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 every week.
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.
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 every week in kilns that can. Ours cannot, and this page is honest about that from the first line.
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.
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 controller, a crystalline recipe, porcelain, pedestals and catchers, a diamond pad, and a notebook.
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 chemistry; the Radish video is the overview; and the first thing both say is that the firing schedule is doing almost all of the work.
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 coloured spots, and those fire on our schedule; grown crystals do not.
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 dusted with alumina. The glaze runs off the foot, down the pedestal, into the catcher, and never touches 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. 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, including the moment the assembly comes apart afterwards.
Some potters fire without catchers by controlling thickness and form so precisely that the glaze stops at the foot; Andy Boswell’s video is that argument, 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.
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 honest 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.
Step four
Take it apart, grind the foot, and read what it grew.
The assembly comes apart with a tap. Cold kiln, pot lifted out on its catcher, a sharp tap at the join between foot and pedestal, and the pedestal comes away with the run. What is left on the foot gets ground flat: a diamond pad or a grinder with a diamond wheel, wet, masked, eye protection, 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 the glaze is not treated as food safe and because the form that shows a crystal best is a tall one with nowhere for the eye to stop.
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 or know we exist. 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, why it runs, and why the schedule grows the crystals. The other four videos are in the comprehensive version.
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 and the run. The last three are the foot, the gloss, and the kiln you wanted to use.
It fired glossy with no crystals at all
The schedule. Crystals grow during a hold on the way down, and a firing that cools normally passes through the window too fast to grow anything. Or the glaze was thin, or too high in alumina.
A programmable controller with a hold at crystal-growing temperature, hours long, on the way down. Thicker glaze. A recipe that is genuinely crystalline, not a crystal-effect glaze from a jar.
Refire with the schedule. Crystalline glazes can be refired, and the second firing often grows what the first did not.
It welded itself to the shelf, or to the catcher
Crystalline glazes are low in alumina by design, which means designed to run. Every crystalline pot runs, and one without a pedestal and catcher runs onto the shelf.
A pedestal under the foot, a catcher under the pedestal, both in the pot’s clay, glued together with white glue, alumina on the catcher. The assembly comes apart afterwards with a tap and a grinder.
If it is on a catcher, tap the pedestal off at the join and grind the foot flat, wet. If it is on a shelf, the shelf is the casualty and the lesson is in the next panel.
One huge crystal, or a few, instead of many
Too few seeds. Crystals grow from nuclei, and with only a few each crystal has the whole surface to itself. A clean, unseeded glaze with a long hold grows one or two monsters.
Seeding: a pinch of a seeding material in the recipe, or a rougher surface, or a shorter hold at the higher end of the crystal window before the long hold. Many small seeds, many small crystals.
Some potters aim for exactly this. If you did not, refire with a seeded surface.
The crystals are there but dull, matte or crazed
Held too long at the low end of the window, or cooled too slowly through it, so the glass around the crystals devitrified. Or the glaze is crazing, which crystalline glazes on stoneware often do.
A shorter hold, or a hold higher in the window. Porcelain, which fits crystalline glazes better than most stoneware.
A quick refire to the top temperature and a faster cool can re-gloss the background without dissolving the crystals. Sometimes.
The foot is a sharp ring of glass
The run reached the foot and the pedestal did its job. What is left after the pedestal is knocked off is a ring of glaze that has to be ground.
A taller pedestal, a bare band waxed above the foot, and a foot ring designed to be ground flat. Crystalline potters trim their feet knowing what is coming.
A diamond pad or a bench grinder with a diamond wheel, wet, with a mask and eye protection. Twenty minutes per pot is normal.
You wanted to fire it here
Our kilns fire cone 5½ on the same schedule every load, with no drop-and-hold, and the schedule is the crystals. A crystalline glaze in one of our firings would run onto a shelf and grow nothing.
A kiln with a programmable controller, at home or at a studio that runs crystalline schedules on purpose. This page teaches it for that kiln.
Fire a crystal-effect brush-on glaze from a jar instead, which fires on our schedule and gives spots rather than snowflakes.
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 coloured 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, on our porcelain, and go home for the glaze firing; bisque is cone 05 and free.
The half that transfers
Pedestals, catchers, alumina, glue and a diamond pad for the foot: everything on this page about managing a glaze that runs is true of our flux glazes too. Our glaze catcher lesson is that half, and it fires here every week.
Questions people actually ask
The ones that come up every week.
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 in an hour; 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, and the page says so plainly. 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 eight bodies include porcelain and light stonewares, but the kiln is the problem, not the clay.
Is it food safe?
Usually not treated as such. Crystalline glazes are unbalanced by design, often craze, and many recipes carry colourants 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 the other firing we teach and do not run.
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.
Written for potters everywhere. Where our studio does things differently, the page says so.
Which version would you like to print?
Both print clean, with the videos turned into labelled boxes you can search for later.