Ceramic materials are the raw powders that go into a glaze bucket: ground minerals, refined chemicals and manufactured glasses. Once you start mixing glazes from recipes instead of buying jars, names like nepheline syenite, Minspar 200 and Gillespie Borate turn up on every list. This guide explains what each common material does, which ones have changed or disappeared, and what the safety data sheets (SDS) say about them.
Home Pottery HQ is a research site. We compare supplier listings, published material data (mainly Digitalfire's reference database) and maker SDS documents; we have not mixed these recipes ourselves. For a shorter answer on what a finished glaze is made of, see our clay and glazes hub.
The main groups of glaze materials
| Group | Job in the glaze | Common examples |
|---|---|---|
| Glass former | Builds the glass | Silica (flint, quartz) |
| Fluxes | Lower the melting point | Potash and soda feldspar, nepheline syenite, whiting, Gerstley and Gillespie Borate, frits, lithium carbonate |
| Stabilizers | Stiffen the melt, keep it on the pot, add matte | Kaolin, calcined kaolin, ball clay, alumina hydrate |
| Colorants and opacifiers | Color and opacity | Cobalt oxide or carbonate, manganese carbonate, iron oxide, copper carbonate, ceramic stains, tin and zircon |
| Additives | Help the raw glaze brush, stick and suspend | Dextrin, CMC gum, bentonite, Epsom salt |
Feldspars and nepheline syenite
Feldspars are natural fluxes that also supply alumina and silica, which is why a feldspar is often one of the largest ingredients in cone 6 and cone 10 recipes. Digitalfire notes that glazes with 35% or more feldspar tend to craze because of the high sodium and potassium content.
Potash feldspar
Potash feldspar is a feldspar in which potassium outweighs sodium. Digitalfire describes it as one of the most important materials for mid and high-fire glazes and says it is often favored for matte surfaces, while soda feldspar is used more for gloss.
For US potters the big change is Custer Feldspar, the long-time standard potash spar from South Dakota. Clay Art Center reports that Pacer Minerals' mines stopped in late 2023, and Seattle Pottery Supply states Custer is discontinued. Suggested replacements include G-200 AU and Mahavir (Clay Art Center) and G200eu (Seattle Pottery Supply). Clay Art Center found G-200 AU worked one for one in most of its glazes but needed adjustment in some, so test any old Custer recipe before a full batch.
Minspar 200
Minspar 200 is a 200-mesh soda feldspar from Spruce Pine, North Carolina, formerly sold as NC-4. Digitalfire lists about 6.5% Na2O, 4.1% K2O and 18.2% Al2O3, and calls its chemistry very similar to the discontinued Kona F-4, so it is the usual swap in older recipes that call for F-4.
Nepheline syenite
Nepheline syenite is an igneous rock, mostly mined in Canada for North American potters, that behaves like a feldspar with less silica. Digitalfire lists roughly 9.8% Na2O, 4.6% K2O and 23.3% Al2O3. It contains no free quartz and melts lower than feldspar, which is why it shows up in many cone 6 glazes. The trade-offs: its high sodium raises expansion (crazing risk), and its slight solubility can make glaze slurries gel over time.
Boron sources: Gerstley Borate and Gillespie Borate
Boron is the main low-temperature flux in pottery glazes. It can come from frits (see our frit guide) or from borate minerals.
Gerstley Borate
Gerstley Borate is a natural calcium borate blend of colemanite, ulexite and a plastic clay, mined in southern California. The mine closed in 2000, and Laguna Clay later milled a stockpile of the remaining ore. Sources disagree about how much is left. Digitalfire wrote in March 2023 that the price had tripled and the material's end was close, while New Mexico Clay's listing, with a note dated November 2025, showed Gerstley Borate "original" in stock from Laguna. Check your supplier before you build a recipe around it.
It is also awkward to use. Digitalfire describes Gerstley Borate as gelling in the bucket, dewatering slowly and losing about 29.5% of its weight as gas in the kiln, which can leave tiny bubbles in clear glazes.
Gillespie Borate
Gillespie Borate is a manufactured substitute made by Hammill and Gillespie. Digitalfire lists about 24.5% B2O3 and 23% CaO, and reports that it makes better slurries than Gerstley without the gelling. It can melt more suddenly and may blister in some old Gerstley recipes, so it is a close substitute rather than an identical one. Laguna Borate and boron frits are the other common routes.
Lithium carbonate in ceramics
Lithium carbonate (Li2CO3) is a strong flux that lowers a glaze's thermal expansion, which helps resist crazing; The Ceramic Shop notes it is also used in some cookware clay bodies. The Ceramic Shop says it is usually used at 5% or less in glazes and can cause pinholes or blisters at higher amounts because it releases gas as it decomposes. Digitalfire lists about 40% Li2O with a 60% loss on ignition, and notes it is slightly soluble.
Lithium carbonate is not a mild material. One SDS for lithium carbonate (Home Science Tools, revised 2018) uses the signal word Danger with H302 (harmful if swallowed), H319 (serious eye irritation), H332 (harmful if inhaled) and H360 (may damage fertility or the unborn child). The Ceramic Shop's listing carries a California Proposition 65 warning for reproductive harm. Read your supplier's SDS before use.
Kaolin, calcined kaolin and alumina hydrate
Calcined kaolin clay
Calcined kaolin is ordinary kaolin that has been fired to drive off its chemically bound water, about 12% of its weight. It keeps the same fired chemistry but is no longer plastic, so it shrinks far less as the glaze dries. Potters swap part of the raw kaolin for calcined kaolin when a high-clay glaze or slip cracks while drying and then crawls in the kiln. Because the water is already gone, roughly 88 parts of calcined kaolin stand in for 100 parts of raw kaolin. Ball clay plays a similar suspending role in some recipes; our ball clay guide covers it.
Alumina hydrate
Alumina hydrate (aluminum hydroxide, Al(OH)3) is a fine white powder that is about 65% alumina once fired. Digitalfire notes it can make glazes more matte and silky, adds some opacity, and is preferred over calcined alumina for kiln wadding, often mixed half and half with kaolin.
Colorants: cobalt, manganese and ceramic stains
What is cobalt oxide?
Cobalt oxide is the strongest colorant potters use. In cobalt oxide pottery the result is blue at almost every temperature, and Digitalfire says potters typically use 1% or less, with high amounts turning black. Commercial cobalt oxide is mostly Co3O4. Cobalt carbonate is a weaker, finer form that disperses more evenly; Digitalfire gives about 93% cobalt oxide content for the oxide against 63% for the carbonate, so you need roughly 1.5 times as much carbonate for the same color.
The Laguna Clay SDS for cobalt oxide (revised 2015) lists H302 (harmful if swallowed), H317 (may cause an allergic skin reaction), H334 (may cause allergy or asthma symptoms if inhaled) and H351 (suspected of causing cancer), and notes an IARC Group 2B listing. Wear gloves and a NIOSH-approved respirator when handling the powder.
Manganese carbonate
Manganese carbonate (MnCO3) gives purples and plums in alkaline glazes and purple-browns in other bases, and is also used to darken clay bodies. It breaks down early in the firing, and manganese can fume at high glaze temperatures, so a working kiln vent matters.
Manganese carbonate carries some of the strongest warnings of any common glaze material. The Nippon Denko SDS (2021) supplied by Archie Bray uses the signal word Danger with H360 (may damage fertility or the unborn child), H370 (causes damage to organs) and H372 (causes damage to the respiratory system and nervous system through prolonged or repeated exposure). Do not handle it without a NIOSH-approved respirator, and never let it dry on benches or floors.
Ceramic stains and body stains
Ceramic stains are manufactured colorants: oxides fired together and ground so they give steadier color than raw oxides. Mason Color Works is the best known US maker. Its reference guide warns that free zinc oxide in a glaze can destroy some stain colors, that some colors want 12% to 15% whiting in the glaze, and that stains have different maximum temperatures, roughly 1,976°F to 2,300°F (1,080°C to 1,260°C).
Body stains are stains suited to coloring clay bodies, slips and engobes rather than glazes. Digitalfire's notes on Mason 6600 Black show about 6% in glazes and 10% to 15% in engobes, but suggest keeping amounts in clay bodies modest because stains change how the clay matures. Our engobe guide covers colored slips.
Dextrin powder and other additives
Dextrin is a water-soluble carbohydrate made from corn or potato starch. In glazes it works as a binder: it hardens the dried glaze so it does not powder or rub off before firing, helps brushing glazes stick to vertical walls, and adds some thickness. Digitalfire suggests about 0.5% to 1.5% in sprayable glazes and 2% to 5% in engobes, dissolved in warm water first. It burns away in the kiln.
A note on barium carbonate
Barium carbonate is used for matte surfaces and some color effects, and it is toxic if swallowed. It has its own page: see our barium carbonate guide.
Safety rules for glaze chemicals
- Get the supplier's SDS for every material you buy and read sections 2 (hazards) and 8 (protection) before opening the bag.
- Weigh and mix dry materials only while wearing a fitted NIOSH-approved respirator; nearly every glaze contains silica, and several colorants are toxic.
- Clean with water: wet sponge, wet mop or a HEPA vacuum. Never dry sweep or blow dust off a bench.
- Wear gloves for colorants, keep food, drinks and phones out of the mixing area, and wash hands before eating.
- Store materials sealed and labeled, out of reach of children and pets.
- Vent the kiln as its maker directs, especially when firing manganese, copper or other colorants that can fume.
Our clay dust safety guide covers silica in more depth.
No raw material makes a glaze food safe or unsafe by itself. Leaching depends on the full recipe, the clay and firing to the stated cone. Never assume a home-mixed glaze is food safe; read our food safe glaze guide before using one on mugs or plates.
Common mistakes with ceramic materials
- Substituting a discontinued feldspar or borate one for one without a test tile.
- Using raw oxides at stain percentages. Cobalt at 5% is far stronger than a blue stain at 5%.
- Ignoring loss on ignition. Carbonates and calcined materials change the weight a recipe really needs.
- Keeping manganese or cobalt in unlabeled containers.
- Mixing in the kitchen or any room used for food.
What you need
Before buying ceramic materials, set up a respirator, a digital scale, a sieve and sealed buckets. Our pottery respirator buying guide compares masks for mixing dry materials, and our pottery supplies guide lists US suppliers that sell raw materials by the pound. For mixing steps, see mixing glaze from powder.
Sources: Digitalfire reference pages for potash feldspar, Minspar 200, nepheline syenite, Gerstley Borate, Gillespie Borate, lithium carbonate, calcined kaolin, alumina hydrate, cobalt oxide, manganese carbonate, dextrin and Mason 6600 stain; Clay Art Center, "The End of an Era: The Extinction of Custer Feldspar" (2025); Seattle Pottery Supply, "Is Custer Feldspar discontinued?"; Ceramic Arts Network, Custer Feldspar and Ceramic Raw Materials guide; New Mexico Clay Gerstley Borate pages; The Ceramic Shop lithium carbonate listing and Mason Stain reference guide; Mason Color Works reference guide; SDS: Laguna Clay cobalt oxide (rev. 2015), Nippon Denko manganese carbonate via Archie Bray (2021), Home Science Tools lithium carbonate (rev. 2018). Specs checked October 2026.