Did some research in using RTV for zinc casting, it won't work the temperature of molten zinc is to high although suitable for pewter casting.
Would be best to make an aluminium mould or use sand casting.
To make an mould from aluminium yoju first need a pattern, for this we would purchase a new zinc anode. Now to make the mould we need to make a wooden box wide enough to slice in half giving us a top and bottom.
Examine the zinc anode for the parting line then fill the bottom box with modelling clay, using a spoon remove any clay surrounding the pattern down to the parting line. One half of the pattern should be exposed while the other half buried in clay.
I use an acorn nut for this next procedure, press some random dimples into the clay, put the top box onto the bottom then seal the box's were they join with clay. Spray on some mould release then pour in a filler of your choice. As a filler my choice is Bondo an auto body filler, but you can use plaster, recommended you paint the plaster once r removed and dried. The paint forms a hard skin on the outside of the plaster making it harder.
Smooth-on sells RTV rubbers in various hardness, they also sell the mould release.
If you decide to use the Smooth on products be sure to watch the videos on how to remove air bubbles.
After the filler used to fill the top half of the paired box's has cured, turn the box over then remove the clay and only the clay leaving your pattern on the cured filler. Once the box is free of modelling clay repeat as you did with the top box before, spray on some mould release the fill the box with filler and let it cure.
Next morning your box should be ready to open, remove the zinc pattern, use the mated cured pairs as patterns to cast your permanent zinc mould from aluminium, you will now see that the dimples you pressed in previously now have a male and female registrations with the two halves fitting perfectly.
Once you have cast these plaster patterns into aluminium copies you can hold them together using a C-clamp while you pour in the molten zinc.
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