White caries, the most formidable variety known, may be produced by nitric acid, and in these cases all the components of the tooth are acted upon and disintegrated as far as the action extends. In proximal cavities attacked by this kind of caries, separate freely on the lingual side, and fill with tin. When such fillings have been removed the dentin has been found somewhat discolored and greatly solidified as compared to its former condition; this solidification or calcification is more frequent under tin than gold, which is partly due to the tin as a poor conductor of heat. Nature will not restore the lost part, but will do the next best thing—solidify the dentin. In some cases, under tin, the pulp gradually recedes, and the pulp-cavity is obliterated by secondary dentin. In other cases the pulps had partly calcified under tin. It has been known for years that tin would be tolerated in large cavities very near the pulp without causing any trouble, and one reason for this is its low conducting power. Attention is called to the fact that gold is nearly four times as good a conductor of heat as tin, and more than six times as good a conductor of electricity. Where tin fillings are subject to a large amount of attrition, they wear away sooner or later, but this is not such a great detriment, for they can easily be repaired or replaced, and owing to the concave form produced by wear the patient is liable to know when a large amount has been worn away. That portion against the wall of the cavity is the last removed by wear, so that further caries is prevented so long as there is any reasonable amount of tin left. If at this time the tooth has become sufficiently solidified, proper anchorage can be cut in the tin or tooth, one or both, as judgment dictates, and the filling completed with gold. A tin filling, confined by four rather frail walls, may condense upon itself, but it is so soft and adaptable that the force which condenses it continually secures the readaptation at the margin; thus there will be no leakage or caries for years. Owing to its softness and pliability, it may be driven into or onto the tubuli to completely close them from outside moisture, and with a hand burnisher the tin can be made to take such a hold on dry, rough tubuli that a cutting instrument is necessary to remove all traces of it.

Tin foil has been found in the market that under a magnifying glass showed innumerable tiny black specks, which, upon being touched with an instrument, crumbled away, leaving a hole through the foil. More than likely, some of the failures can be attributed to the use of such foil. Good tough foil, well condensed by hand or mallet force, stays against the walls of a cavity and makes a tight filling, and ought to be called as near perfect as any filling, because it preserves the tooth, and gives a surface which will wear from five to twenty years, depending upon the size and location of the cavity and tooth-structure. Buccal cavities in the first permanent molars, and lingual cavities in the superior incisors, filled for children from six to eight years of age, are still in good condition after a period of twenty years. Perhaps the limit is reached in the following cases, all in the mouths of dentists: One filling forty years old; one forty-two; four on the occlusal surface, fifty; in the latter case gold had been used in other cavities and had failed several times. Lingual cavities in molars and bicuspids can be perfectly preserved with tin. Tapes of No. 10 foil, from one to three thicknesses, can be welded together and will cohere as well or better than semi-cohesive gold foil, and it can be manipulated more rapidly; therefore, if desirable, any degree of contour can be produced, but the contour will not have the hardness or strength of gold, so in many cases it would not be practicable to make extensive contours with tin, owing to its physical characteristics.

No. 10 will answer for all cases, and it is not as liable to be torn or cut by the plugger as a lower number, but one need not be restricted to it, as good fillings can be made with Nos. 4, 6, or 8. More teeth can be saved with tin than with any other metal or metals, and the average dentist will do better with tin than with gold. It is invaluable when the patient is limited for time or means, and also for filling the first permanent molars, where we so often find poor calcification of tooth-structure. In cases of orthodontia, where caries has attacked a large number of teeth, it is well to fill with tin, and await further developments as to irregularity and caries.

If cavities are of a good general retaining form, that will be sufficient to hold the filling in place; but if not, then cut slight opposing angles, grooves, or pits. Cavities are generally prepared the same as for gold, except where there is a great deal of force brought upon the filling; then the grooves or pits may be a little larger; still, many cavities can be well filled with less excavating than required for gold, and proximal cavities in bicuspids and molars, where there is sufficient space, can be filled without removing the occlusal surface, and here especially should the cavities be cut square into the teeth, so as not to leave a feather edge of tin when the filling is finished, as that would invite further caries and prove an obstruction to cleansing the filling with floss.

In proximal cavities involving the occlusal surface, cut the cervical portion down to a strong square base, with a slight pit, undercut, or angle, at the buccal and lingual corners; where there is sufficient material, a slight groove across the base, far enough from the margin so that it will not be broken out, can be made in place of the pit, undercut, or angle; then cut a groove in the buccal and lingual side (one or both, according to the amount of material there is to work upon), extending from the base to the occlusal surface; in most of these cases the occlusal grooves or pits would have to be excavated on account of caries; thus there would be additional opportunity for anchorage. In place of the grooves the cavity may be of the dovetail form. In nearly all proximal cavities in bicuspids and molars, some form of metal shield, or matrix, is of great advantage, as they prevent the tin from crushing or sliding out. By driving the tin firmly against the metal, a well-condensed surface is secured; and as the metal yields a little, we can with a bevel or thin plugger force the tin slightly between the metal and the margin of the cavity, thus making sure of a tight filling, with plenty of material to finish well. After removing the metal, condense with thin burnishers and complete the finish the same as for gold. Where no shield or matrix is used, or where it is used and removed before completing the filling, it is often desirable to trim the cervical border, for in either case there is more light and room to work when only a portion of the cavity has been filled. Tin cuts so much easier than gold, it is more readily trimmed down level with all cervical margins.

Be sure that all margins are made perfect as the work progresses, and if the cavity is deep and a wide shield shuts out the light, then use a narrow one, which can be moved toward the occlusal surface from time to time.

In filling the anterior teeth when the labial wall is gone, and the lingual wall intact or nearly so, use a piece of thin metal three-quarters of an inch long and wide enough to cover the cavity in the tooth to be filled, insert it between the teeth, and bend the lingual end over the cavity; the labial end is bent out of the way over the labial surface of the adjoining tooth, as shown in Fig. 4. When the labial wall is intact or nearly so, access to the cavity should be obtained from the lingual side, and in this case the bending of the shield would be reversed, as shown in Fig. 5. The shield is not absolutely essential, but it helps support the tin, and also keeps a separation.

It is preferable to save the labial wall and line it with (say) five layers of No. 4 semi-cohesive gold folded into a mat and extended to the outer edge of the cavity; this gives the tooth a lighter shade, and bicuspids or molars can be filled in the same manner. Cases are on record where incisors with translucent labial walls, filled by this method, have lasted from twenty-three to thirty-seven years.