Sand Cast Bowls

Sand cast bowls are made by pouring glass onto a convex form made of damp tamped sand. This is the opposite of the usual sand casting method, where the glass is poured into a concave impression in sand. It is very difficult to get the glass to flow evenly down the curved sides of the form. This occasionally results in a bowl rim which is is irregular. Patterns placed in the bottom of the wooden bowl mold create cavities which fill with glass. The result is a raised pattern on the inside of the finished glass bowl. Following is a description of the entire process.

Patterns are placed face-up in the bottom of the wooden bowl. The patterns represent what will be created in the glass. In this case, a bowl with raised designs on the bottom inside. Next, the sand is added to the bowl and tamped down very well. The sand I use is a mixture of 4 particle sizes to ensure very good transfer of detail from the patterns to the glass. Next, the patterns are removed by flipping over the bowl and carefully lifting out the design features. In this example, a sword fern leaf has a small salamander sitting on it. The salamander is a plastic replica. Patterns have to be solid enough to handle the pressure of the sand tamping. 

The sand molds are poured while the sand is still damp and cold. (If the sand dries out then the glass may heat it up enough to fuse to granules in the pattern which extend the farthest into the glass region.) A thin layer of graphite is then applied by flame-spraying acetylene. (Aerosol graphite sprays don't work well for sand, and powdered graphite only settles on horizontal surfaces.) This is a chemical barrier which prevents the caustic fluxes in the hot glass from melting into and sticking to the sand...just long enough for the glass to cool a bit to a point where the fluxes that make the glass "sticky" are no longer chemically active. Then the pouring is started by "touching down" on the top and slowly adding to the volume as it flows along the sand. A circular filling motion ensures an even distribution of glass to flow down the sides of the pattern.

Pouring continues in a circular motion, adding to the glass supply, never adding to fresh sand. I gently press a flat piece of steel plate on the top of the pour to make a flat spot so the bowl doesn't wobble. The steel is quickly removed as rapid cooling of one area can cause stress in the glass.