How to Make a Damp Box For Clay

Life is busy and having somewhere to store half-finished pottery for a week or a few days is handy.  You may have heard about a helpful piece of pottery kit called a damp box and wondered what it is.  In this article, I will explain what it is and how to make a damp box for clay.

To make a damp box for clay, pour 1-2 inches of plaster into a plastic container.  When the plaster has set, rub it down to get rid of bumps and loose bits.  Pour water onto the plaster to make it damp.  Then place your pottery on the damp plaster and seal the lid of the box.

how to make a damp box for clay
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More About Exactly How to Make a Damp Box

A damp box can be used for a couple of handy things.  Firstly, it can be used to store clay in a more or less workable state indefinitely.  Secondly, it can be used to help you dry your clay out slowly so it doesn’t crack whilst drying.

There are various different types of damp box.  I will explain how to make each different type.  The first one I will explain is the type from which the word ‘damp box’ gets its name.  Not surprisingly the first step is to find a suitable box!

Here are the steps to follow to make an invaluable damp box that can last for years.

What you will need to make a damp box for clay

  • A plastic storage box
  • Some plaster (I use plaster of Paris)
  • Vaseline (or a release agent if you have some)
  • A couple of buckets
  • A pouring jug
  • Thin rubber gloves
  • A pottery rasp
  • A scrubbing pad
  • A respirator mask
  • Rubbing alcohol in a spray bottle (optional)

Here is a quick overview of the steps involved. For the full video on how to make a damp box for clay, check out my YouTube channel.

Simple Steps to Build a Damp Box For Clay

Preparing the Box:

  • Find a plastic box that is going to be large enough to store your greenware.  Choose something that will give you enough room to lift your pieces in and out without damaging them.  If the box is too small, it will be hard to put them in and take them out carefully.  Give yourself 2 or 3 inches clearance all the way around.
  • Put a very thin layer of Vaseline (or release agent) onto the inside of the box.  You only need to cover the bottom two inches.
Preparing your box to make a damp box for clay

Preparing to Mix the Plaster:

  • Measure how much water you need. You can do this by pouring enough water into the plastic container to cover the base of with 1-2 inches.  Then transfer the water into one of your buckets.
  • Some people recommend using warm water.  I would recommend using tepid water.  Not too cold, not too hot.  Warm water will set your plaster quickly.  If it is tepid, it gives you a bit more time to work.
  • Fill your other bucket with water and keep it to one side.  You will use this bucket to wash/rinse things as you go along.  This will stop you from having to rinse plaster down your sink and will save your drains.
  • Put on your respirator mask and gloves.  It’s important not to breathe in the plaster dust and avoid getting it on your skin.
Getting water ready for plaster

Mixing the Plaster:

  • Sprinkle the plaster into the bucket of water.  You can use a jug or a spoon to sprinkle.  Or you can pick up handfuls of the plaster with your gloved hand.  Sprinkle the plaster into the water rather than dumping big chunks in.  Big clumps of plaster will tend to clump together in the water in a lump.  If you sprinkle it, you give the plaster a chance to dissolve into the water.
Sprinkling plaster to make your damp box for clay
  • Keep sprinkling the plaster in until there is enough in the water to create a plaster island.  I never measure a plaster to water ratio out exactly.  You can easily do it by eye.  Once the plaster starts to poke out of the top of the water in a little mound, you have added sufficient.
Mixing plaster
  • With a gloved hand and your mask still on, gently mix the plaster into the water.  You can mix it slowly with a spoon or stick, but it is easier to feel lumps with your fingers.  I’d recommend using thin latex gloves so you can feel the lumps.  Some potters recommend mixing the plaster with a drill and mixing bit.  But this can add a lot of air bubbles to the mixture, which you do not want.  I’d recommend just using your gloved hand.  Stir until it has all dissolved and the mixture has the consistency of double cream.  Once the plaster is all mixed in, you can take off your mask.

Pouring the Plaster:

  • If you are strong you can pour the mix directly from the bucket into the plastic box.  Try to pour it into the corner of the plastic box.  This reduces the amount of air that gets dragged into the plaster mix.  Think about pouring a fizzy drink into a glass.  If you pour it straight into the center of the glass, it gets agitated and bubbles up.  However, if you pour it against the side of the glass it will settle more easily.
  • If the bucket is too heavy you can use a jug to transfer the mix to the container.  Again, you can put a thin layer of Vaseline on the jug to make it easier to clean afterward.
Pouring plaster into the damp box

Letting the Plaster Go Off:

  • Once all the plaster has been transferred, tap the side of the container.  This will help bubbles rise to the surface.  I also put my gloved hand into the mix and give it a very gentle ‘shuggle’.  This gets rid of more bubbles than just tapping.
  • A tip that I picked up from TimSee1, is to spritz the plaster with alcohol.  This will burst the plaster bubbles that pop to the surface.  Rather than spraying your damp box with Whiskey, you can get a bottle of ethanol from a local pharmacy.  I tried this and put it into an old hair product spray bottle.  It works well and means you don’t have to rub the surface down so much later.
  • Leave the container to sit until the plaster has gone off.  This can take as little as half an hour, but I usually leave mine overnight to make sure it isn’t soft.

Finishing Touches:

  • Once the plaster is hard, you have options.  You can either leave the plaster in the container and use a scrubber to rub-down the surface.  Or you can turn the container upside down, and pop the plaster slab out.  This is the reason for putting the thin Vaseline layer onto the container earlier.
  • If you are leaving it in the container, just rub it down with a scrubbing pad.  Remember to clean any plaster splashes on the side of the container too so they don’t flake off later.
  • To pop the slab out of the container, simply invert it and then tap the underside.  It should slide out fairly easily.  Being careful not to drop it, you can use the pottery rasp to file the edges of the slab.  This will get rid of sharp bits of plaster that could flake of and get into your pottery.  Also, scrub down the surface of the slab to get rid of bumps and lumps.
Removing the plaster from the damp box for clay
Rasping the edges of the plaster
  • Once you have done this, put the slab back into the container.  You now have a damp box!  To use the box, simply pour enough water onto the plaster to make the plaster damp.  If excess water is sitting on the plaster surface, soak it up with a sponge.
  • You can then put your greenware onto the plaster and seal the box with a lid.
The finished damp box for clay

If the box is airtight, your pottery can be kept in the damp box indefinitely.  I would recommend checking the moisture levels in your plaster regularly.  You can top the water in the plaster up as time goes on.


A Few More Handy Tips for Making Your Damp Box

  1. A key part to how you make a damp box for clay is to get the plaster the right thickness.  If it is too thick it will be very heavy.  But if it’s too thin it may break with wear.  Between 1.5 and 2 inches is usually right.
  2. Line your plaster mixing bucket with a plastic bag.  When you have finished mixing, you can just throw the bag away.  This will save you from washing your bucket out in the sink.  It’s best to avoid plaster going down the sink if you can.
  3. If you are storing your unused damp box for a while, keep the lid off to let the water evaporate.  If it dries out with the lid on, it can go moldy.
  4. You can, however, get rid of mold in the damp box by spritzing it with a mixture of water and bleach.

Different Uses for Your Damp Box

Damp boxes can be used to keep clay workable or to dry it out slowly.  If you want to dry your clay out slowly to avoid cracks you can put holes in the lid of the box.  The more holes you put in the more air that can circulate.  You can control how quickly the clay dries out by adjusting how much air gets in.

I have two damp boxes.  One is to keep clay damp.  The other is to dry it out slowly.  The box that I use to dry the clay out has a few holes in the lid.  If I want to slow the drying down, I cover the holes up with masking tape.  Should I want to speed the drying up, I remove some of the tape, to increase the number of holes.

How to Make Different Kinds of Damp Box

A damp box is basically anything in your pottery arsenal that helps you control the humidity around your greenware.

There are other variations on the damp box theme, that are very simple to use.  Some of these are as follows:

  • Putting your pottery under an inverted bucket works like a damp box.  You increase the moisture level in the bucket by putting a wet sponge in a bowl underneath the bucket too.  Conversely, you can gradually reduce the humidity by making holes in the top of the bucket.  Buckets are handy because they protect the pottery and can also be stacked, which saves valuable studio space.
  • Simply putting your pottery in a plastic bag is a very simple kind of damp box.  Some potters recommend wrapping the pottery in a wet paper towel or soft cloth.  If you choose to do this, it is a good idea to spray the paper towel with water regularly.  Should the towel or cloth dries out it can end up absorbing water from the clay.

If you have space and resources you can also either make or invest in a damp cabinet.  You can buy these, but they can also be made relatively easily.  Simply cover an old cabinet well with plastic sheeting.

You can use any kind of plastic sheeting, as long as it is not water permeable.  Cover the cabinet in such a way that it traps moisture.  You can store larger pieces in a damp cabinet.  Also, you can add a tray of water in the bottom to boost the humidity.

Final Thoughts

I hope that if you were wondering how to make a damp box for clay, this article demonstrates that it is simple.  It does take a bit of preparation, but the execution is straightforward.  And once you have made it, you will be able to use it for many years.


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