Volcanic Ash, also known as pumice, as tiny particles of volcanic glass, mined from material that has been sent into the air by an erupting volcano and then settled on the ground in layers. This material is non-cystalline because of its fast cool when it is created. It is used for its melting properties, instead of melting it will soften over a range of temperatures. Volcanic ash has varying chemistry and make up, sometimes enough to change the way a glaze containing this material works.

Information gathered from Digitalfire Ceramic Materials Database.