Why heavily pitted pewter?

Question

I just made my first mold using a Smooth-on Co. silicone mold material. The mold looks fine but the finished pewter castings come out covered with heavy pitting. I coated the mold cavity with talm powder or graphic powder but I still get this pitting effect on the finished piece. Can anyone tell me what I'm doing wrong or what I'm failing to do? Thanks.

David Billington · Accepted Answer

I recall having some minor problems like this using Dow Corning Silastic 3120 for pewter casting. I heated the mould to maybe 100C -120C for an hour or 2 and this helped drive out some of the volatile constituents which helped. Bear in mind that this caused the mould to shrink maybe 5% or more depending on the length of time heating. Pewter is above the recommended temperature for the 3120 material but for limited quantities it stands up fine from my and others experience. Don't know anything about the smooth-on product, have you asked the supplier.Joseph Rizzo wrote:

Andy Dingley · Answer

Looks like tiny gas bubbles ?You've either got a new mould and this will disappear after a fewcasts, or else you're pouring too hot for the mould material.  I don'tknow what your "pewter" alloy is, but some of the modern lead-freealloys are too hot for some silicones.If the mould surface starts to break up, you're too hot.

Why heavily pitted pewter?

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