Every raw material you use in a glaze recipe is a source of one or more oxides. Knowing which oxides each material supplies — and in what proportions — is what allows you to read a recipe analytically, substitute materials when something isn’t available, and troubleshoot glaze problems. This guide maps the most common studio glaze materials to the oxides they contribute.
Silica Sources
Silica (SiO₂) is the primary glass former in any glaze. These materials supply silica with minimal other oxides:
Material
Primary Oxide
Notes
Silica / Flint / Quartz
SiO₂ ~100%
Pure silica source; raises melting point
Alumina Sources
Alumina (Al₂O₃) is the stabilizer — it gives the molten glaze viscosity and prevents running. Most alumina sources also contribute silica:
Material
Oxides Supplied
Notes
EPK Kaolin
Al₂O₃, SiO₂
Most common studio kaolin; adds suspension
Calcined Kaolin
Al₂O₃, SiO₂
Fired kaolin; less shrinkage in raw glaze
Ball Clay
Al₂O₃, SiO₂
More plastic than kaolin; adds suspension
Alumina Hydrate
Al₂O₃
Pure alumina source; used for kiln wash too
Flux Sources
Fluxes lower the melting point of the glaze. Different fluxes produce different surface qualities — calcium produces a harder, more stable glaze; sodium and potassium produce brighter, glossier surfaces; magnesium produces a softer, more satin quality at high fire.
Material
Flux Oxide(s) Supplied
Also Supplies
Whiting (Calcium Carbonate)
CaO
—
Wollastonite
CaO
SiO₂
Dolomite
CaO, MgO
—
Talc
MgO
SiO₂
Zinc Oxide
ZnO
—
Barium Carbonate
BaO
—
Strontium Carbonate
SrO
—
Lithium Carbonate
Li₂O
—
Spodumene
Li₂O
Al₂O₃, SiO₂
Feldspars — Multi-Oxide Materials
Feldspars are the workhorses of high-fire glazes. They contribute flux (Na₂O or K₂O), alumina, and silica simultaneously, which is why a simple feldspar glaze can work with very few additional materials:
Material
Oxides Supplied
Notes
Potash Feldspar (Custer, G-200)
K₂O, Al₂O₃, SiO₂
High potassium; standard cone 6/10
Soda Feldspar
Na₂O, Al₂O₃, SiO₂
Higher sodium; glossier surface
Minspar 200
Na₂O, K₂O, Al₂O₃, SiO₂
Mixed feldspar; good casting slip flux
Nepheline Syenite
Na₂O, K₂O, Al₂O₃, SiO₂
Lower silica than feldspar; melts earlier
Boron Sources
Boron (B₂O₃) is a glass former, not a flux — it lowers the melting point of the whole glaze but builds glass structure rather than acting as a flux, and is tracked separately from the fluxes in the unity formula. It’s essential for low-fire and mid-fire glazes and helps reduce crazing in cone 6 work:
Material
Oxides Supplied
Notes
Gerstley Borate
B₂O₃, CaO, Na₂O
Variable material; can cause crawling
Colemanite
B₂O₃, CaO
More consistent than Gerstley Borate
Frit (boron frits)
B₂O₃ + other oxides
Most consistent boron source; varies by frit number
I am a visually impaired ceramic artist. I have been making for around 8 years now. I specialize in functional colorful pottery. Mainly nerikome and other decorative processes.
