Why Tin Alloys?
Tin has long been an old standby as a prepolishing and polishing lap. Its low Coefficient of Friction reduces "grabbing" of the stone and minimizes heating. Tin is just soft enough to allow diamond powder or other abrasives to readily sink into the lap surface under slight pressure, yet hard enough to maintain a flat form. Tin is nontoxic. Pure tin by itself, even if supported on a rigid base, is a bit soft for sharp meets, however. People have addressed this in the past by using type metal or solders as a lap material. These alloys tarnish to a dull grey, and some of the benefits of high tin content are lost. Further, they contain significant amounts of lead. The swarf and mist coming from the spinning lapwheel of course, contain lead. Many of the lead alloys are still quite soft, so they wear and erode faster. The only thing gained by adding lead to a tin lap alloy is cheapness and weight.
95% Tin, 5% antimony has been around as a high temperature solder for decades. A number of prototype laps were made available to readers of the Faceters' Digest, and many people are now trying these laps. If you do not currently subscribe to this daily mailing, send an e-mail to Jerry Dewbre at faceters@ix.netcom.com, with a brief introduction.
Do not use vinegar in the coolant when using lead alloy laps. Lead acetate is water soluble.
We are all forever looking for better laps, better polishes, etc., and I am no exception: A new alloy has been developed with lower antimony concentration, nontoxic in nature, but harder still than 95/5. I am now casting laps from the new alloy, tentatively nicknamed "BATT".
It has been an object in the development of these laps, beginning with the 95/5 alloy to use only alloying ingredients of low or no toxicity. For example, the 95/5 alloy is NSF Approved for direct potable water/ food contact. The following metals and metalloids will not be used in any of these prototype formulations: Arsenic, Selenium, Lead, Tellurium, Cadmium, Indium, Boron, Mercury, Nickel, Cobalt, and Germanium. That leaves a big piece of the Periodic Table to explore.
Choice of Polishes: The new BATT alloy has been formulated to produce hydrophilic passive oxides to improve its use with Linde` and similar alumina polishes, and suspensions of Cerium and Tin Oxides. Its performance with diamond is at least comparable to that of the simple 95/5 SN/Sb alloy. Structure: Polyphase (dendritic grains in a complex eutectic matrix). Soft regions accept grains of polish and can be readily charged with diamond, while hard regions maintain surface geometry needed for obtaining sharp meets. Work on more alloys and metal/metal composites is continuing.
50X
In the above metallograph, note the dendritic grain structure as various alloying components crystallize when the alloy cools. This is not a eutectic solution, but becomes more of a composite, with differing hardness domains, a finer grain structure, and a much stiffer alloy which does work harden, while retaining tin's low coefficient of friction and lack of toxicity. The purpose of this alloy is also to provide better performance on water suspensions of common polishes, as well as with diamond compounds.
This new alloy has undergone testing by experienced faceters, and their reviews of it can be found on the Faceters' Digest.
I will make a few more for people.
