could always use a potato? http://www.smtnet.com/Forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=9994Message56920
But seriously.. best idea is to stay away from any metal as the tin will likely dissolve and contaminate your solder pot. I haven't done this but I would recommend a stone more dense than solder because you wouldn't want some brick floating round the pump space. You would also want it to be clean and crack free so no bits of silica can get free and cause a grinding affect on the pumps, imagine it would act like sand paper. maybe even some bricks that have been Vitreous enamel (this is what our steel solder pot is lined with)as it would act like a conformal coat?
http://en.wikipedia.org/wiki/Vitreous_enamel
Best of luck.
All else fails you could just fill the pot with solder. lead free look into SN100C.
P.S. reducing the solder pot volume could actually cause unforeseen cooling of the solder during production, the volume helps to keep a stable temperature due to thermal mass.
P.P.S Would pay to check where the elements sit, if there is an element on the bottom of the solder pot it would potentially cause additional warm up issues/heat management issues due to the bricks isolating it.
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