Low Fire Clays

[AndreaK]

Hello, 

New to the clay community. Question regarding low fire clays and glazes, are they suitable for mugs, bowls and planter pots? I'm not familiar with the different fires and glazes and so wanted to see if I were to use a low fire clay and glaze if it would withstand the use of say a mug or bowl that had liquid consistently in it if that would be alright. 

Thanks for any information

Andrea

[Callie Beller Diesel]

Hi and welcome!

It is possible to get durable ware at earthenware, midfire and high fire temperatures, but there are different considerations at each of those points. I won’t use the term food safe, because it’s a bit of a misnomer, and legally only covers whether or not the glaze has cadmium or lead in it. When we talk about glaze durability, we want to make sure that the clay is fired to a point where it would have less than 1% absorbion, the glaze doesn’t leach anything, isn’t crazed, resists both scratching and cutlery marking, and is easy to clean. 

With earthenware in particular, you have to be mindful of glaze fit, because earthenware clay remains porous when fired to maturity, and many will glaze the piece all over, and use kiln stilts instead of leaving a bare foot. You have to be extra mindful of your glaze chemistry, because the low temperatures tend to mean the fun, pretty glazes can sometimes be prone to scratching because they’re lower in silica and alumina.

[LeeU]

Just curious...why low-fire earthen ware rather than mid-fire stoneware?

[AndreaK]

The studio that I am looking into that could possibly fire pottery for me in my town only does low fire clays and glazes. There aren't a lot of options in my town for firing pottery so looking into all my options. 

[LeeU]

I came to the conclusion that low fire materials just didn't work for me for items needing to hold liquid..especially over time, and if given/sold to other people.  It meant  spending a fair amount of time (and a definate loss of money) learning how to make the chemistry/glaze ingredients/porosity etc. all working in conjunction. Mid-fire (or high) fire claybodies and glazes were way more likely to result in the desired maturity & glaze fit. I used low fire more for decorative ware that is not subject to freezing temps, no oven/microwave/grill, no liquids or wet food (like a tray for pretzels is OK) , or not something that will get lots of handling (depending on the object and it's purpose).  ALso did a fair amount of reading,  taking a course/class (today it is videos), to learn the basics before spending on trail & error. 

[Callie Beller Diesel]

@AndreaK I think you’re probably relatively local to me, so I assume we’re talking plainsman clay at least. Does the studio you’re looking at do commercial or homebrew glazes, and what kind of work are you thinking you’d like to make?

[C.Banks]

On 4/16/2024 at 10:32 AM, Callie Beller Diesel said:

... we want to make sure that the clay is fired to a point where it would have less than 1% absorbion...

I'm curious how accuarte or outdated my understanding is here.

I always thought under 1% was reserved for porcelain or porcelaneous clay and can overfire quickly.

Under 2% was a good benchmark for stoneware that allows a more broad firing range. I'm even less familiar with eathenware but from what little I know they are dpendent on good glaze fit for surfaces in contact with food.

I only took up my own clay a few years ago so I'm happy to find new answers. I've seen this 1% number before and figured it was time to ask.

Edited April 21 by C.Banks

[Kelly in AK]

It’s a good question. People end up finding their absorption happy place, some are unsatisfied with anything over 1%, others with 0.5%, I am good if the mugs don’t weep or get wickedly hot in the microwave. I believe that’s below 2%. The ASTM standard for vitrified is 0.5%.

My understanding is similar to yours, about firing range. I could be wrong about this, but my general observation is the lower the maturation temperature of the clay, the narrower the firing range. It’s certain the closer you get to vitreous the less wiggle room you have, at any cone. 

In his book Clay and Glazes for the Potter, Daniel Rhodes says, “Fired stoneware should have an absorption of 3 percent or less.” That’s on page 42 of my edition. I was taught, a few decades ago, that absorption below 2% makes reliable pottery but you should shoot for below 1.5%. Talking to potters over the years gives me the impression that the desired number has crept down since Daniel Rhodes wrote his book. 

Now, back to low fire, is it suitable for everyday use? Not without some careful consideration. There are a lot of variables, so many it seems like a subjective question. It depends on what you’re willing to put up with, or what you’re asking of people who use your pots. I use a lot of earthenware in my kitchen, I wouldn’t expect someone buying my work to deal with that, so I don’t sell anything unless it’s non-absorbent. The issues are strength, absorption, and glaze fit (which really goes back to the first two). The solutions are finding/making a clay body that works and then glazes that fit it. 

[AndreaK]

On 4/19/2024 at 11:12 AM, Callie Beller Diesel said:

@AndreaK I think you’re probably relatively local to me, so I assume we’re talking plainsman clay at least. Does the studio you’re looking at do commercial or homebrew glazes, and what kind of work are you thinking you’d like to make?

Yes I'm looking at using plainsman clay as it's made in my city. The studio that I'm looking at uses their low fire snow clay for wheel thrown pieces and then low fire glazes. They are saying that it is good for mugs and bowls, however everything that I am reading online is kind of saying the opposite. I am really looking to make pieces that would be food safe and useable pieces. I'd be able to do everything in my home other than the firing as I don't have a kiln. 

[Min]

@AndreaK, lowfire and earthenware clay has been used for centuries around the world for domestic ware. How durable this particular  body is would have to be tested, I know some of the lowfire talc bodies were very weak. In North America  many potters have gravitated to midrange for various reasons, one of which is it's possible to have wares that don't leak or overheat in the microwave even if the glaze has crazing or pinholes and it's fine to leave the bottom of the pots unglazed. For lowfire (including earthenware for this conversation) in order for the ware to not weep / leak when used for mugs etc it is necessary to completely glaze the pot. (I do know someone who uses terra sig on the foot but that's another topic). A huge advantage of using lowfire / earthenware is the energy savings when glaze firing and the wear and tear on the kiln elements etc. 

Plainsman supplies really thorough info on all their claybodies. For Snow the link to it is here, click on each of the subjects and it will expand to give the details. For Snow the absorption figures are below. You can see that it has a huge absorption of 28% when fired anywhere from  cone 06 all the way to cone 2. You can also see that the absorption doesn't drop below 12% even when fired to cone 6. What this means is there is never going to be enough fluxing of the clay to vitrify it enough to be leak proof without a glaze. 

Functional ware made from this clay will need to be stilted when glaze fired. From the Snow page (linked above) it looks like Spectrum 700 glaze fits this body well with Snow being bisque fired to cone 04 and glaze fired to 05.