Baked Cast Slate

Culture & Context

I used slate from a local quarry in Valentia Island, Ireland.

Valentia slate has many notable historic applications in and outside of Ireland, and holds significance when doing a deep dive into traditional Irish materials, and Irish material culture. 

This material was an attempt to work with slate in a new and contemporary way, bringing a breath of fresh air to how designers can work with locally sourced stone.

Process & Production

Wear a dust mask while handing slate dust, wear gloves when handling sodium silicate.

Recipe Details

Air dry cast slate:

• Collect slate silt.
• Rinse silt with clean water.
• Dry silt to remove moisture, preferably in an oven, until it is a dry powder.
• Sift powder to break up clumps, achieve uniform consistency.
• Measurements by weight: 70% slate, 15% water, 15% sodium silicate. (experimentation + samples is recommended)
• Combine water and sodium silicate, mix well. 
• Combine the diluted sodium silicate with the slate powder.
• Mix thoroughly to remove clumps. 

• Add mix to non-porous mold. (porous molds will extract the water and sodium silicate from mixture and intervene with bonding process, resulting in a crumbly product)
• Put in oven and bake at 150-180C. Time in oven depends on casted piece. Baking the mixture does a few things. The goal is to get the water in the mixture to evaporate into a gas and expand, creating various air pockets. The mixture will “rise” similarly to a cake. Likewise, air pockets will also collapse slightly after being taken out of the oven. 
• Let dry for at least 24hrs. 
• Your cast piece can now be trimmed using a saw to remove uneven surfaces and expose the even-toned, “cake-y” slate. 
• Let dry until solid. (Drying time depends on the size of casted piece, drying time could take up to 2 weeks or more for larger pieces. CO2 improves the hardening process, and accelerates drying time. Otherwise, use a drybox, with a fan and dehumidifier. The goal is to remove all the water from the solution, and allow the sodium silicate to bond all the slate together)