Copyright 2006 T. Sheil & A. Sheil All Rights Reserved
(Editor's Note: This article is one of several which were provided to Bob Bard for a book he never finished. We do not the author's identity. Nonetheless, it is a good article on casting and the properties of casting metals.)
In comparing notes on casting with fellow collectors I was amazed at the variety of techniques and metal alloys being used, and how little is understood of the tin-lead alloys. For this reason I am passing on my limited experience in hope that it will be of value to others.
"Variations in the tin-lead composition caused me untold grief with poor figures and short mold life, until I had a better understanding of the characteristics of these alloys,
In the above diagram you will note that where tin is added to lead which melts at 621°?., it lowers the melting point of lead along line, A.C. When lead is added to tin which melts at 450°F., it lowers the melting point of tin along line, B.C. At point C where these two lines meet, there is an alloy of the lowest melting point of these metals, tin and lead which is known as the eutectic composition and consists of 63% tin and 37% lead, with a melting point of 36l°F. You will note that, except for the eutectic alloy, the other mixtures of tin-lead do not melt sharply at any one temperature, but will pass through an intermediate range of plasticity when cooling from a liquid to the solid state.
The influence of additional elements is varied and pronounced. As an example: Bismuth forms a 3-element eutectic and this requires 52% Bismuth. This melts at 295°F., and requires 18% cadmium. The addition of small amounts of silver forms a somewhat lower melting eutectic than the pure tin-lead eutectic, whereas a little antimony raises rather than lowers the temperature of the tin-lead alloy.
Impurities in tin-lead alloys exert a profoundly deterious effect on
flowing qualities and on the general appearance of the finished casting.
This is particularly true when using reclaimed lead alloys. Just to show
a few examples.
1. Zinc requires higher temperatures and the resulting casting will be rough, grainy, frosty and porous.
2. Aluminum will react like zinc.
3. Cadmium in very minute amounts imparts a sluggish property to the molten alloy and castings have a porous, dendritic character*
4. Arsenic in minute quantities causes small blisterlike spots or "lamellae" to appear on the casting.
5. Nickel reacts the same way as does Arsenic.
6. Copper will produce a hard, grainy and brittle casting.
7. Bismuth reacts similar to copper.
8. Antimony - The effect of antimony is a controversial question. However, a 40/60 tin-lead mixture plus 2% antimony will produce a very hard and brittle casting. This is beneficial when casting small parts such as rifles, bayonets etc*
At this point one might ask "What ratios of tin and lead produce the best figures?" Personally, I use the 63/37 composition, because of its low melting point and cannot detect any difference in the quality of the casting when comparing it with castings of pure tin.
A good source of tin is your local commercial refrigeration mechanic who is presently discarding all of his pure block tin carbonated water lines in favor of stainless steel or plastic.
Of all of the materials I have tried for molds, the Silicone rubbers have been the best. However, because of their cost and the time required for curing, I prefer the special plaster used by the dentists for making impressions. It is called, "Cocal". The reasons for this preference is that it gives finer details, withstands higher temperatures, sets up faster and has a longer mold life than does regular Plaster of Paris.
Rather than using a removable wooden frame for preparing
molds, I prefer to use the retaining rings from Mason jar caps. These
become a permanent part of the mold with the threads, grooves,
holding the plaster firmly in
place. This metal frame not only protects the mold should it be dropped,
but also allows the mold to be clamped in a vise for quick and easy
pouring. For larger figures, such as horses, square metal down spouting
cut in l" lengths serve very nicely. These can be further
improved by soldering copper tubes to each side so that the two halves can be
held together by long
1/4 -20 bolts.
In preparing the first half of the mold, I fill my frame with melted parafene instead of clay. As it cools, the figure to be molded is embedded to the predetermined parting lines, with the sprue and air vents being added after it sets up with a hot knife. The advantages to this method over modeling clay is that it gives a firmer base, is faster, cleaner and does not stick to the figure or the mold.
When trying to create a new part or figure, I have gone back to my dentist again and had him save the scraps of blue wax which he uses for making inlays. Using my present molds, the wax is melted down and cast in the same manner as with tin-lead. These wax castings then can be modified and sculpt with a knife or hot needle until the desired figure is created in wax. For the first figure or master, the wax figure is embedded completely in plaster and after it sets up the wax is removed by heat leaving a hollow, one piece, detailed mold. This is basically the "Lost Wax" method. After casting the tin-lead alloy, the plaster mold is broken away leaving a detailed casting without parting lines. Additional carving or engraving can be done at this time after which the regular two part mold procedure can be followed.
It would be interesting to receive ideas from others on the methods they are successfully using.