Glazing · Crystalline glazes

The most spectacular glaze in ceramics, and the one most likely to weld a pot to a shelf.

A crystalline glaze is designed to run so that crystals can move, and a firing schedule that drops and holds on the way down is what grows them. The schedule is doing almost all of the work, and it needs a programmable kiln. Ours fires one schedule at cone 5½, so this page teaches it for a kiln that can.

Four steps with a tick list at each: the chemistry, the pedestal-and-catcher assembly, the schedule, and taking it apart. Six things that go wrong, including wanting to fire it here, and five videos from potters who fire it.

This is the short version: why it runs, the assembly, the schedule, the foot, each with a tick list. The chemistry, the variables and the questions are in the comprehensive version.

HoursOf hold on the way down. That is where the crystals grow
Not hereOur kilns run one schedule. This is for a kiln that holds
x5Videos, from potters who have nothing to do with us
x4Steps, each with a tick list you can work off
Jump to a section
Why this page has words and video The whole idea in three lines The technique in six moves What you need, starting with the kiln 1. Why it runs 2. Pedestal and catcher 3. The schedule 4. Taking it apart Five potters, five kilns that hold The one video to watch When it goes wrong How it works at The Clay Hole Questions people actually ask Related lessons
Illustration of a smiling bearded potter in glasses and an apron holding up a deep blue vase covered in pale blue starburst crystals, in a brick workshop with shelves of dark pots and a small kiln behind him

The chemistry, the variables, the studio position and five videos.

Read it, then watch it

Why this is written down and filmed, both.

Crystalline is a firing schedule with a glaze attached, and a schedule is a table of temperatures and holds, which is what a page holds. What a page cannot show is a pot glued to a pedestal in a catcher, or the schedule typed into a controller by someone who has done it a hundred times, or what a crystal looks like when it grew right. That is the videos, from potters who fire this in kilns that can. Ours cannot.

Use both, in this order

Read what the glaze is and why it runs. Read the schedule. Watch the pedestal being glued and the schedule being programmed. Then find a kiln that holds, because ours does not.

The whole idea

A glaze designed to run, and a schedule that grows crystals on the way down.

High zinc, high silica, low alumina: a glaze that flows like water at the top and grows zinc silicate crystals if the kiln drops to a window near 1,050 to 1,100 degrees Celsius and holds there for hours. Every pot stands on a pedestal in a catcher and gets its foot ground. It needs a programmable kiln; ours runs one schedule at cone 5½, so this is taught here and fired elsewhere.

The five-minute version

The whole technique in six moves.

If you have a kiln that holds and you just want the shape of it, this is the shape of it. Everything below is these six moves, slowed down and with the places they fail marked.

1Know why it runsLow alumina lets the crystals move and lets the glaze run. Every crystalline pot runs. Plan for it.
2Pedestal and catcher, gluedA bisqued ring of the pot’s clay under the foot, a catcher under that, white glue between, a wet alumina wash on the catcher.
3Glaze thick, on porcelainThicker than feels right, on a smooth white vitrified body. Thin crystalline is glossy nothing.
4Fast climb to the topCone 9 or 10 for most recipes, some mid-range. As fast as the kiln will go.
5Drop, then hold for hoursQuickly down to the crystal window, then a hold of several hours, sometimes stepped. The hold is the crystals.
6Tap off the pedestal, grind the footWet, never dry, with safety glasses and a half face respirator with P100 filters. Twenty minutes per pot. Then a vase nobody can stop looking at.

Before anything

What you need, starting with the kiln.

Six things, and the first one is the whole question. Without a kiln that holds on the way down, the other five are a shelf of runny vases.

A kiln with a programmable controllerThe one thing on the list that cannot be improvised. Our staff run every glaze load on one schedule, so we do not fire it here; a home kiln with a controller can hold on the way down.
A crystalline recipe, or a bagged oneFrom Glazy or a crystalline potter, or one of the bagged crystalline glazes sold for the purpose. Not a crystal-effect jar. Mixing dry powder is where the dust is: a fitted respirator with P100 cartridges, a damp towel under the scale, and a wet wipe down afterward. Our mixing lesson has the rules.
PorcelainOr a white stoneware. The crystals want a white, smooth, vitrified ground under them.
Pedestals and catchersBisqued rings and saucers of the pot’s clay, white glue, alumina hydrate. Our catcher lesson is the catcher half.
A diamond pad for the footWet, never dry, with safety glasses and a half face respirator with P100 filters. A power grinder is outdoors only, with a face shield.
Patience and a notebookThe schedule is the recipe. Every firing gets written down against what it grew.

A kiln with a programmable controller, a crystalline recipe, porcelain, pedestals and catchers, a diamond pad, and a notebook.

1

Step one

Understand the glaze: designed to run, so that crystals can move.

Crystals grow in a melt fluid enough to let them. A normal glaze has enough alumina to stay on a wall. A crystalline glaze has almost none and is loaded with zinc and silica, so it flows like water at the top and, held at the right temperature on the way down, grows crystals like frost on a window. The chemistry that grows the crystals guarantees the run.

That is why every crystalline pot stands on a pedestal in a catcher, why the foot gets ground afterwards, and why the whole discipline belongs to potters with their own kilns and a shelf of ruined test pots. Robert Weiss’s seven minutes below is the science of growing crystals, and the Radish video is the overview.

It is also why this page is educational. Our kilns fire cone 5½ on a fixed schedule every load, with no drop and no hold on the way down, loaded by our staff for everyone. A crystalline glaze in that firing would run onto a shelf and grow nothing. The crystal-effect glazes sold in brush-on jars are a different animal, chunks of frit that melt into colored spots, and those fire on our schedule; grown crystals do not.

Tick list, the idea0 of 3
2

Step two

Prepare the pot: porcelain, thick glaze, a pedestal glued into a catcher.

The pot is an assembly. A pedestal, a bisqued ring of the pot’s clay a little narrower than the foot, glued to the foot. A catcher, a bisqued saucer of the same clay wider than the run, glued to the pedestal and brushed with a wet wash of alumina hydrate and water. The glaze runs off the foot, down the pedestal, into the catcher, so it does not reach the shelf.

Porcelain or a white stoneware, because the crystals show against white and fit porcelain better than most stoneware. Glaze thick, thicker than feels right, brushed or dipped or poured, heavier toward the top of a tall form because it will move down. That is for your own crystalline glaze in your own kiln; studio glazes stay at our normal coat. A bare band waxed at the foot so the run starts from a clean line. Adam MacMillan’s twenty-three minutes below is the whole preparation, from catcher methods to joining the pedestal.

Some potters fire without catchers by controlling thickness and form so precisely that the glaze stops at the foot; Andy Boswell’s video is about firing without them, and it is worth watching for the same reason it is worth watching someone walk a tightrope. Our glaze catcher lesson is the catcher half of the assembly.

Tick list, the pot0 of 3
3

Step three

The schedule: climb fast, drop, hold for hours.

This is the technique. A fast climb to the top, cone 9 or 10 for most recipes, held briefly to melt the glaze fully. Then a fast drop to the crystal window, around 1,050 to 1,100 degrees Celsius depending on the recipe. Then a hold there for hours: three, four, six. The hold is the crystals. Some schedules step between holds to grow rings inside them.

Every variable in the schedule changes the pot. Hold higher in the window and the crystals are fewer and larger; hold lower and they are more and smaller. Hold longer and they grow; hold too long and the glass around them goes dull. Seed the surface and there are many; leave it clean and there is one. MeditativePottery’s video below puts real schedules on screen, and the advice from every crystalline potter is the same: fire the same recipe on the same clay with a different schedule ten times, and write every one down.

None of this is possible without a controller that holds on the way down. That is the whole reason this is a home-kiln technique and not a shared-studio one, and it is not a rule we made; it is what the crystals need.

Seeding

A crystal needs somewhere to start. A perfectly smooth, clean, thin glaze offers almost nowhere, and grows one or two huge crystals; a glaze with a pinch of a seeding material in the recipe, or a slightly rough surface, or a short hold at the top of the window before the long hold below it, offers hundreds, and grows a field of small ones. How many crystals you get is decided before the kiln is even lit, and it is the second most powerful variable after the hold itself.

Tick list, the schedule0 of 4
4

Step four

Take it apart, grind the foot, and read what it grew.

The assembly comes apart with a few light taps. Cold kiln, pot lifted out on its catcher, safety glasses on, light sideways taps at the join between foot and pedestal, and the pedestal comes away with the run. If it does not let go, or the pot flexes, stop tapping and grind the join away wet. What is left on the foot gets ground flat: a diamond pad, wet, never dry, with safety glasses and a half face respirator with P100 filters, twenty minutes a pot.

Then read it. No crystals means the schedule missed the window or the glaze was thin. One giant crystal means too few seeds. Dull crystals mean too long a hold or too slow a cool. A welded catcher means the pedestal was short or the glaze was thick at the foot. Each is in the diagnostic below, and each changes one line of the next schedule.

A crystalline pot that grew right is the most spectacular surface in ceramics and the reason people put up with the rest of this page. It is almost always a vase or a bottle, because a crystalline glaze is for looking at, never for food or drink, and because the form that shows a crystal best is a tall one with nowhere for the eye to stop.

Tick list, after0 of 3

Watch it done

Now watch five people grow crystals.

Watch this one, then go find a kiln that holds.

None of these potters work here. Between them they cover the basics, the chemistry, the schedule with real numbers on screen, the pedestal and catcher assembly, and a potter who fires without catchers at all. Nothing but the still frame loads until you press play.

Fourteen minutes on what a crystalline glaze is, how to make one and how to fire it, ending on the recipe and schedule. The other four videos are in the comprehensive version.

The basics in fourteen minutes Ceramics 106: The basics on Crystalline Glaze Radish (Salmon)
An overview of what a crystalline glaze is, how to make one and how to fire it, ending on the recipe and schedule. Watch on YouTube
The chemistry, briefly Scientific Process of Crystalline Glazing with Robert Weiss Robert Weiss
Seven minutes from Robert Weiss, a ceramics mentor, on the science of growing crystals: heat work, kiln performance, supervising the firing, creating the crystal patterns, and safety. Watch on YouTube
The firing schedule Crystalline glaze journey 5: What are the firing schedules MeditativePottery
Ten minutes of firing schedules for crystalline glaze, with the controller settings and hold times shown as text on screen. Watch on YouTube
Pedestal and catcher Crystalline Pottery - How to prepare a piece for glazing by Adam MacMillan Adam MacMillan
Twenty-three minutes on getting a pot ready for crystalline glaze: catcher methods, making and grinding the separate rings, joining the pedestal, and setting it up to separate cleanly after firing. Watch on YouTube
No catchers Crystalline Glazing - No Catchers! WHAT?! Andy Boswell
Fourteen minutes from episode four of Andy Boswell’s Life of a Potter series: glazing crystalline pots and going without catchers, with tips along the way. Watch on YouTube

When it goes wrong

Six ways it goes wrong, and what to do about each.

The three failures that happen on the first firing.

Pick the one that looks like your pot. The first three are the schedule, the run and the seeding. The last three are the gloss, the foot and the kiln you wanted to use.

Pick the one that looks like your pot. The other three, the gloss, the foot and the kiln you wanted to use, are in the comprehensive version.

How it works here

At The Clay Hole, crystalline is taught and not fired.

This page is written for potters everywhere, and the firing it describes needs a kiln we do not run. Here is where the studio stands.

Three things to know here

Our kilns fire cone 5½ on one fixed schedule, loaded by our staff, with no drop-and-hold, so we do not fire crystalline glazes and this page is for a kiln that can. Crystal-effect brush-on glazes from a jar are a different thing and fire fine here. The catcher half of the technique is in the glaze catcher lesson.

Why not here

The schedule is the crystals, and our kilns run one schedule for everyone: cone 5½, the same every load, loaded by our staff. A drop-and-hold firing is a different firing for a whole kiln, and a crystalline glaze in a normal one runs onto a shelf and grows nothing. It is not a rule; it is chemistry.

What does fire here

Crystal-effect brush-on glazes, the kind sold in jars with chunks in them, are frit fragments that melt into colored spots on our normal schedule. They are not grown crystals, and they are lovely, and they need a catcher if the jar says they move.

Where it does happen

A kiln with a programmable controller at home, or a studio that runs crystalline firings on purpose. The pots can be thrown and bisqued here and go home for the glaze firing; bisque is cone 05 and free. Our porcelain is a cone 6 body, so it suits a mid range crystalline recipe only: a cone 9 or 10 recipe needs a high fire porcelain from somewhere else.

The half that transfers

A catcher under the pot, an alumina wash on it if you have one, and a wet diamond pad for the foot: what this page says about catching a glaze that runs holds for our flux glazes too. Our glaze catcher lesson is that half, and that half applies here.

Questions people actually ask

The ones that come up most.

What is a crystalline glaze?

A glaze so high in zinc and silica, and so low in alumina, that when it is held at the right temperature on the way down from the top of the firing, zinc silicate crystals grow in it, visible ones, sometimes inches across, like frost on a window. The same low alumina that lets the crystals move lets the whole glaze run, which is why every crystalline pot stands on a pedestal in a catcher.

Why is the firing schedule doing all the work?

Because crystals grow in a temperature window on the way down, and only if the kiln sits in that window long enough. A normal firing climbs to the top and cools straight through the window without stopping in it; a crystalline firing climbs fast, drops quickly to the window, and holds there for hours, sometimes stepping between two or three holds. Same glaze, different schedule, completely different pot.

What temperature does it fire to?

Most crystalline glazes are fired hot, cone 9 or 10, and then held around 1,050 to 1,100 degrees Celsius on the way down. There are mid-range crystalline recipes that top out only a little above our firing, and they need the same drop-and-hold. Our kilns fire cone 5½ on a fixed schedule with no hold on the way down, so this is not something we fire here.

Can I do this at The Clay Hole?

No. Our kilns run one schedule, cone 5½, loaded by our staff, and a crystalline glaze in that firing would run onto a shelf and grow nothing. This page teaches the technique for a kiln with a programmable controller, at home or at a studio that runs crystalline firings on purpose. The crystal-effect brush-on glazes sold in jars are a different thing and fire fine here; those are chunks of frit that melt into spots, not grown crystals.

What is a pedestal?

A small ring or cylinder of the pot’s clay, bisqued, that the pot stands on inside a catcher. The glaze runs off the foot, down the pedestal, and into the catcher, and afterwards the pedestal is tapped off the foot and the foot is ground flat. It is a glaze catcher with a stalk, and our catcher lesson covers the catcher half.

Which clay?

Porcelain, mostly. Crystalline glazes show best on a white, smooth, vitrified body, and they fit porcelain better than most stoneware, which they tend to craze on. A white stoneware works. Our clay bodies include a cone 6 porcelain and lighter stonewares, but the kiln is the problem, not the clay.

Is it food safe?

No. Treat a crystalline glaze as decorative only, never for anything that touches food or drink. The Food Safe Lookup on theclayhole.com covers the glazes we carry. Crystalline glazes are unbalanced by design, often craze, and many recipes carry colorants at levels nobody would put on a plate. They are for vases, bottles and things you look at, which is what almost every crystalline potter makes.

Related lessons

Six lessons that sit next to this one.

Three to read next.

Crystalline is a catcher, a schedule and a recipe. These are the pages for each, and another firing that happens in someone else’s kiln.

GlazingMaking a Glaze CatcherThe catcher half: a dish under the pot, same clay, wider than the foot. Firing and fixingWhat a Firing Actually DoesWhat a firing does, and why the cooling curve decides the surface. GlazingMixing Your Own GlazesWhat mixing involves, and the dust rules that come with it. Firing and fixingFixing Fused and Sharp PotteryThe foot, afterwards. GlazingRunning Hot ChowderA flux glaze that fires here. Every pot under one goes on a catcher. Other ways to find, fire and formSalt and Soda FiringAnother firing that happens in someone else’s kiln.

That is the whole technique

Designed to run. Pedestal in a catcher. Climb fast, drop, hold for hours. Grind the foot.

Your first crystalline firing will grow nothing because the schedule missed the window, and your second will grow one crystal the size of a saucer because you did not seed it. That is normal, and the tenth one is why people build a kiln in the garage. Throw the vase here, bisque it here for free, and fire it somewhere that holds.

If you want the chemistry, the schedule variables and the answers to the questions everybody asks, switch to the comprehensive version at the top. Otherwise, go find a kiln with a controller.

Back to Clay Hole University The half that fires here: the catcher

Written for potters everywhere. Where our studio does things differently, the page says so.