For food safety and durability, I use clays that meet or come as close as possible to ASTM International’s definition of impervious. I also generally avoid crazing or crackling in my glazes.

In moving to Maine this year, I’ve had multiple reasons to switch clays, so I’ve been researching and testing new clays. In this post, I’ll explain why I look for the clay characteristics I do, and what I’m finding so far this year.

1. Introduction

2. Clay absorption rate

3. Coefficient Of Expansion (COE): crazing

4. Workability and warping

Introduction

Food safety of ceramic pieces is determined by the continuity or unbroken-ness of the glaze layer between the food and the ceramic body; and the absorption rate of the ceramic body itself.

I ran into a gross demonstration of these factors immediately when I started making pieces for sale. I was testing a new coffee mug, and was appalled to find coffee not only staining a network of glaze cracks (“crazing”) on the inside of the mug, but seeping all the way through the walls to form tiny black droplets at the crazing intersections on the outside of the mug. *Eww.*

There were two problems at work in that mug: glaze crazing and clay body absorption rate. I’ll cover them in the opposite order, because I see clay absorption rate as most important.

Here’s my chain of logic, which I’ll detail in the sections below.

With a continuous (non-crazed) glaze surface, liquid would never reach the clay; properties of the clay wouldn’t matter.
But it’s very difficult to ensure that pieces will not craze long-term, not to mention that many people like the appearance of crazed glaze. In fact, whole families of glaze are formulated intentionally to craze.
Ceramic ware can be food safe even with crazed glaze, at least if the underlying ceramic is impervious (very low absorption rate), based on testing by Dr. Ryan Coppage.
Even if the underlying ceramic also absorbs liquid, Dr. Coppage has indicated to me via email that dishwasher cycle temperatures kill all bacteria, even in the clay itself. But that’s where I personally draw the line – many people don’t use dishwashers, and moisture getting into the walls of the piece is also problematic for other reasons.

In short: avoiding crazing removes the concern, but is unreliable and not always desirable – so I view very low clay absorption rates as mandatory.

Clay Absorption Rate

To avoid absorbing water and other liquids, the clay itself needs to have a low absorption rate once it’s fired.

How low? Unfortunately for both ceramicists and consumers, there’s a large disconnect in the handmade ceramics world. It’s clear that minimum absorption is good, but very few clays meet the lowest absorption numbers. As a result, craftspeople have different absorption rates as goals (0.5%; 1%; 3%), and many commercial clays (i.e., clays that individual craftspeople generally buy, as opposed to the clays ordered or forumulated in-house by ceramics factories) don’t even meet the least-stringent 3% bar.

Seattle Pottery Supply describes the range succinctly:

There are three water absorption classifications as defined by the American Society for Testing and Materials (ASTM): Impervious porcelain, vitreous porcelain, and ceramic. Impervious porcelain has less than 0.5% water absorption rate, vitreous porcelain has 0.5-3% and ceramic has 3-7%.

When you are making dinnerware (plates, cups, bowls) that will come in contact with food you would want a clay with the lowest absorption rate possible (<.5%) This is because clays with a low absorption rate will not absorb water even if there is a bad fit between the clay and glaze (cracks in the glaze).

When making work that will hold liquids but not necessarily food (vases, bird baths, fountains) an absorption rate can be slightly higher (.5-3%). While the clays have a higher absorption rate they should not absorb enough water to seep through the piece onto the table.

Here’s a key quote from Davidh4976 at ceramicartsdaily.org (emphasis mine), who followed up with ASTM:

I finally got around to asking ASTM and received a reply from them. While they do have standards for how to test for absorption, they do not have any standards covering the amount of absorption that is appropriate for functional ware or tableware that comes in contact with food or for any usage of ceramics.

The FDA, U.S. States, and other countries, have standards that say tableware in retail food services need to be non-absorbent, but that includes considering the effect of glaze on the surface to make it non-absorbent. They also say that there should be no crazing. All of which bypasses any specified requirements for absorption of the clay body.

Based on these guidelines and my “Eww” experience above, I look for clays that meet the most stringent ASTM classification: less than 0.5% absorption when fired. Even looking at clay companies’ published COEs, these clays are surprisingly difficult to find: only porcelains and a very few stoneware clays pass this bar.

Additionally, independent clay testing by a new group called the Ceramic Materials Workshop (CMW) finds that many clays have higher absorption rates than the numbers published by their manufacturers. So ideally, I’m looking for clay that’s been tested and verified by CMW.

Coefficient Of Expansion (COE): Crazing

Crazing is the pattern/network of fine cracks in glaze on a ceramic piece. It’s often very attractive, and in many cases produced intentionally.

Research by Ryan Coppage, PhD on crazing shows that ceramic ware is food safe even with crazed glaze: “the use of heat, soap, and water kills or removes bacteria in even crazed surfaces, though they are commonly (and mistakenly) thought to be totally non-food safe.”

Crazing is caused with the clay and the glaze of a finished/fired piece have different expansion/contraction rates. This is not the same as the shrink rate of the clay; I’ve heard many potters mistakenly blame the shrink rate because crazing may already be visible when a piece comes out of the kiln. The relevant characteristic is the Coefficient Of Expansion (COE, or sometimes CTE for Coefficient of Thermal Expansion), which describes how much a clay or a glaze expands or contracts based on heating and cooling. This explains why a COE mismatch between clay and glaze can cause crazing later in the life of the piece, e.g. after dishwashing cycles.

To minimize crazing, I’ve read that the COEs of the clay and glaze should be within 1.25 of each other. It’s difficult to calculate the COE of glazes, but they generally start at around 7 and go up from there. So, I try to find clays with a COE of 5.75 or higher.

I avoid glazes that are formulated to intentionally craze, e.g. many celadons. But in order to support as wide a range of glazes as possible – until/unless I get to the point of mixing my own glazes and calculating their COEs – I minimize the likelihood of crazing by looking for clay with a high COE.

Workability and Warping

I make thin-walled, precise pieces. Wheel-throwing these pieces requires a delicate touch and careful trimming, and the resulting pieces can be severely impaired by warping. Different clays vary widely in terms of how easy they are to throw, trim, and handle while drying, and how readily they warp.

Here are some clays I’ve worked with this year, with my brief notes/reviews.

Laguna BMix (^5, ^ 10, and wood-fire; all without grog)

I used BMix because it was readily available at both studios I inhabited at the beginning of this year. I’ve just thrown a box of the wood-fired variation for a wood firing, but otherwise I’ll be trying to avoid it because of technical properties and frustrating workability.

BMix Cone 10: COE is slightly lower than I’d like; absorption higher than I’d like:

COE: 5.2
Absorption: Laguna lists as 1% +/- 1% (so, as much as 2%), and Ceramic Materials Workshop tests it at 1.42.

BMix Cone 10 Wood: good COE, absorption OK

COE: 5.8
Absorption: 1% +/- 1% (no CMW test)

BMix Cone 5: COE good; absorption too high:

COE: 5.75
Absorption: 2.3%; CMW much higher at 3.95%.

All three variations throw similarly, and are relatively unforgiving with a major “cream cheese” factor. They are very sticky, take on water readily and generate masses of thick, sloppy slip.

Pieces tend to warp as they dry, even if regularly rotated. I have only been able to achieve precise round forms by periodically straightening them during drying, drying bowls and plates rim-down, etc.

BMix is a little boring when fired, but can flash well in environmental firings.

BMix ^10 with nice flashing from a soda firing at Watershed Center For Ceramic Arts, Jan 2025

Laguna/Axner, #900 ^10

Good workability and absorption; worryingly low COE

COE: 4.2. I’m concerned about this long-term; it’s low enough that I expect to see crazing on this clay with glazes that don’t normally craze. That said, my favorite clear Dolo-Matte glaze and Malcolm Davis shino recipes both seem to be craze-free on it so far.
Absorption: Laguna lists as 0.6, and Ceramic Materials Workshop finds it to actually be lower at 0.34. That’s really good- better than many porcelains.

I find 900 to be a normal, relatively friendly stoneware. It has enough grog to hold its shape well during throwing, and I didn’t have to do much to manage warping; pieces tended to stay round/flat while drying and stay that way through firing.

The color is a bit odd/frustrating: it’s milk chocolate brown when wet, but fires a mid-light gray with tan/brown edges in reduction. My color results match CMW’s tests, and both are very different from the dark gray/brown shown on the Axner website.

KY Mudworks, Roo ^6

Ok workability, ok absorption, hopeful on COE

COE: unpublished, but hopefully high: CMW tested the similar-looking Tony Beaver at 6.56.
Absorption: KY lists as <1%. Unfortunately, I’m suspicious of this number based on CMW’s testing of Tony Beaver, which KY lists as <0.5% but CMW tests at a whopping 4.5%.

My box of Roo was particularly wet, and I found it awful to work with at first: very floppy and relatively sticky, a terrible combination. I had better luck once I let it dry and firm up a bit. Trimmed well, and fired nicely at cone 6.

Laguna, Porcelain 16 ^6

Excellent numbers, but sticky, tricky, warpy.

COE: 5.76
Absorption: Laguna lists as 0.12%; CMW tests at 0.29%.

I used this clay last year and early this year because the numbers are so good, and because Laguna’s tech expert told me it is their most durable clay. But like many porcelains, it’s a pain to work with. It tends to form a dry, crack-prone skin very quickly, making handbuilding techniques difficult. It’s extremely sticky, making it challenging to throw evenly: I had to use a lot of water to keep thin walls from grabbing my fingers and folding/tearing, in turn necessitating frequent passes with a rib to remove the slip/water layer. Even trimming was challenging: if even slightly too wet, pieces tend to grab the trimmer.

I find that my P16 pieces slump/sag when fired to cone 6; I have better luck firing it to cone 5, but that could cause it to miss its low absorbency number. Slightly gray when fired; not as white as some porcelains. Fired surface seems to be susceptible to crystallization especially if used in slow-cooling firing cycles e.g. crystalline glazes; even burnished surfaces got an unpleasant toothy feeling that I felt the need to sand down.

Laguna, Frost Porcelain ^5

COE: not listed
Absorption: not listed

I tried Frost briefly last year because it was recommended to me for my Bones sculptures, based on its whiteness and nice fired finish. It is beautiful when fired, but I encountered severe problems with cracking. 14/17 of the sculptures I made, as well as several of my thrown pieces, developed cracks anywhere between initial drying and final ^5 firing, in unexpected locations. So much for that.

Standard Clay, 551 V.P. Porcelain

Fancy name, apparently good numbers, good workability

COE: unpublished, but CMW’s tests of two other Standard porcelains indicate it’s probably 6+.
Absorption: Standard lists at 0.3%. CMW hasn’t tested this one, but their tests of other Standard ^6 and ^10 porcelains closely support Standard’s numbers.

I was pleasantly surprised when I tried switching from Laguna P16 to Standard 551 – it’s significantly easier to work with, mainly in being less sticky. This means less water required while throwing, which means less floppiness and less need to do slip-removal passes. I was able to do shaping pulls even without water on the surface. Similarly (and I think as a result), easier to trim even when slightly wet. Also, it’s much less prone to forming the cracking skin, so handbuilding tasks are much easier.

Tends to warp while drying, but so far I’ve found it to respond well to straightening – in fact, I even had a large bowl that warped during bisque firing un-warp during glaze firing and come out perfectly straight/round. So far, excited to keep using this one.

Michelle Wen also has a useful review of 551 here. Thanks Michelle!

Clay Research and Testing, 2025