Toxic. Crystal glazes are an exciting array of coloured glazes that "explode" with jewels of colour during the firing process. They consist of a base glaze with pebbles, granules, and specks of contrasting glazes. During the firing, the crystals spread out to produce a burst of colour. The base glaze may be gloss or matte; the crystals may be the same type of glaze or a contrasting type. In any case, the mixture is designed to melt and smooth out in the firing. Crystal Glazes are a lead-bearing glaze and should not be confused with the non-toxic crystal glazes called Crystalites. Fire to cone 06.