Web Sodium Bicarbonate + Cornstarch https://materiom.org/recipe/47
Take 1/2 cup of tapioca starch and 1 cup of sodium bicarbonate and mix together in a saucepan. Add 3/4 of water and mix together until everything is one viscous mass.
Place saucepan onto medium heat and keep mixing until mass becomes dough-like
Take out into a bowl and place a towel over it to cool for 5 minutes. In the meantime turn oven on to 150 (fan)
You may wish to create a unique shape for when placing the mixture in a mould or glass/baking tray. Clay stone covered in oven-proof cling film can work as objects to create spaces between the mixture.
When mixture is placed in the mould - place in the pre-heated oven and check every 20 minutes for the surface to be hard and have a very light matte finish rather than a slightly damp surface.
Place oven proof cling film at the bottom of your mould or glass/baking tray. You may wish to create a unique shape for your mixture before it is placed in the oven. For example clay stones wrapped in oven-proof cling film can work as objects to create spaces between the mixture.
Carefully take baked mixture out of the mould when done.