Glazes 101: What They Are and How They Work
If you've ever held a mug and wondered why it feels smooth and glassy, why it doesn't absorb your coffee, why it can be matte or shiny or somewhere in between — that's all the glaze doing its job. And once you understand what glaze actually is, a lot of the mystery disappears.
This post is for absolute beginners. No chemistry degree required. Let's start at the very beginning.
What Is a Glaze?
The simplest way to think about glaze: it's liquid glass that you paint onto clay.
Before firing, glaze looks a lot like slightly thick, milky water. It's a suspension of powdered minerals and water — not unlike the consistency of thin paint. You brush it, dip your pot into it, or pour it over the surface. At this stage it looks dull and chalky. Pretty unimpressive, honestly.
Then it goes into the kiln.
At high temperatures — somewhere between 1000°C and 1300°C depending on your glaze — those minerals melt together and fuse into a thin layer of glass that bonds permanently to the clay surface underneath. When the kiln cools, you open the door and what was a chalky powder coating has transformed into something smooth, glassy, and beautiful.
That transformation is the magic of glazing. The powder just disappears.
Why Do Potters Use Glaze?
Two reasons: function and beauty.
Function first. Unglazed clay is porous. Imagine a terracotta flowerpot — water soaks straight through it. That's great for a plant, terrible for a coffee mug. Glaze seals the surface, making it waterproof and hygienic. You can eat off it, drink from it, wash it. This is why functional pottery (mugs, bowls, plates) is almost always glazed on the inside at minimum.
Then beauty. Once you've solved the functional problem, glaze becomes your canvas. It can be clear (showing the clay body underneath), opaque white, deep cobalt blue, iron-brown, or almost any colour you can imagine. The surface can be glass-smooth and reflective, silky and satin-like, or dry and matte. Glaze gives pottery much of its personality — and honestly, it's where a lot of the fun lives.
What's Inside a Glaze?
Think of a glaze recipe like a cooking recipe. Different ingredients each play a specific role, and the balance between them determines what the finished glaze looks, feels, and behaves like.
Most glazes are built from three types of ingredients working together:
1. The Glass-Former (Silica)
Silica is the backbone of every glaze. It's what actually becomes glass when it melts. Think of it as the flour in a cake — everything else supports it, but without it you don't have a cake at all.
Pure silica melts at around 1700°C, which is far hotter than any pottery kiln. So on its own, it's useless. That's where the next ingredient comes in.
2. The Flux
A flux is a melting agent. Its job is to lower the temperature at which silica melts — bringing it down into the range your kiln can actually reach. Different fluxes work at different temperature ranges, and you'll see them everywhere:
- Whiting (calcium carbonate) is a common high-fire flux. You'll see it in many cone 6 recipes.
- Gerstley Borate or Gillespie Borate is a boron-rich low-temperature glass-former. Boron melts at far lower temperatures than silica, so it pulls the overall firing temperature down into a practical range. It's often listed as a flux in older references, but it works by forming glass at low temperatures rather than by dissolving other oxides the way a true flux does.
- Feldspar (like Nepheline Syenite) is both a flux and a glass-former — it's a workhorse ingredient in many glazes.
Think of a flux like salt in a recipe: it fundamentally changes how the other ingredients behave.
3. The Stabiliser (Alumina)
Silica and flux together would melt beautifully — and then immediately run off your pot and onto your kiln shelf. Not ideal! Alumina (usually introduced through clay minerals like Kaolin) stiffens the melt, giving the glaze body and keeping it in place on vertical surfaces.
Alumina is also what makes glaze durable. More alumina = harder, more scratch-resistant surface.
Think of it as the egg in your cake batter: it holds everything together.
A Simple Recipe, Decoded
Here's the base recipe for our Translucent Satin glaze, with the role of each ingredient labelled. Don't be put off by the table — it's simpler than it looks:
| Ingredient | % | Role |
|---|---|---|
| Nepheline Syenite | 40% | Glass-former + Flux |
| Gillespie Borate | 18% | Low-temperature glass-former |
| Kaolin (EP Kaolin) | 17% | Stabiliser — introduces alumina |
| Silica | 13% | Glass-former |
| Whiting | 12% | Flux |
| Total | 100% |
How to read each line
Every line in a glaze recipe tells you three things: what to weigh, how much relative to everything else, and what job it does in the melt.
The percentages are ratios, not fixed gram amounts. A recipe that says 40% Nepheline Syenite doesn't mean 40 grams — it means Nepheline Syenite makes up 40 parts out of every 100 parts of the base. Whether you mix 100g or 5kg, the ratios stay the same.
This is actually really freeing once it clicks. To make a 500g test batch of this recipe, just multiply each percentage by 5:
| Ingredient | % | Grams (500g batch) |
|---|---|---|
| Nepheline Syenite | 40% | 200g |
| Gillespie Borate | 18% | 90g |
| Kaolin | 17% | 85g |
| Silica | 13% | 65g |
| Whiting | 12% | 60g |
| Total | 100% | 500g |
The mechanics of this conversion — how percentages become gram weights, and how to handle colorant additions on top — are covered in detail in Reading a Glaze Recipe.
What the ingredient balance tells you
Here's something potters love about working from recipes: before you even mix anything, the numbers already tell you how a glaze will behave. Look at the proportions:
- Low-temperature glass-former + fluxes (Gillespie Borate + Whiting + Nepheline Syenite = ~70%) — Gillespie Borate brings the melt temperature down by forming boron glass at relatively low temperatures, while Whiting and Nepheline Syenite act as true fluxes. Together they give this glaze a generous melting drive — it will flow and fuse readily, producing a smooth, fluid surface.
- Moderate alumina (Kaolin at 17%) — enough to hold the melt in place on a vertical surface, but not so much that it turns matte. This is the satin zone.
- Glass-formers split between Nepheline Syenite and Silica — Nepheline Syenite acts as both flux and glass-former, while the added Silica extends the glass network and improves durability.
If you saw a recipe with 30% Kaolin instead of 17%, you'd predict: stiffer melt, more matte surface, very stable on vertical walls. The numbers let you read the character of a glaze before it ever touches a kiln — which is a really satisfying skill to develop.
Adding Colorants — The Fun Part
The base recipe above produces a clear, translucent glaze. On its own it's not very exciting — but it's a perfect platform for colour.
Colorants are added on top of the 100% base. They're not part of the base calculation. A recipe that says "add 2% cobalt carbonate" means: for every 100g of dry base materials, add 2g of cobalt carbonate.
Think of the base glaze as a window. The colorant is the tint you apply to the glass. The clearer and more transparent the base, the more faithfully it transmits the colorant's true colour.
Common colorants and what to expect
| Colorant | Typical % | Colour (electric kiln) | Character |
|---|---|---|---|
| Iron oxide | 1–2% | Pale amber, honey | Warm, subtle. Beautiful over textured clay. |
| Iron oxide | 4–8% | Rich brown, tenmoku black | Dark and opaque at high amounts. |
| Cobalt carbonate | 0.25–0.5% | Pale sky blue | Very powerful — a tiny amount goes far. |
| Cobalt carbonate | 1–2% | Deep, saturated blue | Classic pottery blue at this range. |
| Copper carbonate | 1–3% | Soft to vivid green | Shifts dramatically in reduction firing. |
| Rutile | 3–8% | Cream, buff, broken texture | Creates mottled, flowing movement in the surface. |
| Manganese dioxide | 2–5% | Purple-brown, mauve | Soft and organic. Handle as fine dust with care. |
Amount controls intensity — not just hue
This is one of the most important things to understand about colorants: the percentage controls how saturated and deep the colour becomes, not just whether colour is present.
At 0.25% cobalt carbonate, you'll get a wash of pale blue — the kind of thing you might find on celadon-inspired work. At 2%, the same colorant produces a deep, inky blue. Double the cobalt again and you start losing nuance — the colour becomes almost black.
Iron oxide works the same way. At 1–2% it gives a golden amber. At 8–10% the surface turns dark brown to black. Potters use this range deliberately — a recipe like a tenmoku glaze intentionally pushes iron to its maximum. Once you get a feel for this, you'll start reading colorant percentages almost intuitively.
The base glaze changes everything
Here's something that surprises a lot of beginners: the same colorant looks completely different depending on the base it's added to.
- Transparent base (like this Translucent Satin) — the colorant reads clearly and cleanly. Cobalt gives you true blue. Iron gives you true amber. The clay body can even show through.
- Opaque base (one with added Zircopax or Tin Oxide) — the colorant sits on a white background. Colours look lighter, more pastel. Blue becomes powder blue. The effect is more like painting on white paper than tinting glass.
- Matte base — colours often look more chalky and saturated at once. The dry surface catches light differently and can make some colours look almost velvety.
Opacifiers — not colour, but equally important
Alongside colorants, you'll often see opacifiers in recipes. These aren't colours — they're white minerals that block the transparency of the base:
- Zircopax (zirconium silicate): 8–12%. The most common opacifier. Produces a bright, cool white.
- Tin oxide: 5–10%. A softer, slightly warmer white. The classic opacifier for majolica earthenware.
Opacifiers are frequently combined with colorants. Add 10% Zircopax plus 0.5% cobalt to a clear base and you get a soft, opaque periwinkle. Add it with iron and you get a warm buff or terracotta. This is the foundation of most commercial pottery glazes.
Layering and mixing
Two more things beginners often discover by happy accident:
Mixing two colorants produces a blend, just like paint — but the results aren't always predictable. Cobalt and iron together can produce grey-green or brown-grey depending on amounts. Cobalt and copper can produce a rich teal. Experimentation is genuinely part of the craft here.
Layering two different glazes (applying one over another) creates effects neither glaze produces alone. The outer glaze melts into the inner during firing, and where they meet, unexpected colours and textures emerge. This is a whole technique in itself — and one of the reasons experienced potters keep notebooks full of test results. It's worth starting your own.
Matte, Satin, Glossy — What's the Difference?
The surface texture of a fired glaze comes down to the balance between its glass-formers, stabilisers, and fluxes — specifically the ratio of silica to alumina in the melt.
- Glossy glazes have a high silica-to-alumina ratio (roughly 8:1 or above). The silica-rich melt flows freely and cools to a smooth, reflective surface. Most durable and easiest to clean.
- Matte glazes have a lower ratio (roughly 5:1 or below). The alumina-rich melt resists flowing, and as the glaze cools it can partially devitrify — forming microscopic crystals that scatter light rather than reflect it. That's what creates the dry, velvety feel.
- Satin glazes sit in the middle (roughly 6–8:1). Enough silica to melt well, enough alumina to soften the sheen. A favourite for functional ware because it's forgiving to the touch and hides fingerprints.
The primary lever is alumina content — more alumina (introduced through kaolin or other clay minerals) pushes the surface toward matte. Less alumina, or more flux, tips it toward glossy. It's one of the most satisfying dials to experiment with once you understand what's happening.
Water: The Hidden Variable in Every Glaze
Every recipe you'll ever use lists dry ingredients only. But in practice, glaze is always mixed with water before application — and how much water you add matters more than most beginners expect.
What water actually does
Water is purely a carrier. Its job is to hold the dry minerals in suspension while you apply them to the pot, then evaporate completely during drying and early firing, leaving behind only the mineral coating. Because water plays no role in the fired glaze, you might assume the amount you add doesn't matter much. It does.
- Too much water: thin application, poor coverage, glaze that slides off vertical surfaces before it can dry.
- Too little water: a paste-thick suspension that applies unevenly, goes on too heavy, and may flake off the bisque before it even reaches the kiln.
Getting the water content right before you start glazing saves a lot of frustration.
Specific gravity: the consistent way to measure
Experienced potters don't estimate water content by eye. They measure it using specific gravity (SG) — the density of the glaze relative to pure water.
Pure water has an SG of 1.00. A typical dipping glaze sits between 1.40 and 1.50, meaning it's 40–50% denser than water.
Measuring SG is simple without any special equipment:
- Fill a 100ml graduated cylinder with your mixed glaze.
- Weigh it on a digital scale.
- Subtract the weight of the empty cylinder.
- Divide by 100. The result is your SG.
If 100ml of glaze weighs 144g, your SG is 1.44. If it weighs 152g, you're at 1.52 — probably time to add a little water.
Standard specific gravity ranges by application method
Different ways of applying glaze need different consistencies. These are the ranges most formulators use as a starting point:
| Application method | Typical SG | Notes |
|---|---|---|
| Dipping | 1.40–1.50 | The most common starting point. 1.44–1.46 is a reliable sweet spot for most bisqueware dipping glazes. |
| Pouring | 1.38–1.45 | Slightly thinner so the glaze flows and drains cleanly without pooling. |
| Spraying | 1.35–1.42 | Thinner still, to pass through a spray gun without clogging the nozzle. |
| Brushing | 1.50–1.60 | Thicker consistency helps build up coverage coat by coat without running. |
These are guides, not hard rules. A glaze heavy in barium carbonate or zinc oxide is denser by nature and may need a higher SG to apply at the same thickness. A lighter-formula glaze (mostly feldspar and silica) often performs well at the low end of the range. The target coat thickness on bisque — once dry — is roughly 1.5–2.5mm, which gives you a practical way to sanity-check your SG.
Bisqueware absorbs, raw clay doesn't
How the clay responds to the glaze suspension also affects your target SG. Bisqueware is porous. When you dip a bisque pot, it acts like a sponge — drawing water out of the suspension and pulling the dry particles against the surface. The coat builds quickly, and the pot itself tells you when it's absorbed enough (the surface stops looking wet). This makes bisque glazing relatively forgiving.
Raw (unfired) clay is much less absorbent. The glaze sits in its wet state on the surface for longer, which means runs and drips are harder to control. Raw glazing generally benefits from a slightly higher SG and more careful application.
Glaze settles and changes over time
A mixed glaze bucket is not a set-it-and-forget-it situation. Heavy minerals settle toward the bottom — sometimes forming a dense cake that takes real effort to break up. Water also evaporates from an uncovered bucket, gradually raising the SG.
Get into the habit of checking SG at the start of every glazing session. If a glaze was perfect at 1.44 two weeks ago and now reads 1.50, add a small amount of water, stir thoroughly, and check again. Keep a note of your target SG for each glaze — it's one of the most useful things you can track alongside your test records.
Some potters add a small amount of deflocculant (typically 0.1–0.2% Darvan 7, or a few drops of sodium silicate) to help keep glaze in suspension between uses. A deflocculant reduces settling without altering the water content or SG, so it's worth experimenting with for glazes that settle hard.
What Is "Cone" All About?
You'll see glazes labelled cone 6, cone 10, or cone 06 (note the zero — it matters!). Cone is a system for measuring heat work inside a kiln.
- Cone 10 is high-fire (~1300°C). Very durable results.
- Cone 6 is mid-fire (~1220°C). The most popular range for hobby potters with electric kilns.
- Cone 06 is low-fire (~1000°C). Often used with commercial underglazes and bright commercial glazes.
Here's a visual overview of the full range, with the cones you're most likely to encounter:
Cone Temp (°C) Range Common Use Cone 022 585 Low Fire Very low-fire enamels and lusters Cone 06 999 Low Fire Earthenware, commercial glazes, underglazes Cone 04 1063 Low Fire Terra cotta, low-fire stoneware Cone 6 1222 Mid Fire Most popular for studio and home electric kilns Cone 8 1263 High Fire Transition between mid and high fire Cone 10 1285 High Fire Gas, wood, and salt kilns, reduction firing
A glaze is formulated for a specific cone range. Using a cone 06 glaze at cone 6 would likely burn it out entirely. Using a cone 10 glaze at cone 6 means it never fully melts. Always match your glaze to your firing temperature — it makes a bigger difference than almost anything else.
The Big Picture
Here's the whole journey in one place:
- You mix powdered minerals with water → liquid glaze
- You apply it to bisque-fired clay → chalky, dull coating
- The kiln heats up → minerals melt together
- The kiln cools → glass layer fused to clay surface
- You open the kiln → finished, transformed pot
That's it. Everything else — recipes, chemistry, formulation — is just dialling in exactly what happens in steps 3 and 4.
Start by using a well-tested recipe (like the ones in our Glaze Library), mix it correctly, apply it evenly, and fire to the right temperature. The results will teach you more than any book can. Enjoy the process!
Want to go deeper? Read Reading a Glaze Recipe to understand exactly how to convert those percentages into weights for your batch.