A few pots have been returned to the gallery in the last 6 months because of leaking. What I’ll address here are the main causes and how to test your clay’s absorption percentage.

Primary reasons for leaking.

Large or small, visible or sub-visible cracks
Under-vitrified clay (high absorption rate)
Crazed glaze over under-vitrified clay

Let’s start with the FDA’s requirements for ceramics that can be used for food.

Maximum of 0.5% (one half percent) absorption rate
Glaze with no crazes

This is primarily aimed at restaurant ware. I’m not aware of anyone looking at hand made ceramics. But… makes sense that we know what the “rules” are when we’re making our decisions about clay/glaze combinations used with food pots, or pots that will hold liquids for longer time periods.

Let’s start with the clay.

As stated above the FDA says 0.5% absorption rate is required for food related clay.

The Ceramic Materials Workshop (link below) has a page where they have tested many common commercial clays absorption rates. Many are not the same as what the manufacturer says. And, the clay that the supplier used for their tests may not be the same as what’s currently being mined. Remember, this is earth stuff deposited over eons. Not made in a lab to required specs. So… it’s good to test our clays.

Ceramic Materials Workshop

Checking your clay’s absorption rate.

NOTE: A common error regarding vitrification is to use a clay that says it’s good from cone 4,5,6 to cone 10. That clay will have a high absorption rate at the lower cones and thus leak.

Prepare a sample of your clay(s) to test. Small pots—thrown, pinched, folded slabs—that will hold water are best as you can use them later for another test.
Fire it unglazed to your normal glaze temp. If you have hot and cool spots in the kiln put samples in each labeled accordingly.
After the glaze firing, weigh each sample. You need a scale that can measure to .01 of a gram. Available on Amazon for $20-30. (The one I have is no longer available.) Write that weight down.
Then put the clay sample(s) in a pot of wat
er and boil for 3-5 hours. Different sources give different time directions.
When the heat is turned off on the boiling water put the sample(s) in cold water until they are room temperature.
Remove from the water and pat dry with a damp sponge. Careful not to use something dry that will suck a bit of water from below the surface.
Now weigh the sample(s) again and write that down with the pre-boil weight.
Divide the pre-boil weight from the post-boil weight. (The small number by the big number.)
That’s your clay’s absorption rate.

Summary

Remember the FDA is looking for 0.5%. The only clays that I see in the Ceramic Materials clay lists that are consistently under 0.5% at the advertised temperature are porcelains (Cone 5 & 10) and most cone 10 stonewares.
Some advertised Cone 5 clays are at 1% or lower at Cone 6.

I don’t know what the minimum absorption rate is for your specific clay and firing temp to not leak. You have to do the test.

Do you really HAVE to do these absorption tests? No. You can find out if your clay/glaze combos are leaking without gathering this info. But having the absorption test gives some parameters for testing the pots. If the absorption rate is 1% or less, you can quite sure your clay won’t leak with or without glaze. If it’s 3% or higher there’s a good chance it will leak.

How to check if your pots that hold liquids for long periods are leaking?

First take your samples and put water in the unglazed, glaze temp samples and leave them for 72 hours. It can take that long for water molecules to work their way a matrix of microscopic openings in the clay. You may not see a puddle, but the clay may be moist to the touch, or cool to the touch.
If the clay passes this test make liquid holding pots with glazes to test. To me the easiest is vases, though any other vessel will work.
The test is to put water in samples of specific clay/glaze combos and leave them for a minimum of 72 hours. It can take that long for water to work its way through any tiny cracks in glazes and microscopic pores in the clay.
If your kiln fires unevenly, test pots from the hot and cool parts of the kiln.
If you’re a hand builder, tiny cracks in the seams can cause leaking. Make sure all your seams are smooth to the touch. After verifying a specific clay/glaze combination I’d test every hand built pot that’s intended for long liquid containment overnight. Especially vases.
Any pot with a crack is going to leak faster, probably overnight, than an uncracked pot where water molecules slowly find their way through the glaze/clay micro-matrix. (I’ve see moist pot bottoms after 3 days with some clays.)

Summary

If your bare clay is vitrified and not leaking at all your firing temperatures, it should be fine with all your glazes. I’d still at least do the test for each glaze once.
If your bare clay is slightly moist after 72 hours you may get away with it if you have a really stable non-crazing glaze. I’d test this kind of combo with water for a week to be sure water molecules can’t find their way out.

Best Case

Non-leaking vitrified clay
Non-crazing glazes inside pots

Happy Potting,

Let’s eliminate leaving wet spots on customer tables.

Wyn Matthews

50+ year potter

805.207.3122

me@wynmatthews.com

www.wynmatthews.com

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