Mark Lauckner artist in glass
The Glass Foundry

Sand Cast Bowls

10-inch bowl with cedar leaf and beetles 10-inch red beetle candy dish 5-inch cornflower blue goldfish soap dish

10-inch bowl with cedar leaf and beetles

10-inch red beetle candy dish

5-inch cornflower blue goldfish soap dish

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.


Click on photos to see enlargements

Sand is added to the bowl and tamped down well.
Removing pattern

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. 


Sand Bowl Molds
Starting the pour.

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.


Finished pouring
Creating the flat spot

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.

Fire Polishing
Loading Spent mold

Occasionally, fire polishing is done to any areas that look like they need smoothing off, then the casting is loaded into the annealer on a charred wooden paddle. Any contact with metal can crack the glass when it is being handled at this crucial temperature. The sand remaining in the bottom of the bowl just sweeps off after the cooling process. The spent sand mold is re-processed and used over and over again.

detail in sand cast bowl

This photo shows the level of detail next to the tip of a ball-point pen, in the resulting sand cast bowls. 



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last update
December 12, 2013