Lowfire White Clay Absorbs Water from Dishwasher?

[Leaman Pottery]

I am experimenting with the maiolica watercolor decoration technics and decided to use white clay. Got Standards LF White 100 rated at 04-06. The mugs came out fine, and were great in the microwave the first use out of the kiln, but every time I've tried to heat up my tea in the microwave since then the handle gets so hot I can't touch it. Obviously I can't sell. Used Arbuckle firing temps and base glaze. Bisque to 04 - Glaze to 05. Is this problem normal? Is there a fix? The mugs are cut directly off the bat, so no foot ring. 

[Min]

Lowfire clay typically has absorption in the 15% range (+/- a few %). This means any surface not covered in a well fitting (non crazing) glaze is going to absorb moisture. Glazing the bottoms of the pots then firing on stilts and smoothing the stilt marks off is one way to go. I have a friend who works with earthenware, she uses terra sig on unglazed areas. Some lowfire clays can tolerate a higher firing than ^04 which might help tighten the body up to reduce the absorption. Nice brushwork on your pots.

[Leaman Pottery]

Thanks for the kind words regarding the brushwork. Ahh. Linda Arbuckle used terra sig on her foot rings at a workshop she taught, and I attended. Now I know why. I will give that a try. Thanks so much for your help.

[Denice]

I made quite a few  exterior murals using low fire red terracotta,  I used a Cone 1 Majolica glaze recipe.  The red terracotta was vitrified at Cone 1,  we have icy cold winters and I needed to make tiles  that wouldn't freeze and break.  That clay is no longer available but you might test some white terracotta and see is you can tighten it up.  If you need the glaze formula you can contact me.     Denice

[Min]

Terra sig will help reduce the porosity of the clay but if you are making ware that can go in the microwave then test it to be sure it’s adequate.  Might need a combination of things, firing higher (if tolerated by the clay) plus Terra sig , or consider glazing the pot all over and stillting  it and/or having a footring and glazing inside it and terra sig the foot. All clays have considerations, its a question of testing what works for yours. 

[Rae Reich]

I have had the same microwave overheating result with commercial low-fire pieces, even when they are glazed all over. I don’t think that the problem is with retained water. My theory is that the clay is less dense than high-fire so the heat in handles dissipates more slowly and plate rims stay untouchable longer too. Part of my reasoning comes from noticing that glass measuring cup handles and Pyrex handles (dense) can usually be grasped bare-handed. 

Bottom line - put a microwave warning on them - Something like, “Use Caution When Removing From Microwave” like on microwave food.
I don’t think there’s a clay body remedy. 

[Leaman Pottery]

Since I can't put the warning on the ware, I guess I can add it to the back of the hangtag and that will have to do. I am also going to fire on stilts. Thanks for the input from everyone.  Appreciate the time you took to review the question and reply. 

[Pyewackette]

I read somewhere (of course I can't remember where but it was recently) that adding a small amount of frit to an earthenware body would improve vitrification.  Of course they didn't say, or if they did I've forgotten, what a "small amount" is nor whether you would be expected to fire at a higher temp that might melt the clay.  Something like frit 3124 vitrifies at lower than cone 04 - would that help or would you need something that vitrifies closer to actual cone 04 to help reduce absorption in a cone 04 clay body?  Or is it mythology?

Quote

This is a USA pottery frit. Ferro now calls it Frit 3124-2.

This borosilicate frit is high in calcium. It melts are very low temperatures and among the most useful of all common frits because of its glaze-like balanced chemistry. This frit has a chemistry somewhat similar to 3134 (the latter adds CaO, Na₂O and B₂O₃ at the expense of all the Al₂O₃ and some SiO₂.

 

[Min]

For sure you can make a fritware body with zero  or as low absorption as you want, although it's an expensive way to make a tight claybody. If you want a rabbit hole to go down here might be somewhere to start. https://digitalfire.com/glossary/zero3

[Pyewackette]

@Min I don't want to go to zero absorption, but I would like to tighten up an earthenware body up enough that it doesn't actually drip.  I'm actually not sure where this body is functionally yet on that spectrum.  I've got some of it slaking in a bucket right now. It had languished at the studio for at least a year, I had repeatedly asked for it to be pugged back into throwable shape but apparently they were going to throw it out so I asked for some of it and brought it home.  Dry but not totally dry.  Too dry to work.  Not sure when it'll be ready to wedge up so I can throw some of it.  I WANT it to be permeable to water, but not actually drippy.  Is there another way to reduce absorbency that's maybe a little more practical? 

What might happen if you mix low fire with a higher firing clay like stoneware? I would imagine there would be issues of what cone you could actually fire it to - I wouldn't think that would help if you were still firing to cone 04 because the stoneware won't vitrify until cone 5ish... not that I have any real knowledge on the subject, just doesn't seem logical to think that would help without also manipulating the firing temp and then what would happen to the low fire part.

I have searched assiduously for information on mixing clay bodies and I keep coming up empty.  Sure, test tiles would be in order (or test forms at least since I'm interested in jar-like forms that will release water but not drip) but where to start ...

Remember I'm in a clay desert here so I have very limited (if any) access to different clay bodies.  My only choices locally are the studio bodies and apparently they're phasing the red earthenware out.  Even the white earthenware hasn't been in workable condition since I've been there, starting last Sept.  It occasionally appears in the throwing room but not in workable condition.  It's in there again right now after another extended absence - no idea what shape its in, I didn't check when I saw it, I'll check later today. 

So right now there is only the generic white stoneware, the white earthenware that hangs around without actually being usable, and the red earthenware that's in even worse shape than the white.  We haven't had the studio version of B-mix since last year though its still on the list, and the white earthenware just comes and goes without actually being workable.  The red earthenware is apparently slated for disposal and I intend to try to bring it ALL home if they haven't already tossed it (which they hadn't as of Friday).  I'm going for an unglazed jar form that will release moisture but not to the extent that it actually drips.

EDIT:  BTW I hadn't looked at the link you provided when I first answered and I just went to look at it again.  It looks like that might be REALLY helpful. At least interesting, LOL! Thanks!

[Min]

Okay, from the link on Fritware I posted Hansen mentions it took 5% Ferro 3110 (which is a high sodium frit) added to an earthenware (red clay) body to tighten it up adequately and 20% for a white lowfire body. To test your scrap earthenware I would dry out some of clay thoroughly then weight out 200 grams. Add 10 grams of frit (5%) and slake down the clay with the frit into a slurry then blend it up really well. Pour the slip onto plaster and when it’s dry enough make a test  bar.  After firing measure the absorption and go from there insofar as increasing frit if necessary to get an absorption you are happy with and not overfluxing it causing bloats and brittleness. Adding just 5% frit shouldn’t effect plasticity of the clay too much but more than that and you might need to add some bentonite. 

BTW bone dry clay will slake overnight, much quicker than slaking damp clay.

[Pyewackette]

28 minutes ago, Min said:

bone dry clay will slake overnight, much quicker than slaking damp clay

TOO LATE!  I totally forgot that fact.  However there is probably another 100 lbs or so of the stuff at the studio that I'm hoping to lay hands on.  So I guess I'll slice that up (if they'll give it to me) and let it dry out totally and then slake that separately.

I can't figure out how to join a group - it seems insightlive doesn't let you do much of anything without a subscription?  It's cheap enough I guess ($36 for almost 2 years, why 720 days instead of 2 years??? LOL!) but I'm not sure how much I would currently benefit from a subscription.  I did bookmark the recipe page and I'm checking on semi-local availability of the materials (semi-local being 5 to 7 hour drive, or 12 if I count Baton Rouge.  Well there's blues there too!).

I don't think there's any question at present of me mixing up a clay body from scratch until I get my Peter Pugger, so that may be getting accelerated.  I WAS going to try to put that off until I got my Big Kiln.  But now it seems I have a reasonable need for it sooner rather than later.  The little kiln is likely 2 or 3 months out, I expect about the same on the pugmill.

Thank you so much for your help!

[Callie Beller Diesel]

Insight live is a glaze calculator that uses Digitalfire as a database. It’s been around a long time, so I tend to favour its database. The amount of free stuff available on that website is a gold mine. The countdown is so you know if you’ve still got a week before it’s time to update your subscription. You can always access your recipes, you just can’t alter them in the calculator if it’s expired. There’s more background information on a lot of materials and their providence on digitalfire.

I have a subscription, and if you’ve seen Min post recipes or comparisons here, that’s the interface. I find the interface a bit dated, and you have to know what some of the numbers mean to know why they’re significant. Its manual is a bit clunky, tbh, but does offer good working examples. I find I have to speed up the videos because the slow pace drives me nuts. It’s got a better layout for comparing glazes to limit forumulas, or to other glazes. If you have a good working knowledge of glaze chemistry, having easy access to limit formulas to compare to is nice, and you don’t have to import materials data into your library to start working: it’s all there.

 But. If you’re looking for an effective, user friendly free calculator to get started with (there’s a subscription level that offers more information), Glazy is a gentler introduction. But for some materials, you have to download the info into your library from theirs, especially if you’re working with European materials, or some specific material analyses (like older Custer). The background info on the materials isn’t there, but they do have Stull charts, which I find I like. The info on how to read them properly is in the help links. 

[Pyewackette]

@Min Really what I'm looking for are materials to help me understand what the calculators are showing me.  Also to better understand the properties of different clay bodies and their components.  I've been circling the edges of ceramics for 50 years and I only yesterday discovered that everything I thought I knew about low fire earthenwares appears to be nonsense - the whole it'll melt over cone 04 and you can never use it to make utilitarian ware because its fragile and it leaks appears to be patently untrue (or at least only true as long as you are restricted to very low firing temps).  I mean it didn't seem logical to me, if earthenware was always so fragile how did anybody ever manage to cook anything in it or store anything? 

I think I need to know more about, well, pretty much EVERYTHING before a calculator would be of much use to me.

[LeeU]

Nope--sorry--that will not do!    If I bought a mug or if a friend gave it to me and the handle was too hot coming out of a microwave, no tag or card or sticker (long gone of course) would justify selling or gifting the item with that known issue. Suppose a little kid was getting mommy's tea for her? I can see it now--drops the cup and the scalding water hits her bare feet.  Or a breaking piece  flies up and nicks her leg--not as far-fetched as it sounds--my ankle tendon got cut that way.

On 4/24/2022 at 8:58 AM, Leaman Pottery said:

I guess I can add it to the back of the hangtag and that will have to do.

 

[Min]

2 hours ago, Pyewackette said:

@Min Really what I'm looking for are materials to help me understand what the calculators are showing me.

This is a really good question! I'm going to start a new thread using your question for the post so it doesn't get lost in this thread. 

Re your claybody question, "Clay Materials - for the Self-Reliant Potter" by Hendrik Norsker is a good place to start reading. It's written for people who are interested in exploring their own claybodies but it also speaks to the types of claybodies potters use, the materials that are used in them, the origins of the materials etc. It's available free to read here.

[Rae Reich]

On 4/25/2022 at 12:47 PM, Pyewackette said:

only yesterday discovered that everything I thought I knew about low fire earthenwares appears to be nonsense - the whole it'll melt over cone 04 and you can never use it to make utilitarian ware because its fragile and it leaks appears to be patently untrue (or at least only true as long as you are restricted to very low firing temps). 

Dear Pye - the way I learned anything about earthenware was to make stuff, make assumptions, make mistakes, learn stuff, take notes! Your experience and observations are making sense of technicalities and limitations  (as your parenthesis shows).