I've used a 50/50 mix of sodium silicate and water to coat my tufa,
investment and cuttlebone molds for many years. I've never added
borax to the mix, but I can see how it might make the surface
smoother and perhaps provide an additional element of protection from
hot metal. However, from my experience, several thin coats of the
sodium silicate/water mix keeps the molds from deteriorating so you
can use them repeatedly. Larger molds tend to deteriorate more than
smaller ones, as the mass of molten metal holds its heat longer. In
the U.S. Southwest, these molds are passed down or traded.
Generally, your original casting is traditionally done in bronze.
This casting is finished and used for sandcasting, or for impressing
into another tufa, investment or cuttlebone mold. The original is
always kept as the master. Many of the prefabricated 'sandcast'
models you see advertised are from pieces bought in the '70's from