"functional" Low Fire Clay

[clayshapespottery]

I'm thinking of switching from the cone 6 porcelain I use,  to a lower fire clay for the summer as I'll be working at a satellite studio at my cottage where electricity is extremely expensive. I'm in Toronto - and am thinking of using one of Tuckers more sturdy low fire clays, (a red one) which the notes say is just as functional as stoneware. What are people's thoughts about "functional" low fire clay? I don't have access to as wide a range of clays as many US potters do - but we have a pretty good selection from two different suppliers here. I'd love to know general experience with functional low fire clays, and of course, any advice from someone local to the Toronto area who has used locally available clays, would be appreciated.

[neilestrick]

Any clay can be used for functional work, but the issue with most low fire clays is that they are still porous after firing. This means that if there is any crazing of the glaze, the pot will weep moisture. That can lead to ruining people's furniture, or nasty stuff growing under the glaze. With good glaze fit and handwashing, earthenware pots can be 'functional' for years to come.

 

As for the Tucker's clay, I would ask them what they mean by 'just as functional as stoneware'. Do they simply mean it's non-toxic, or do they mean that it actually vitrifies?

[clayshapespottery]

Thank Neil. There are earthenware clays now that are more functional, and sturdier than they used to be - as in good glaze fit (as you say) and even dishwasher safe. I was at Tuckers this morning, and discussed it with them and feel confident their clay is a good fit with the commercial glazes I use for low fire work (Spectrum, and Tuckers own glazes) - to avoid what you are describing. I'm going to test this new clay, that I haven't used before. BTW - years ago when I did my first pottery course, I used a low fire clay in the class to make a set of small plates, and glazed them with the Spectrum glazes provided in the class. I still use these plates almost every day at my house - and always put them in the dishwasher. Nothing wrong with them at all. Good glaze fit, no crazing, and sturdy as can be. I guess I'm answering my own question.

[Mark C.]

I do not think of low fire clay having the strength to hold up in the long run.

That is Daly use for decades. All my low fire clay functional items long ago chip or broke.

Yes you can baby them in the dishwasher but the test of use will gauge this story better.

My thought is all the time you put into them is the same no matter what clay body and end firing them.

I know cone 6 is more durable like cone 10 clays

The real test will be what's left in 30 years from your efforts.

I already know the answer

I,m with Neil on this one

 

I have a friend who was a full time low fire potter in the 80s

He quit pottery in the 90s

The only work left is mantle pieces for him as all the work over time gave up thru various weaknesses

[MatthewV]

I suspect that even at the higher electric rates, firing to ∆6 is still economical.

 

I personally would use the opportunity to try raku and pit firings and explore a different direction.

[neilestrick]

The cost difference between firing bisque or cone 6 is only going to be a couple dollars per firing. The main cost difference is replacing the elements, since you get 2-3 times as many firings when doing low fire. Either way it's pretty cheap, though, when you calculate the cost per pot.

[bciskepottery]

http://smokieclennell.blogspot.com/

 

For a short time, Tony Clennell was working with a low-fire red clay from Tuckers; he might be able to give you some advice. Tony is up around Ontario. He is a wood-firer; gave up on earthenware/electric kiln and returned to the wood kiln. Email him and he might offer some first hand experience. He is a good friend of the owner of Tuckers.

[perkolator]

The cost difference between firing bisque or cone 6 is only going to be a couple dollars per firing. The main cost difference is replacing the elements, since you get 2-3 times as many firings when doing low fire. Either way it's pretty cheap, though, when you calculate the cost per pot.

This 100%

[glazenerd]

I do not know the clay, but I do know the man who formulated it. He would have thought about clay particle size/s, and compensated for that unless he had to hit a certain low cost price point. Best route is to buy a minimal amount and test it for vitrification and absorption. No need to go through the usual test protocols; but smply fire a 4 x 4 to recommended temp without glaze and check it. Weigh it before you boil it in water for two hours, pat it dry and weigh it again immediately afterwards. Any increase in weight means it is absorbing water, which also means it is not vitrified. If you like the clay and want to fix the issue, then you can wedge in 3-5% molochite (325m): which is super fine particle size. Extra work, extra time: have to decide if its worth it.

Nerd

[clayshapespottery]

thanks everyone - lots of good advice here and in the end, you are all probably right that I shouldn't worry too much about the extra cost of firing with cone 6. My original plan was just to bisque at this satellite studio, so I could transport the pieces home with less chance of breakage...but I know that I won't be able to wait until I go home! I'll want to glaze fire out in the country too. And yes - Neil - the element wear and tear is a major concern too - as out in the country replacing things like that is a pain. But I have bought some of this low fire clay and will test it here in the city first. Am definitely thinking about getting set up for raku too, out at the cottage - can't do it in the city. Looking forward to playing with new clays. I got a speckle stoneware today too, which I've never used - it's definitely going to be fun to try new things.

[Stephen]

I am curious, what is the difference in cost? Is it a few bucks a load or ?

[clayshapespottery]

Stephen  - I have no idea since I haven't done any firing yet, but power is expensive where my cottage is - double what it costs for power in the city. Wear and tear on elements is another concern - actually, the biggest concern. Low fire is much easier on elements

[Stephen]

I figure a blended average of 100 firings between bisque (1/3rd) and glaze (2/3rds)and a set in my kiln is about $400 or $4 added to about $10 electric for cone 04-6 firings in our 8.7cf kiln. We get about 50 pots out of a glaze load so that's 65 glaze firings for a little over 3000 pots for the $400 elements. That's about 13 cents a pot for the elements. That seems OK. Bunch of large pots cost more but usually I would imagine it all averages out.

 

The electric savings I guess I could see as a better reason but it's still such a low cost savings it seems like it would be better to go for the strength of cone 6 for functional ware. Maybe Tuckers was really just touting the strength of their low fire clay more than recommending actually using it for functional pots.

 

When a functional pot fails it a real pain in the A$$ for the person using it. Either something is going to get ruined by liquid seeping all over everything or its going to break while using it, ruining whatever was in it and maybe even injuring someone. Who knows if the person using it down the road has any idea its low fire and just assumes it can stand up to any use their other functional pottery can.

[Mudslinger Ceramics]

Do you have sales orders to fill with your C6 porcelain?  If so, you'll have to wear the expense of firing while you're at the cottage.

 

If you don't have to think of orders...... then leave the porcelain at home and spend the summer experimenting with all the low fire techniques, clays and processes that you don't get to do normally.....and not just more of the same stuff you already do.

Try raku, black firing, pit and bin firing, local clay slips and engobes, burnishing, hand building.  Maybe build a small wood fire kiln and see what happens to your C6 clay and glazes when they're exposed to a live flame.  

Make sculpture, wall plaques, ornamental pieces, garden pots (where porosity is a virtue!)

 

You know......lemons to lemonade.   Couple of months, out of town and experimenting with all the techniques and processes you don't normally use......sounds good to me.

 

Irene

[clayshapespottery]

Great advice Irene! Of course I would not use low fire clay to fill porcelain orders - but you are correct that I want to experiment and have fun this summer. Really looking forward to trying some new clays and ideas - and maybe even raku!

[dee3]

I read this the other day, thought it might be of interest.

 

 

This is a very relevant thread about fired clay strength (MOR). Please have a read.

From: Ceramic Arts Discussion List [CLAYART@LSV.CERAMICS.ORG] on behalf
of Pete Pinnell [ppinnell1@UNL.EDU]
Sent: Thursday, December 20, 2001 11:15 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: clay body strength

For the final project in my Clay and Glaze class this semester, we mixed about 50 clay bodies for testing, including red and white earthenwares, stoneware, porcelain, and sculpture bodies. Besides other tests, we extruded numerous bars of each body and broke them to measure MOR (Modulus Of Rupture, which is a measure of the bending strength). There are other strength tests that can be done (chipping tests, for instance), but MOR is a quick and easy way to predict how well a body will hold up to the bumps of everyday use.

Out of all these tests, there were a number of interesting trends:

1. Any amount of grog weakens clay bodies, especially in sculpture
bodies that are essentially underfired. Some of the sculpture clays were so weak at cone 04 that we couldn't measure them- the bars broke at initial contact before any stress was applied. Any texture in the clay tended to have the same result, though the texture from using 50 mesh fireclay seemed to have only a minimal effect. Really fine grogs- those less than 80 mesh- also had little effect.

2. Glaze made a huge difference in strength. Crazed glazes lowered results 50% or more from the strength of the same bar unglazed. Uncrazed glazes raised the strength of the bars from 50 to 100 %. I had read this before, and assumed that it was mostly related to the lack of surface flaws on a smooth glaze (cracks like to start at a flaw- take away the flaws and it's more difficult for a crack to start). What I found interesting is that the amount of compression also mattered. We glazed the porcelain bars with three different versions of my Pete's Clear glaze, which ranged from mild compression for the original version to a very low expansion version that places the clay in a very high compression. Consistently, the higher compression versions produced higher MOR results.

3. Clays have to be fired to maturity to get good strength. Even firing porcelain bodies to cone 9 rather than 10 lowered strengths a good deal. As an aside, I define maturity as the point at which a body achieves its best strength and glaze fit, and no longer suffers from marked moisture expansion. Absorption, in my opinion, is not a good indicator except within one clay body group (such as "high fire porcelain"). Porcelains may need to have less than 1% absorption to avoid moisture expansion problems, while mature white earthenwares can have upwards of 20% absorption (which is why those cheap white tiles on our shower walls don't develop delayed crazing).

4. "Smooth" counts for more than "glassy", which seems to contradict one bit of standard wisdom I've heard in the past.

5. Quartz seems to be a problem- at least in a minor way. Porcelain
bodies that used a combination of pyrophyllite and quartz were stronger than those which used only quartz as a filler. It's a bit of a mixed bag, though, because glazes on pyrophyllite bodies tended to craze more.

What were the strongest clays? This will surprise you- it certainly did me. The strongest clays, consistently, were (drum roll, please) red earthenware clays fired to a full cone 04.

Yep, that's right. Plain old Redart based, smooth red earthenwares. They were stronger than smooth, brown or gray stonewares, and even stronger (over all) than porcelain, which I had assumed would be best.

Yes, it was very important to fire them to a full cone 04: cone 06
didn't hack it. Surprisingly, taking them to cone 1 did not increase
MOR, though they certainly were denser and felt more solid and chip resistant. Within red earthenwares, we got consistently higher strength from those using wollastonite as a secondary flux (5 to 10%), rather than talc. It seemed best to use red clay in amounts of 50 to 70%, and while Redart alone (for the red clay portion of the body) gave the best strength, we got much better workability (and only a tiny bit less strength) by using a mixture of red clays, such as Redart mixed with Ranger Red (from Texas) and Apache Red (from Colorado).

As with porcelain, the clay was made much stronger with glazes that fit, and higher compression glazes were strongest of all. Our all-time champion (for strength, NOT workability) was the following recipe, glazed with Linda Arbuckle's Majolica and fired to a full cone 04.

Redart, 60%
KT 1-4 Ball Clay, 30%
Wollastonite, 10%

I thought you might find this interesting. I only teach a Clay and Glaze class one semester every three years, so while I plan to do some follow up tests (these tests raised as many questions as they answered), don't look for those results any time soon!

Pete Pinnell
University of Nebraska at Lincol

[momvon]

do i need to leave the top open on my kiln for raku firing like i do for ceramics when firing to bisque