanic action, which they believe to be therapeutic, solidifying the tooth-structure.

"Tin possesses antiseptic properties which do not pertain to gold for arresting decay in frail teeth; it not only arrests caries mechanically, but in chalky (imperfect) structure acts as an antacid element in arresting the galvanic current set up between the tooth-structure and filling-material." (Dr. S. B. Palmer.) If the metal is acted on, the tooth is comparatively safe; if the reverse, it is more or less destroyed. The galvanic taste can be produced by placing a piece of silver on the tongue and a steel pen or piece of zinc under it; then bring the edges of the two pieces together for a short time, rinse the saliva around in the mouth, and the peculiar flavor will be detected.

"In 1820 attention was called to the injurious effects of the galvanic current on the teeth, and dentists were advised never to use tin and amalgam in the same mouth.

"A constant galvanic action is kept up in the mouth when more than one kind of metal is used in filling teeth, and galvanism is often the cause of extensive injury to the teeth. The most remarkable case I ever saw was that of a lady for whom I filled several teeth with tin. After a time decay took place around some of the fillings. I removed them and began to refill, but there was so much pain I could not proceed. I found that by holding a steel plugger an inch from the tooth I could give her a violent galvanic shock. I observed that the exhalation of the breath increased the evolution of galvanism." (Dr. L. Mackall, American Journal of Dental Science, 1839.)

"When a faulty tooth in the upper jaw had been stopped from its side with tin, the interstice between it and the adjoining tooth being quite inconsiderable, while the upper surface of a tooth not immediately beneath it in the lower jaw was stopped with the same metal, I have known a galvanic shock regularly communicated from one tooth to the other when by the movement of jaws or cheeks they were brought near together." (Dr. E. Parmly, American Journal of Dental Science, 1839.)

"An interesting debate here sprung up on the action where two metals are used in one filling, such as gold and tin, the saliva acting as a medium, and where the baser metal is oxidized by exhalents and by imbibition through the bony tooth-structure." (Pennsylvania Society of Dental Surgeons, 1848.)

"A patient came to me and complained of pain in the teeth. Upon examination I found an amalgam filling next to one of tin. With a file I made a V-shaped separation, when they experienced immediate relief from pain." (Dr. Nevill, American Journal of Dental Science, 1867.)

In regard to the decay of teeth being dependent on galvanic action present in the mouth, Dr. Chase, in 1880, claimed that a tooth filled with gold would necessarily become carious again at the margin of the cavity, wherever the acid secretions constantly bathe the filling and tooth-substance. A tooth filled with amalgam succumbs to this electro-chemical process less rapidly, while one filled with tin still longer escapes destruction. The comparative rapidity with which teeth filled with gold, amalgam, or tin, are destroyed is expressed by the numbers 100, 67, 50. He prepared pieces of ivory of equal shape and size, bored a hole in each, and filled them. After they had been exposed to the action of an acid for one week, they had decreased in weight,—viz, piece filled with gold, 0.06; amalgam, 0.04; tin, 0.03.

"With tin and gold, some have the superstition that the electricity attendant upon such a filling will in some way be injurious to the tooth; it matters not which is on the outside, when rolled and used as non-cohesive cylinders each appears. We say that neither experimentally, theoretically, nor practically can any good or bad result be expected from the electrical action of a tin-gold filling on tooth-bone, and neither will the pulp be disturbed." (Dr. W. D. Miller, Independent Practitioner, August, 1884.)

"When the bottom of a cavity is filled with tin which is tightly (completely) covered with gold, there is practically no galvanic action and there is no current generated by contact of tin and gold,—i.e., no current leaves the filling to affect the dentin. That portion of tin which forms the base is more positive than a full tin filling would be. The effect is to cause the surface exposed to dentin to oxidize more than tin would do alone; in that there is a benefit. In very porous dentin there is enough moisture to oxidize the tin, by reason of the current set up by the gold." (Dr. S. B. Palmer.)

Electricity generated by heat is called thermo-electricity. If a cavity with continuous walls is half filled with tin and completed with gold, or half filled with silver and completed with gold, and the junctions of the metal are at 201/2° C. and 191/2° C., if the electrical action between the tin and gold be 1.1, the action between the silver and gold will be 1.8, thus showing the action in silver and gold to be nearly two-thirds more than in the tin and gold, a deduction which favors the tin and gold.

Rubbing two different substances together is a common method of producing an electric charge. Is there not more electricity generated during mastication on metal fillings than when the jaws are at rest? Friction brings into close contact numerous particles of two bodies, and perhaps the electrical action going on more or less all the time through gold fillings (especially when other metals are in the mouth) accounts for a powdered condition of the dentin which is sometimes found under cohesive gold fillings, but not under tin.