Master Mold Materials


Here we discuss different materials we have used to make master molds and some of the experiences we have had.


Some quick definitions:

Mold or casting - We are referring to whatever is being used to give a final shape to fused glass that is being slumped. This mold has to be able to survive kiln slumping temperatures.

Master mold - Whatever is being replicated with the use of mold making material such as Best Mix to make a mold. Master Molds may or may not be used more than once to make a mold depending on their durability.




Plastics are great for use as master molds because they are water proof and smooth. The smoother the surface, the easier it is to remove the cast mold from the master. Plastics work well since they flex when removing the mold from the plastic.



There are a wide variety of wood products available. My favorite is medium density fiber board (MDF). This material is is very consistent in its density and is dry so shrinkage is not an issue as it can be with other wood products. I can also glue (Titebond II) up two or more pieces of MDF, rough cut a circle with my jigsaw and turn it on a wood lath for circles.

A key to creating a master mold with wood is water proofing it before casting over it. ANY exposed wood will soak up water, swell and make the casting around the exposed area crumbly.

Wood needs to be completely sealed against water leaking into the wood. I primarily use Spar Varnish from Ace Hardware. The gloss marine type soaks into the wood very well and creates an excellent seal. It does take a long time as each coat take 24 hrs to dry and MDF wood needs 3 to 5 coats. I then finish it off with green sealer.

Spray on lacquer also works well and dries quickly. It does not work well with the green sealer, so I do not use it as often.



We have used modeling clay carved or molded to the desired shape as master molds. You need to make sure that your finished shape doesn't have any undercuts that would lock slumped glass into the mold, and also that the clay is anchored so that it doesn't float up off the surface when pouring the casting.

Molds made of pottery clay have been problematic and are not recommended. The porous surfaces are difficult to seal. Time, understanding and experience using this kind of clay is strongly recommended.



For large or more hand-crafted master molds, styrofoam can be a option. There are many different densities of styrofoam available so care must be taken in determining which kind will work the best for your application. As with wood, styrofoam needs waterproofing.