Trimming, handles and drying · Damp box

A tub, a plaster slab and some water.

A damp box holds work at leather hard for weeks instead of days, which is most of what stops a project stalling. It is a lidded tub with a plaster slab in the bottom, and because the slab is plaster, it is built and kept at home. The plaster-free version for your studio shelf is here too.

This page walks the build with a tick list at every step: the tub, the plaster by weight, the pour, the cure, charging it, covering it, and the studio-shelf version. Six fixes, most of them about water or plaster, and five videos.

This is the short version: the tub, the pour, the charge, and the plaster-free studio version, each with a tick list. The reasoning, the reclaim note and the questions are in the comprehensive version.

1½″Of plaster in the bottom of the tub. Thinner cracks, thicker is wasted.
WeeksHow long leather hard work keeps in it
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 build in six moves What you need on the bench 1. The tub and the plaster 2. The pour 3. Charging and covering 4. The plaster-free studio version Five potters, five boxes 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 person in an apron and black gloves smoothing freshly poured white plaster in a clear plastic tub on a workbench, with stacks of clay slabs beside it and finished pots on the shelves behind

Every step, the reasoning, the reclaim note and five videos.

Read it, then watch it

Why this is written down and filmed, both.

A damp box is a recipe with one judgment call in it. The recipe is the tub, the plaster ratio, the thickness and the drying time, and that is a list. The judgment call is how wet: what a charged slab looks like against a saturated one, and what standing water does to a pot left on it overnight. You have to see it once, and the videos below show the pour and the slab.

Use both, in this order

Read the recipe. Watch the pour and the slab. Build it at home with the tick list open. This one is a plaster project, so it never happens at the studio, and the page carries a plaster-free version for your studio shelf.

The whole idea

A tub, a plaster slab, some water, a lid. At home.

Pour an inch and a half of plaster into a lidded tub, cure it, soak it, pour the standing water off, and leather hard work stays leather hard for weeks. The box lives at home, because wet plaster and plaster dust never come into the studio; a sealed tub with a wet towel in the corner is the studio-shelf version.

The five-minute version

The whole build in six moves.

If you have mixed plaster before 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.

1Pick a tub with a lid that sealsClear plastic, a snap-shut or gasket lid, deep enough for the tallest thing you make plus the slab.
2Mix plaster by weightOne hundred parts pottery plaster to seventy parts water. Water first, plaster sifted in, slaked, stirred slowly.
3Pour it in the tub, an inch and a half deepOn a flat floor. Tap the tub. Do not touch it for an hour while it heats and cools.
4Cure it, then charge itA day open to the air. Then pour water on until the plaster stops drinking, and pour every drop of standing water off.
5Mesh or newspaper over the plasterSo clay and plaster never touch. Pots set down, never dragged.
6Lid on. Recharge when it goes paleEvery week or two in a dry climate. Vinegar in the water keeps the mold off.

Before you open the bag

What you need on the bench.

Six things, and one of them is vinegar. The tub is the decision; everything else is a bag of plaster and an afternoon.

A lidded plastic tubClear, so you can see what is in it. A lid that snaps or seals. Deep enough for your tallest piece plus two inches of slab. A tub you do not mind giving to plaster forever.
Pottery plaster and a scaleNo. 1 pottery plaster, mixed by weight, one hundred to seventy. Our plaster mold lesson has the mixing method and the safety rules in full.
An N95, safety glasses, gloves and a bucket you can flexAn N95 or better and safety glasses for the powder, gloves for the skin, and a bucket you can flex so set plaster comes out of it without ever going near a sink.
Plastic mesh or newspaperA sheet of thin mesh, the kind sold for craft canvas or gutters, or damp newspaper, laid over the plaster so pots never sit on it directly.
White vinegarA splash in the recharge water. The difference between a damp box and a mold farm.
A garage or a porchWhere the plaster gets mixed and poured. Never the studio, never the kitchen sink.

A clear lidded tub, pottery plaster and a scale, an N95 and safety glasses, gloves, a flexible bucket, mesh or newspaper, white vinegar, and a garage.

1

Step one

Pick a tub that seals, and work out how much plaster.

The lid is the damp box. A plaster slab in a tub that does not seal is a plaster slab on a shelf. Look for a clear tub with a lid that snaps shut or has a gasket, deep enough for the tallest thing you make plus two inches of slab.

Then the plaster. An inch and a half to two inches of slab is right: thinner cracks and holds less water, thicker is wasted. To know how much, pour water into the tub to the depth you want, then pour that water into your mixing bucket and weigh it. Then take two thirds of that weight: that is your seventy parts, and the plaster is one and three sevenths of it, by weight. Two thirds looks wrong and it is right, because plaster does not vanish into the water, it takes up room of its own, and a bucket of slurry ends up about half as big again as the water you mix it with. Mix ten percent extra. The ratio and the slaking and the slow stir are exactly as in our plaster mold lesson, and so are the safety rules: an N95 or better and safety glasses for the powder, gloves, never a hand in a setting bucket, and nothing plaster ever down a sink.

The place matters as much as the tub. This is a garage, porch or driveway job. Plaster dust, plaster tools and plaster rinse water never come into a clay studio, ours or anyone else’s, and the reason is in the first card of the studio band below.

Tick list, the tub0 of 4
2

Step two

Pour it into the tub, tap it, and leave it alone.

Cast the slab in the tub it will live in. Set the tub on a flat floor, mix the plaster by weight, and pour it in low and steady. Tap the sides for a minute so the bubbles rise. Then walk away for an hour: it will heat as it sets, and moving a tub of setting plaster is how a slab cracks before it has done a day of work.

Some potters cast the slab separately in a tray and drop it into the tub afterwards, which lets you replace it later without giving up the tub. That works too; the slab needs a day to cure before it is moved, and it wants to fit the tub with a little room round the edge. Casting in the tub is simpler and the slab cannot tip.

Then a day with the lid off, so the plaster cures and gives up its own water. A slab charged the same day it was poured is a swamp. The next day it is pale, hard, and ready to be charged for the first time.

Tick list, the pour0 of 4
3

Step three

Charge it, pour the standing water off, and cover the plaster.

Dark and damp, never shiny. Pour water over the slab and watch it drink. When it stops, pour off every drop that is standing. A charged slab is dark, cool and matte. A slab with a puddle rewets everything you put in and grows things.

A splash of white vinegar in the charging water is the difference between a damp box and a mold farm; a few drops of bleach does the same. Pick one and keep to it, because bleach and vinegar together give off chlorine gas. Then lay something over the plaster so clay never touches it: thin plastic mesh, a sheet of damp newspaper, a few strips of wood. Pots dragged across bare plaster pick up chips, and pots left standing on wet plaster get wetter at the base than at the rim. The cover solves both.

Now it works. Anything leather hard goes in and stays leather hard. Freshly thrown work goes in only after it has firmed up in the open air, because the box holds a stage rather than choosing one. Lid on, and the box goes somewhere cool and out of the sun. Recharge when the plaster goes pale, every week or two in a dry climate, and open the lid for an hour now and then to let the air change.

Tick list, charging0 of 5
4

Step four

The plaster-free version for the studio shelf.

Wet plaster and plaster dust never come into the studio, so this is what does. A lidded tub with a wet sponge or a wrung-out towel in a small open container in one corner. It holds a piece at leather hard for days instead of weeks, which covers most projects that span two visits.

The rules are the same in miniature: the lid has to seal, nothing wet touches the clay, and the sponge gets rewetted when it goes dry. A plastic bag with a damp paper towel inside it does the same job for a single piece and fits on any shelf, which is the method many lessons in this University that span two sessions already recommend. Name and date on the bag or the tub.

For anything that needs weeks (a big sculpture, a set of pieces that have to match, a project that stalls), the answer is the plaster box at home, and the pieces travel. Wrap them in damp cloth and plastic for the trip; a leather hard pot in a sealed bag survives a car journey without complaint.

Tick list, the studio version0 of 5

Watch it done

Now watch five potters, three of them building a damp box.

Watch this one, then go find a tub.

None of these potters work here or know we exist. Between them they cover the whole box in five minutes, a slower walk through the build, the measurements, the case for building one, and the plaster slab on its own. Nothing but the still frame loads until you press play.

Under five minutes: a tub, a bag of plaster, a pour and a lid. The other four videos are in the comprehensive version.

Start here: the whole box in five minutes How to Make a Plaster Slab and Damp Box Little Street Pottery
A tub, a bag of plaster, a pour and a lid, in under five minutes. The video’s chapters run from mixing and pouring the plaster, to reclaiming clay on the slab, to making the damp box, so this is the short version of the build and not of everything on this page. Watch on YouTube
The slower version How To Make A Damp Box for Clay The Pottery Wheel
Twelve minutes, described by its maker as straightforward and complete instructions for making one. The maker says a damp box can keep clay workable indefinitely, or let pieces dry out slowly, and can last for years. Watch on YouTube
With the measurements How to Make a Damp Box Easy | Keep Clay, Ceramics and Pottery Wet | Using Plaster with Measurements Blue Nose Trading
Made outdoors for a 24 by 17 inch tub with a slab about an inch and a half thick. The description gives the water it took, 1.9 gallons, and links a mixing chart that scales the water and plaster to any other size of tub. Watch on YouTube
Why, in three minutes PLASTER in your studio? Why you need a DAMP BOX! Outpost Pottery
A working potter’s short case for a plaster damp box, with a diagram of the plaster to water ratio. Watch on YouTube
The plaster slab on its own How to Make a Plaster Slab for Wedging and Recycling Clay Rachel Hamilton
Casting a free-standing plaster slab in a pan, for wedging and reclaiming clay, with nearly five minutes of the ten on the arithmetic for how much plaster and water a mold of your size needs. It is the same kind of slab that goes in the bottom of a damp box. Watch on YouTube

When it goes wrong

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

The three failures that happen in the first month.

Pick the one that looks like your box. Most of them come back to water or to the plaster, which is the whole subject.

How it works here

At The Clay Hole, the damp box lives at your house.

This page is written for potters everywhere, and the box it describes is a home project. Here is where the studio stands.

Three things to know here

There is no communal damp box, because wet plaster and plaster dust never come into the building. On your shelf, a bag with a damp paper towel holds a piece at leather hard for a week; a sealed plaster-free tub a little longer. Weeks-long projects go home to the plaster box, wrapped damp for the trip.

Why there is no communal one

Plaster in clay blows a crater out of the fired pot, weeks after the firing, on a shelf, on a pot that was perfect. In a shared studio a chip travels through the reclaim into everybody. So wet plaster and plaster dust stay out of the building, and that includes the slab in the bottom of a damp box.

On your shelf

A plastic bag with a damp paper towel holds a single piece at leather hard for about a week. A sealed tub with a wet sponge in a container holds several for a little longer. Both are plaster-free and both are welcome. Name and date on everything; unlabeled work on a shelf is how projects go missing.

Traveling work

Leather hard pieces wrapped in damp cloth and then plastic travel home and back without trouble. That is how a weeks-long sculpture gets the studio’s tools and the home damp box both. Bone dry pieces travel in a box with padding and nothing damp near them.

What the box is for, here

Matching handles to mugs across two visits. Coil-built work that takes four sessions. A figurine whose arms were made on a different night from its body. Our drying awkward pieces lesson is the other half of the same problem.

Questions people actually ask

The ones that come up every week.

What does a damp box actually do?

It holds clay at whatever stage it went in, for weeks. A leather hard pot stays leather hard. A half-built sculpture stays workable between sessions. Handles pulled on Tuesday match a mug thrown on Thursday. The plaster slab is a reservoir: it releases water into the sealed air until the box is at full humidity, and at full humidity clay stops drying.

Do I need plaster?

For a real damp box that holds work for weeks, yes; the plaster is the reservoir and nothing else holds and releases water the same way. For a few days, a sealed tub with a wet sponge or a folded wet towel in a container in the corner does most of the job, and that plaster-free version is the only kind that can live on a studio shelf.

Why can I not have one at The Clay Hole?

Because it has wet plaster in it, and wet plaster and plaster dust never come into the building. A single chip of plaster in the reclaim blows a crater out of somebody else’s pot months after the firing. Your plaster damp box lives at home. At the studio, a bag with a damp paper towel holds a piece at leather hard for a week, and a plaster-free tub holds it a little longer.

How much water?

Enough that the plaster drinks and stops, then pour off every drop that is standing. A dark, damp slab with no shine is charged. A shiny slab with a puddle rewets everything you put in it. In a dry climate you will recharge every week or two; you can tell because the plaster goes pale.

Can the pots sit straight on the plaster?

Not for long. Two things go wrong: a pot dragged across the slab picks up chips, and a pot left on wet plaster gets wetter at the base than the rim. A sheet of thin plastic mesh, a piece of damp newspaper or a few strips of wood keep the clay just off the surface and solve both.

How long will work keep?

Weeks, sometimes months, if the box seals and the slab is recharged. Leather hard is the stage that keeps best. Wet, freshly thrown work is better left in the open to firm up first, then boxed. Bone dry work does not come back; a damp box holds a stage, it does not reverse one.

Can I use it for reclaim?

A plaster slab on its own is the classic reclaim tool: slop poured onto it firms to wedging consistency in a day. In the damp box the slab is doing the opposite job, so use a separate slab for reclaim, and keep both away from the studio, per the plaster rule.

Related lessons

Six lessons that sit next to this one.

Three to read next.

A damp box holds a stage. These are the pages about the stage, and the projects that need holding.

Hand buildingMaking Your Own Plaster Slump MoldThe mixing method and the safety rules, in full. FoundationsThe Five Stages of ClayLeather hard is the stage a damp box holds. This is the page that explains it. Trimming, handles and dryingDrying Awkward PiecesThe other half of the problem: letting go of the stage on purpose. Preparing your clayReclaiming Your ClayWhere a plaster slab does its other job, at home. Hand buildingA Beginner's Guide to SculptingThe project that needs a damp box most. Trimming, handles and dryingHow to Pull HandlesHandles that match mugs made on another night.

That is the whole build

Tub. Plaster. Charge it. Cover it. Lid on. At home.

Your first damp box will probably be charged too wet and grow something interesting in the corner. That is normal, and vinegar fixes it. After that it is the tool you did not know you were missing, and the reason your handles finally match your mugs.

If you want the reasoning, the reclaim note and the answers to the questions everybody asks, switch to the comprehensive version at the top. Otherwise, go find a tub with a lid.

Back to Clay Hole University The plaster method, in full

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