Trimming, handles and drying · Covering work
One of these is a brake. The other is a wall with holes in it.
Every piece you make dries from the outside in and from the thin parts first, and the only tool you have for steering that is what you put over it. A plastic bag stops evaporation dead wherever it touches the clay and lets it run wherever it gapes. A cotton sheet slows every square inch of the pot by the same amount. They are not two versions of the same thing. They are two different tools, and most pieces want one of them, and some want both.
This page covers what each one actually does to the moisture in a pot, a decision table for the pieces you make most, the sheet method with its stacking dial, the bag method and its two mistakes, the combination for big or awkward work, and the studio’s own sheets, bags and rules. Six fixes and five videos.
This is the short version: what each one does, the decision table, and the two methods. The combination for big or awkward pieces, the reasoning, the questions and four of the five videos are in the comprehensive version.
Read it, then watch it
Why this is written down and filmed, both.
Drying is invisible. Nothing moves, nothing changes color for hours, and by the time a rim curls or a handle cracks, the decision that caused it was made three days earlier. That makes it a subject that reads better than it films: the reasoning about where water leaves a pot and how each cover changes that is a paragraph, and a paragraph is what you want on your phone at the shelf.
What films well is the handling. How loosely a bag is tented, how a sheet is draped so it touches nothing but the rim, what a piece looks like at the moment it is ready to come out from under cover. So read the table, then watch somebody wrap and unwrap, and you will remember both better than either alone. Then go cover tonight’s piece with the tick list open.
Use both, in this order
The two-tool idea and the decision table first. Then the first video. Then your shelf.
The whole idea
Plastic blocks. Cotton slows. Pick the one the piece needs.
Water leaves a pot from the surface, fastest from thin parts and edges. A plastic bag stops that completely wherever it touches and does nothing wherever it gapes, so the pot dries in patches decided by where the bag happened to land. A cotton sheet lets air through the whole cloth at one rate, so every part of the pot slows by the same amount. Use cotton when the piece has thin and thick parts that need to finish together. Use a bag when you need the piece to stop drying for days. Use both when it is big, awkward, or precious.
Cotton sheets live in the basket by the shelves for Purple Level Members and Black Level Members. Bags in three sizes are for everyone. Neither costs anything.
The two tools
What each one actually does to the water in a pot.
Two paragraphs of physics, and then you will never confuse them again.
Plastic: a barrier you have to manage
Plastic does not breathe. Wherever the bag touches the clay, evaporation from that patch stops completely and the surface underneath stays as wet as the day you made it. Wherever the bag gapes, the pot dries as if nothing were over it. So a bagged pot ends up with a moisture map decided by where the plastic happened to fall: wet under the fold, dry at the open mouth, damp in the middle. That is workable, and most of the studio does it every week, but you are steering the piece by fiddling with a bag, and the bag moves every time you check it.
What plastic is unbeatable at is stopping. A piece bagged loosely with the mouth closed will sit at nearly the same stage for days. A leather hard mug waits for its handle; a half-built sculpture waits for next week; a lump of clay waits for the next class. When the job is to press pause, plastic is the tool.
Cotton: a brake that works everywhere at once
Fabric is permeable. Air passes through the whole sheet at the same rate, so every square inch of the pot underneath is slowed by the same amount, and the evenness stops being something you manage with folds and becomes something the cloth does for you. A thin rim and a thick foot both lose water more slowly, which gives the foot time to catch up before the rim has shrunk away from it. That catching up is the entire reason cotton exists on a pottery shelf.
And it stacks. One sheet slows a piece a little; two slow it more; three or four, on a thick sculpture with thin ears, slow it to a crawl while still letting it dry evenly. A plastic bag has never had a dial. A stack of sheets is one.
The one thing cotton cannot do
Stop. A piece under a sheet keeps drying, just evenly and more slowly. If you need it to wait a week, cotton alone will not do it, and that is when the bag goes over the sheet.
Which one, for which piece
The decision table.
Have this open at the shelf. It covers most of what gets made here on a class night.
Have this open at the shelf.
| The piece | Cover | How long | Why |
|---|---|---|---|
| A mug or bowl, even walls, trimming tomorrow | Nothing, or a loose bag mouth open once it is firm | Overnight to trim | Even pieces dry evenly on their own. A bag is only to hold it at trimming stage if you cannot get back for a few days. |
| A mug waiting for its handle | Loose bag, mouth closed | Up to a week | You need it to stop at leather hard. Plastic stops; cotton does not. |
| A mug with its handle just attached | One cotton sheet | Two or three days | The handle is thinner than the wall. Cotton lets the join and the wall lose water at the same rate. |
| A plate or a wide flat piece | Cotton sheet, flipped daily | Three to five days | Rims dry first and curl. Cotton slows the rim to the speed of the center, and flipping evens the two faces. |
| A pot with a thick base and thin walls | Two sheets, then one | A week or more | The base is the slowest part. Stacked sheets hold the walls back until it catches up, then pull one off. |
| A sculpture with thin parts on a thick body | Two or three sheets, plus a bag over the sheets | Two weeks and up | Ears, fingers and edges finish days before the core. Cotton evens, the bag over it slows the whole thing to the core’s pace. |
| Anything big, tall, or awkward | Cotton under, plastic loosely over | A week to a month | The combination. Fabric evens the surface, the bag keeps the studio’s air movement off it. |
| Slab work joined at the corners | One sheet, then open air | Three or four days | Corners dry first and pull apart. One sheet for the first few days, then let it go. |
| A half-built coil pot between sessions | Damp cloth loosely inside a bag, not touching the clay | Until you are back | Holding, not drying. This is the studio’s stand-in for a damp box. |
| Bone dry and waiting for the bisque | Nothing | Until it is fired | It is done. A cover on a dry piece just hides it from the shelf sweep. |
Step one
Find the thinnest part, the thickest part, and the day you are coming back.
Thin, thick, when. Three questions decide the cover. Where is the thinnest part, because that is what will finish first and crack away from the rest? Where is the thickest, because that is the pace the whole piece has to dry at? And when are you back, because a piece that has to wait a week wants a different cover from one you will trim tomorrow. Run a thumb round the rim, tap the base, look at the handle or the ears, and answer all three before the cover goes on. If the piece is even and you are back tomorrow, the answer is often nothing at all.
Thin, thick, when. Find the thinnest part, which finishes first; the thickest, which sets the pace; and the day you are back. Even piece and back tomorrow: nothing. Thin and thick together: cotton. Waiting days: a bag. Big or awkward: both. The awkward pieces lesson has the long version.
The building is part of the reading. Overnight and on weekends it cools to about fifty five degrees, which slows drying, and the air near the shelves moves whenever the doors do. A piece on a top shelf by the door dries a full day faster than one low and in the middle, so a cover chosen for the shelf is not always the cover the piece needs.
Storage bats are plastic here, always. A wooden bat under a wet pot pulls water out of the base at its own speed, which is exactly the uneven drying every cover on this page exists to prevent, and the wood warps. Cover the top of the piece; let the plastic bat handle the bottom, and flip the piece when it is firm enough so the base gets air.
Our awkward pieces lesson is the long version of this step, with a ladder of settings from open air to fully wrapped. This page is the cover half of that ladder.
Step two
Drape it loose, let it touch only what it has to, and use the stack as a dial.
Loose, and let it settle. Take one sheet, shake it out, and lay it over the piece so it settles under its own weight. Do not tuck it, do not press it into the pot, do not wrap it tight. The cloth touching the rim is fine; the cloth pressed against a wet wall prints its weave into the clay and pulls a skin of surface off when it is lifted. On a wide piece, let the sheet drape to the bat all round so air has to come through the fabric to reach the pot. On a tall piece, a second sheet over the first closes the gaps the first one left.
Loose, and let it settle. One sheet shaken out and laid over the piece so it settles under its own weight, touching the rim and nothing else, draped to the bat all round. Stack a second or third sheet to slow a thick piece further, and pull one off every few days as the thick part catches up. Sheets go back to the basket dry.
The stack is the dial. One sheet takes the edge off the exposed faces without really slowing the piece. Two slow it noticeably, which is right for a piece close to leather hard that needs to even out before trimming. Three or four is for the sculpture with fingers or the pot with a base like a brick. Start with more than you think, then pull one sheet off every few days as the thick part catches up, down to one, then none.
Flip flat work. A plate under a sheet dries from the top face; turn it over each day so both faces lose water, and put the sheet back. A bowl comes off its bat and onto its rim as soon as it can take it, because the base is the one part getting no air at all until you do.
When the piece is done with the sheet, shake the sheet out and fold it back into the basket dry. A damp sheet folded into the basket is how mildew starts, and mildew is the one thing that gets a sheet thrown away.
Step three
Tent it. Never let the plastic lie on wet clay, and decide whether the mouth is open or shut.
A tent, not a wrap. Pick the bag that is a size bigger than the piece, so it can stand off the clay rather than lie on it. Set the piece on its bat, bring the bag over the top like a tent, and let it hang to the bat without touching the walls; a chopstick or a wooden dowel stood beside a tall piece holds the plastic off. Then the one decision: mouth open, and the piece keeps drying slowly and unevenly; mouth closed and tucked under the bat, and the piece stops. Open for a night or two of gentle slowing, closed for waiting.
A tent, not a wrap. A bag one size bigger than the piece, tented over it so the plastic hangs to the bat without lying on the walls, with a dowel beside a tall piece if it needs holding off. Mouth open to slow the piece a little; mouth closed and tucked under the bat to stop it for days. For holding work between sessions, a damp paper towel inside the bag, not touching the clay.
The two mistakes are both about contact. A bag pulled tight against a wet wall leaves that patch wet while everything round it dries, and the patch shows up later as a crack, a warp or a glaze flaw over a place that fired differently. And a bag that touches the rim of a freshly thrown pot pulls the rim out of round when it is lifted. Tent it, every time.
For holding, which is what most bagged pieces here are doing, add a damp paper towel or a folded damp rag inside the bag, standing beside the piece or lying on the bat, never on the clay. That keeps the air in the bag at full humidity and holds leather hard for a week or so. We do not have damp boxes for members, and this is the studio-shelf stand-in; our damp box lesson is the home version.
Bags are reused. Shake the slurry out, hang or lay it to dry, and put it back in the right size pile. A wet bag folded onto the pile turns the whole pile into a sponge.
Step four
Cotton against the piece, plastic loosely over the whole arrangement.
Slow and even, both. For anything big, tall, or built from thick and thin parts, use both tools at once. Cotton goes on first, draped as in step two, one sheet or a stack. Then a bag, or a sheet of plastic, goes loosely over the whole thing, sheets included, mouth open at first and closed if the piece needs to stop. The fabric evens the surface; the plastic keeps the studio’s moving air off it. You get slow and even at the same time, which is the combination that potters in dry climates swear by, and Utah is a dry climate.
The sequence for a big piece runs the other way from how it went on. Plastic comes off first, after a week or so, once the thin parts have stopped racing ahead. Then the sheets come off one at a time, over another week or two, and the piece finishes in open air. Rushing the last step is how a sculpture that survived three weeks under cover cracks on the shelf in its last two days.
A large piece under all of that is still a large piece on a shelf, and shelves are shared. Name and date on the bat, and a note if it needs to stay wrapped: an unlabeled bundle on a shelf gets unwrapped by somebody who thinks it was forgotten.
A hand-built sculpture with no thin parts at all can skip the cotton and go straight to a loose bag; a thin-walled thrown form with nothing attached can skip the plastic and take one sheet. The combination is for the piece that is both.
Watch it done
Now watch five people wrap and unwrap.
Watch this one, then go cover your piece.
None of these potters work here or know we exist. Between them they cover why pieces crack, how to wrap, how to unwrap in stages, and how a shared studio runs its shelves. The still frames load while you read; the player and its cookies wait until you press play.
Twelve minutes on wrapping and unwrapping in stages. The other four videos are in the comprehensive version.
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. Most of them are the cover doing exactly what it does, on the wrong piece.
Pick the one that looks like your shelf.
A crack starting at the rim, or a rim that curled up on a plate
The rim dried first, shrank first, and pulled against the wetter clay below it. Open air on a wide piece, or a bag that gaped exactly at the rim.
Cotton on anything wide or flat, draped to the bat, and flat work flipped daily so both faces dry. A rim that is thin because of the throwing wants a sheet from the first night.
A hairline at leather hard can be scored, slipped and burnished closed. A crack at bone dry is a crack; it opens in the bisque.
The weave of the bag pressed into the wall, or a wet patch that dried differently
The plastic was lying on the clay instead of tented over it. The patch it touched never dried while everything round it did.
A bag one size bigger, tented to the bat, a dowel beside anything tall. Plastic touches the bat and nothing else.
A print in leather hard clay ribs out. A patch that dried differently is usually invisible after glazing; dry the piece the rest of the way under cotton so it evens up.
A crack where the handle meets the mug, on the shelf or after the bisque
The handle was thinner and drier than the wall when it went on, or it dried faster afterwards. Either way the join shrank at two speeds.
Handle and mug at the same stiffness when joined, then one sheet over the whole mug for two or three days so the join and the wall lose water together. Our handles lesson covers matching them.
Caught at leather hard: work slip into the crack, compress it, and sheet it. Caught after the bisque: the mug is a pencil cup.
Under cover for a week and still soft
A closed bag does what it is for: it stops drying. Or a thick base under a stack of sheets is doing exactly what the stack asked it to.
A closed bag is for waiting, not for drying. When you want it to move, open the mouth, then take the bag off, then thin the stack of sheets.
Open the bag tonight. A thick piece will take days more under one sheet, and that is normal; nothing bad happens to a piece that dries slowly.
Black or green spots on a sheet, or on a bagged piece
A sealed, damp, warm space is where mildew lives. A wet sheet folded into the basket, or a piece sealed in a bag for weeks with a wet rag inside.
Sheets back in the basket only when dry. A bagged piece opened for an hour every week or so. The damp rag rewetted and the bag aired.
Wipe the piece with a damp sponge; it burns out completely in the bisque. A mildewed sheet gets washed hot or thrown away, not folded back into the basket.
You came back to trim and it had gone past leather hard
Open air, a warm shelf by the door, or a bag with the mouth left open on a piece you needed to hold. The studio dries things fast.
Anything that has to wait more than two days for you goes in a bag with the mouth closed. Anything that has to wait a week gets a damp paper towel in the bag as well.
A pot that is past trimming is still a pot. Sand the foot at bone dry with a damp sponge and a rib, or leave the base as thrown and call it a design. Our waking up stiff clay lesson is for clay, not pots: a dry pot does not come back.
How it works here
Sheets in the basket, bags in three sizes, no damp boxes.
This page is written for potters everywhere. Here is where our studio stands.
Four things to know here
Cotton sheets live in the basket back by the shelves and are for Purple Level Members and Black Level Members; they go back to the basket dry. Green Level Members and Yellow Level Members are welcome to bring their own, one hundred percent cotton and the highest thread count they can find. Bags in three sizes are for everyone and get shaken out and reused. There are no damp boxes for members: a bag with a damp paper towel inside is the studio version.
The sheets
A basket of high thread count, one hundred percent cotton sheets sits back by the shelves. Purple Level Members and Black Level Members take one, drape it, and fold it back into the basket dry when the piece is done with it. Stacking two or three is a real setting, not a waste.
Bringing your own
Green Level Members and Yellow Level Members are very welcome to bring their own. Two rules and no others: one hundred percent cotton, and the highest thread count you can find. A poly blend does not breathe the same way, and a coarse weave prints its texture into soft clay.
The bags
Three sizes: ten by ten, thirteen by fourteen, and sixteen by twenty inches. Pick the bag for the piece rather than the nearest one, tent it, and shake it out and dry it before it goes back on the pile. They are for every Level.
No damp boxes
We do not have damp boxes for members, because plaster never comes into the building. A loose bag with a damp paper towel inside it, not touching the piece, holds leather hard for about a week and takes up less of your shelf.
The building
It cools to about fifty five degrees overnight and on weekends, which slows drying, and shelves near the doors dry faster than shelves in the middle. Read the shelf as well as the piece.
Bats and labels
Storage on plastic bats only; wood warps and pulls water out of the base unevenly. Name and date on the bat, and a note if the piece needs to stay wrapped, so nobody unwraps it thinking it was forgotten.
Questions people actually ask
The ones that come up at the shelves.
Which is better, a sheet or a bag?
Neither. A bag stops drying, unevenly. A sheet slows drying, evenly. Use a sheet when thin and thick parts need to finish together, a bag when the piece has to wait for you, and both when it is big or awkward.
How many sheets?
One takes the edge off an exposed rim. Two is right for a piece close to leather hard that needs to even out. Three or four is for a thick sculpture with thin parts. Start with more than you think and pull one off every few days.
Can I use a bedsheet from home?
Yes, at any Level. One hundred percent cotton and the highest thread count you can find. Poly blends do not breathe the same way and coarse weaves print into the clay.
Should the bag be open or closed?
Open to slow the piece a little for a night or two. Closed and tucked under the bat to stop it for days. Never lying on the clay either way; tent it.
How long can a piece stay in a bag?
About a week at leather hard with a damp paper towel inside, longer if you open it for an hour each week and rewet the towel. Weeks-long projects want a damp box at home; we do not have them here because they are plaster.
Why did my plate curl up at the edges?
The rim dried before the center and shrank first. Cotton over it from the first night, draped to the bat, and flip it every day so both faces lose water.
Do I cover a bone dry piece?
No. It is done. A cover on a dry piece just hides it from the shelf sweep and makes it look forgotten.
Does the studio dry things faster than home?
Usually. The air is dry, the doors move, and the shelves near them get a breeze. It cools overnight and on weekends, which helps a little. Assume fast, and cover accordingly.
Related lessons
Six lessons that sit next to this one.
Three to read next.
Covering is one third of drying. These are the pages about the other two thirds, and the pieces that need covering most.
That is the whole decision
Even piece, open air. Thin and thick together, cotton. Wait days between sessions, a bag. Big and awkward, both.
Most cracks that get blamed on the clay, the kiln or the weather were decided on the night the piece went onto the shelf, by what did or did not go over it. A sheet costs nothing to use and a bag costs nothing to open an inch. Spend the thirty seconds.
If you want the moisture physics, the six fixes and the other four videos, switch to the comprehensive version at the top. Otherwise, go cover the piece you left on the shelf tonight.
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.