Glazing · Test tiles

A test tile is a question you get to ask the kiln for four dollars and eighty cents.

Every glaze on our wall is already fired onto a numbered tile, and for choosing a glaze off the shelf you never need to make one. But the moment you want to know what two glazes do on top of each other, what three coats look like against two, what a glaze does over a carved texture or over your underglaze, or what a glaze you mixed at home does at cone 5½, the studio’s tiles stop answering. Your own tile is the only way to ask. This page is how to make one that gives an honest answer, and how to keep the answers so you never ask the same question twice.

This page covers when the studio tiles are enough and when they are not, the tile shapes that actually work, a batch of thirty in an hour, numbering that survives the kiln, glazing in a grid, the log that turns a pile of tiles into a library, and the one-variable rule for dialing a result in. Six fixes and six videos.

This is the short version: when you need a tile, how to make a batch, how to number and glaze it, and the log. The tile shapes, the dialing-in method, the questions and five of the six videos are in the comprehensive version.

30Tiles from one slab, in about an hour, which is a season of questions
1Thing changed per tile. Two changes and you have learned nothing
x6Videos, from potters who do not work here
x4Steps, each with a tick list to work off
Jump to a section
Why this page has words and video The whole idea in four lines When the studio tiles are enough, and when they are not Tile shapes that give honest answers 1. Make a batch 2. Number them before the bisque 3. Glaze them in a grid 4. The log 5. Dial it in Six videos on test tiles The one video to watch When it goes wrong How it works at The Clay Hole Questions people actually ask Related lessons
An illustration of a potter with teal and copper hair at a wooden bench, brushing glaze onto a small test tile, with rows of colorful tiles in front of her and a bearded man at a pottery wheel behind her

When, the shapes, the batch, the numbers, the grid, the log, dialing in, what goes wrong, six videos.

Read it, then watch it

Why this is written down and filmed, both.

Test tiles are two skills. Making them is quick and physical and films perfectly: a slab, a cutter, a stamp, a bend, and there they are on the bat in a row. Using them is slow and mental and does not film at all: a numbering system, a log sheet, the discipline of changing one thing per tile, the habit of looking at last year’s results before firing this year’s. This page carries the second skill in full, because nobody films a notebook, and points you at six videos for the first.

Read the system first, so you know what a tile is for before you make one. Then watch the batch get made. The words give you the questions to ask and the videos give you the hands, and people who do both remember the whole practice rather than half of it. Then roll a slab with the tick list open.

Use both, in this order

The when band and the log band first. Then the first video, which shows tiles made from thrown rings in eighteen minutes. Then the slab roller.

The whole idea

Make thirty tiles that stand up. Number them before the bisque. Change one thing per tile. Log every one.

The studio’s numbered tiles answer what a glaze looks like alone. Your own tiles answer everything else: combinations, coats, texture, underglaze, your clay body, a glaze mixed at home. Cut a slab into tiles with a foot so they stand, a texture on one half, and a hole to hang them by. Scratch a number into each before the bisque. Glaze them in a grid where every tile changes exactly one thing from its neighbor. Write each one into a log with the glaze, the coats, the clay and the firing, and photograph the sheet. Then repeat what worked and never guess again.

Before you make one

When the studio tiles are enough, and when only your own will do.

Every brush-on glaze and underglaze here is fired onto a numbered tile in the Glaze Bible and on the wall, and the Mega Glaze Tool on your phone gives you a color shortlist, with the tile number of each glaze to look up in the Glaze Bible. For choosing a glaze, that is the whole job done for you. Here is where it stops.

For choosing a glaze off the wall, the studio’s tiles are enough. For any of these, make your own.

Two glazes on top of each other

The wall shows each glaze alone. What a celadon does over a tenmoku, or a matte over a gloss, is a new glaze that exists nowhere but on your pot. Test it.

How many coats

Most of the wall is three coats. One coat can be a thin wash; four is likely to run. If you plan to use a glaze thin or thick, tile it at that thickness.

Over texture and carving

A glaze that pools breaks over a ridge and settles in a groove. The studio tile is flat. A tile with your texture on it tells the truth.

Over underglaze or slip

Underglaze changes a glaze’s color and sometimes its melt. A tile with a stripe of the underglaze you plan to use, and the glaze over half of it, saves a whole pot.

On a different clay

The wall tiles are on one clay. Porcelain, speckled buff and Berlin Gray all pull a glaze a different direction. Tile it on the clay the pot is made of.

A glaze mixed at home

Anything not from our shelves has to be tiled on a catcher before it goes near a pot, after a conversation with the studio team. Our mixing lesson has the rules.

  • Two glazes on top of each other.
  • A glaze at one or two coats instead of three.
  • A glaze over your texture, carving, underglaze or slip.
  • A glaze on a clay other than the one the wall tile is on.
  • Anything mixed at home, which goes on a catcher after a word with the studio team.

The shape

Three tile shapes that give honest answers, two that lie, and one thing every tile needs.

A flat square on the shelf shows you what a glaze does on a floor. Almost nothing you make is a floor.

The L-tileA slab strip scored and bent so it stands on its own foot, with the upright face showing the glaze on a wall and the foot showing where it runs. The workhorse. Thirty from one slab.
The thrown ringA cylinder thrown thin, cut into segments at leather hard. Each segment is a curved wall with a real throwing surface, the closest thing to a mug you can fit thirty of on a shelf.
The textured stand-upAn L-tile with a texture stamped or carved into the top half of the upright, so one tile shows the glaze flat, on texture, and where it pools at the foot. Three answers from one tile.
The flat square that liesLies about running, because gravity never gets a say. Fine for underglaze color swatches. Wrong for anything that melts.
The tile too small to readAnything under two inches on its face. A speckle, a break, a pool: none of it shows at that size, and you end up firing the pot to find out anyway.
The holeEvery tile gets a hole near the top. Hung on a nail or strung on a ring, thirty tiles take up no shelf space and stay in order. A tile in a drawer is a tile you will not look at.
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Step one

One slab, thirty tiles, an hour.

Strips, bent, textured, holed. Roll a slab a quarter inch thick on the slab roller (it needs the slab roller certification first), in the clay your pots are made of. Cut it into strips two inches wide and four inches long with a ruler and a knife. On each strip, score a line an inch from one end and bend it to ninety degrees, so the tile stands on a one inch foot with a three inch upright. Press a texture into the top half of the upright with a stamp, a rib edge or a piece of lace, leave the bottom half smooth, and cut a hole near the top with a straw. Stand them up on a bat, and you have thirty questions ready to ask.

Strips, bent, textured, holed. A quarter inch slab in your own clay, cut into two by four inch strips. Score an inch from one end and bend it up so the tile stands on a foot. Texture the top half of the upright, leave the bottom smooth, cut a hole near the top with a straw. Thirty from one slab, standing on a bat.

Bend the foot toward the textured side, so when the tile stands, the upright faces you with the texture at the top and the foot in front of it where a run will land. Compress the bend with a finger on both sides; a cracked bend snaps off in the kiln and the tile falls face down onto the cookie, glaze and all.

A thrown ring is the other batch. Throw a cylinder five inches tall with walls as thin as your mugs, leave it slightly soft, and cut it into two inch segments with a knife at leather hard. Each has a real throwing surface and a real curve. One of the videos below makes tiles both ways in fourteen minutes.

Make tiles from every clay you use, not just the one you are testing today. A shelf of tiles with the clay stamped on the back is what lets you answer next year’s question with this year’s firing.

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Step two

A number that is part of the clay, not painted on top of it.

Scratched, on the back, before the kiln. A tile with no number is a tile you will fire and then be unable to read, and it happens to everyone once. Scratch a number into the back of the foot with a needle tool while the clay is leather hard, deep enough to read after glazing, or press it with number stamps. Then write the same number in the log with the clay body next to it. The number is the tile’s name for the rest of its life; the glaze gets recorded against the number later, not on the tile.

Scratched, on the back, before the kiln. Scratch a number into the back of the foot at leather hard, deep enough to read after glazing, or press it with stamps, and write the number and the clay into the log. Glaze details go in the log against the number, never on the tile.

An underglaze crayon or pencil on bone dry clay, before the bisque, is the other honest method: write on the back of the foot where no glaze will go, and it fires in permanently. A marker does not survive the kiln, and a number written in glaze on the front is a number hidden under the second coat.

Use one running series and never restart it. Tile 1 through 30 this month, 31 through 60 next, so a number can only ever mean one tile. Add a letter for the clay if you use several: B for B-Mix, P for porcelain, S for speckled. B41 tells you the clay and the tile before you look at the log.

Bisque them like any greenware, which here means bone dry and free. Staff load every kiln here, so leave the tiles on the bisque firing table.

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Step three

Every tile changes exactly one thing from its neighbor.

One row, one question. Set the bisqued tiles out on the table in rows and decide what one thing each row is testing. A row for coats: tile one gets one coat of a glaze, tile two gets two, tile three gets three. A row for combinations: the same base glaze on every tile, with a different second glaze over the top half of each. A row for underglaze: a stripe of the same underglaze across each, painted on the raw tile at leather hard or bone dry and bisque fired before any glaze goes over it, with a different clear or celadon over it. Glaze the whole tile the way you glaze a pot, brush-on and coat by coat, and stop the glaze a quarter inch above the foot so a run has somewhere to go.

One row, one question. Lay the tiles out in rows, and give each row one question: coats, or a second glaze over the top half, or a glaze over an underglaze stripe that was painted on the raw tile and bisque fired first. Every tile in a row changes exactly one thing from its neighbor. Glaze them coat by coat like a pot and stop a quarter inch above the foot.

Two glazes on a tile go top and bottom, not side by side. The top glaze runs down into the bottom one exactly as it will on a pot, and the overlap is the result you wanted to see. Note which went on first; the same two glazes in the other order are a different result, and a different tile.

Keep the foot bare and dry. It is the part that touches the shelf, and a tile that fuses to a shelf is a tile somebody has to grind off. Wax it if your Level has a wax station, or ask an instructor how to keep it clean, and put every tile on a kiln cookie or glaze catcher, every time, because a run lands on the foot; our glaze catcher lesson is a short make.

Write the glaze against the number in the log as you go, not after. Thirty tiles glazed in an afternoon are thirty tiles you will not remember by evening, and the tile that looks incredible is always the one whose entry is blank.

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Step four

A tile without a log entry is a pretty rock.

Number, clay, glaze, coats, under, firing, photo, verdict. The log is a notebook, a spreadsheet, or the notes app on your phone; it does not matter which, as long as it is the only one and you can find it in a year. Every tile gets a line: the number, the clay, the glaze or glazes with their jar numbers and the order they went on, the coats, anything underneath, where and how it was fired, and a photograph once it is out. Then a column that matters more than all the others: what you thought of it, in one line, and what you would change next time. That last column is the difference between a record and a plan.

Number, clay, glaze, coats, under, firing, photo, verdict. One notebook, spreadsheet or notes file, and only one. A line per tile: number, clay, glaze and jar number, order, coats, anything underneath, the firing, a photo, and a one-line verdict with what to change next time. Photograph the fired tiles laid out on the sheet, numbers up.

Photograph tiles laid out on the log page itself, numbers visible, the day they come out of the kiln, in daylight by a window. A photograph of thirty tiles on a sheet with the numbers next to them is worth more than thirty separate photographs you will never match up, and the daylight matters because glaze color under the studio lights is not the color of the glaze.

Record the firing, not just the glaze. Our glaze firing is cone 5½, but where a tile sits in the kiln changes the result, and a test that ran on a top shelf might not run on a bottom one. If you are told where it went, write it down; if not, write the date so it can be matched to a load.

A log is worth keeping in the Mega Glaze Tool’s terms: the jar numbers it uses are the numbers on our shelves, and a tile logged with the same numbers can be found again in thirty seconds. Our Mega Glaze Tool lesson shows where the numbers live.

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Step five

Look at the tile, change one thing, make the next tile.

Change one thing. Only one. The first batch is never the answer; it is the map of where the answer is. A combination that is nearly right but a little too runny gets a second tile with one fewer coat of the top glaze. A texture that reads beautifully on the tile but disappears on the pot gets a tile with the texture carved deeper. Each round changes exactly one thing from the tile that came closest, and each round is logged against the tile it came from, so the log reads as a chain: 41 was close, 52 fixed the run, 63 is the one. Three rounds is usually enough. Ten is a hobby.

Resist the urge to fix two things at once. A tile with one fewer coat and a different underglaze tells you nothing about either, because you cannot know which change did what. It feels slower to change one variable; it is faster, because every tile teaches you something you can use.

When a tile is right, glaze a small pot exactly the way the tile was glazed, coats and order and all, before you commit a big one. A tile is a wall three inches tall; a vase is a wall twelve inches tall, and a glaze that stopped a quarter inch short on the tile can run an inch on the pot. Our glaze problems lesson is for reading what went wrong when it does.

Then the tile goes on the ring or the nail with its number facing out, and the next time you want that result, you look at the tile, read the line in the log, and do that. That is the entire payoff, and it is enormous.

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Watch it done

Now watch six videos on making and reading tiles.

Watch this one, then go roll a slab.

None of these potters work here or know we exist. Between them they cover both batches, what a test tile needs, combination testing, and results from three brands we carry. The still frames load while you read; the player and its cookies wait until you press play.

Eighteen minutes on making test tiles from thrown rings. The other five videos are in the comprehensive version.

Start here: tiles from thrown ringsHow to Make Glaze Test Tiles, A Beginner’s GuideFlorian Gadsby
Eighteen minutes on making test tiles from thrown rings: throwing, separating the rings from the bat, a light trim, cutting the individual tiles and fettling them clean. The chapter list has nothing on numbering or logging; the steps here cover that. Watch on YouTube
What a tile needs, and six ways to make oneMaking The Most Effective Pottery Test TilesOld Forge Creations
Eighteen minutes on what a test tile needs (clay body, surface and edge quality, glaze thickness, how many, what to record) and six ways to make one: handbuilding, throwing, slipcasting, press molding, extruding and slab building. Watch on YouTube
The thrown ring, and combinationsThrowing Test Tiles and Testing Glaze CombinationsJonthepotter
Twelve minutes on making test tiles on the wheel, then recording glaze combinations and a testing strategy. Watch on YouTube
Four ways to make tilesHow to Make Glaze Test Tiles | John BrittCeramic Arts Network
Seven minutes from a longer course, Understanding Glazes with John Britt. It shows four ways to make tiles: extruded and numbered, hand built, slab rolled and textured, and thrown and cut. Watch on YouTube
Both batches, one after the otherMaking Pottery Glaze Test Tiles - Wheel Thrown and HandbuiltPottery Crafters
Hand built tiles first, then wheel thrown ones, then glazing and a look at the results. Fourteen minutes. Watch on YouTube
Combinations from brands we carry99. Glaze Combo Test Tiles 01 - Stunning Blue & Green Results! (Mayco, Amaco & Spectrum)Far Forest Ceramics - Jeanette Stevens
Blue and green combinations of Mayco, Amaco and Spectrum glazes on white stoneware tiles fired to cone 6, with before and after results and the number of coats. Mayco, Amaco and Spectrum are brands we stock, but our glaze firing is cone 5½, so expect ours to differ. Watch on YouTube

When it goes wrong

Six things that go wrong, and what each one is telling you.

The three things that go wrong first.

Pick the one that looks like your shelf. Half of these are a missing number or a missing log line, which is the whole subject.

Pick the one that looks like your shelf.

How it works here

Our tiles are already fired. Yours are for the questions ours cannot answer.

This page is written for potters everywhere. Here is where our studio stands.

Four things to know here

Every glaze on the wall is already fired onto a numbered tile: the Glaze Bible by the mold room, the wall itself, and, on your phone, the Mega Glaze Tool, which gives a color shortlist and the tile number of each glaze. Make your own for combinations, coats, texture, underglaze and your own clay. Tiles bisque free and are measured for the glaze firing like any piece, which for one tile means the flat charge, and several tiles on one 6 inch kiln cookie, a quarter inch apart and 125 cubic inches or less in total, pay as one piece. Home-mixed glazes go on a catcher, after a word with the studio team.

The tiles we already have

Every brush-on glaze and underglaze in the building is fired onto a numbered tile. The Glaze Bible lives on the table by the mold room, the wall carries the tiles, and the Mega Glaze Tool on your phone gives you a color shortlist and the tile number of each glaze. For choosing a glaze off the shelf, you never need to make one.

When you make your own

Combinations, coats, texture, underglaze, a different clay, or a home-mixed glaze. Anything the wall tile cannot show you. Most potters who test make a batch now and then and never run out of questions.

What it costs

Bisque is free. The glaze firing is measured like any piece, five cents a cubic inch, and a test tile is small enough that one tile is one flat charge. Several small tiles that fit on one 6 inch kiln cookie, a quarter inch apart and 125 cubic inches or less in total, pay as one piece, and that is the only time pieces combine. Put a batch on one cookie instead of firing them one at a time.

Home-mixed glazes

Anything not from our shelves goes on a catcher, and only after a conversation with the studio team about what is in it. A glaze nobody here has seen fired can take a shelf with it, and the catcher is the price of the experiment.

Your clay

Tile the clay your pots are on. Our clays pull the same glaze different directions, and the wall tiles are on one body. A speckled buff tile under a celadon looks nothing like the wall.

Storage

Fired tiles go home, or hang on a ring on your shelf. They are yours. A tile left loose on a shared shelf reads as abandoned and goes to the lost pots.

Questions people actually ask

The ones that come up at the glaze wall.

Do I need test tiles at all here?

Not for choosing a glaze off the wall. Every glaze is already fired onto a numbered tile. You need your own the moment you want a combination, a thickness, a texture, an underglaze, a different clay, or a glaze mixed at home.

What size and shape?

A two by four inch slab strip, bent an inch from one end so it stands on a foot, with texture on the top half of the upright and a hole near the top. Flat squares lie about running; tiles under two inches are too small to read.

How do I number them so it survives?

Scratch or stamp the number into the back of the foot at leather hard, or write it there in underglaze crayon or pencil on bone dry clay. Marker burns off and a number on the front disappears under the glaze.

What goes in the log?

Number, clay, glaze and jar number, the order they went on, coats, anything underneath, the firing, a photograph, and a one-line verdict with what to change next time. One log, only one.

How many coats should the tile have?

Whatever the pot will have. If you test three coats and glaze the pot with two, you tested a different glaze. A row of tiles at one, two and three coats answers the question once for good.

Why does the pot not match the tile?

The pot is taller, thicker, curved, on another clay, or got different coats. A tile is a small pot, not the pot. Glaze a small pot exactly like the winning tile before a big one.

What does a tile cost to fire?

Bisque is free. The glaze firing is measured like any piece, and a tile is small enough that one tile is one flat charge. Several small tiles that fit on one 6 inch kiln cookie, a quarter inch apart and 125 cubic inches or less in total, pay as one piece, so put a batch on one cookie.

Can I test a glaze I mixed at home?

On a catcher, after a conversation with the studio team about what is in it. Never straight onto a shelf, and never on a pot before a tile.

That is the whole practice

Make thirty. Number them. Change one thing. Write it down.

Potters who test are not more talented than potters who do not. They have a shoebox of tiles and a notebook, and when a pot comes out of the kiln looking exactly the way they meant it to, that is why. The first batch feels like homework. The second batch feels like cheating.

If you want the tile shapes, the dialing-in method, the six fixes and the other five videos, switch to the comprehensive version at the top. Otherwise, roll a slab and cut thirty.

Back to Clay Hole University Getting the Most Out of the Mega Glaze Tool

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