3 questions about kiln wash, and 1 about grog

[Claybot]

Do I need to use kiln/bat wash on a bat if this is my process: fire my dry greenware pot; paint the pot with underglaze; fire the pot again? I thought maybe the kiln wash on a bat might only be necessary if a pot was covered with glaze, not underglaze. (Low fire clay and underglazes, if that makes a difference.)

 

If a bat already has kiln wash on it and my pots are only underglazed, never glazed, do I need to sand the bat after the underglaze firings and paint on new kiln wash? I imagine the bat would eventually be covered with lots of rings if I didn't, like coffee mug stains, but is that a bad thing? (Low fire clay and underglazes, if that makes a difference.)

 

If I wipe the kiln wash on my bat with my hand, a cloud of powder puffs into the air. Is that normal, or did I do something wrong when I mixed it?

 

Which is harder and less susceptible to chipping: clay with grog or clay without grog? If the answer depends on the clay, then low-fire terracotta clay.

 

Thanks for your help!

[Chris Campbell]

Do I need to use kiln/bat wash on a bat if this is my process: fire my dry greenware pot; paint the pot with underglaze; fire the pot again? I thought maybe the kiln wash on a bat might only be necessary if a pot was covered with glaze, not underglaze. (Low fire clay and underglazes, if that makes a difference.)

 

I am asumming by 'bat' you mean a kiln shelf?? If you are not glazing the work you don't need it. Kiln wash simply absorbs glaze drips and makes them easier to clean off. It also helps high fired porcelain move and not stick to the shelf.

 

If a bat already has kiln wash on it and my pots are only underglazed, never glazed, do I need to sand the bat after the underglaze firings and paint on new kiln wash? I imagine the bat would eventually be covered with lots of rings if I didn't, like coffee mug stains, but is that a bad thing? (Low fire clay and underglazes, if that makes a difference.)

 

Same answer as above ... If you are not glazing or high firing ...no need to apply kiln wash.

 

If I wipe the kiln wash on my bat with my hand, a cloud of powder puffs into the air. Is that normal, or did I do something wrong when I mixed it?

 

You probably applied it too thickly.

 

If you apply underglaze to the bottom of your pots you might want to use kiln wash as the color sometimes transfers onto the shelf.

[Claybot]

I am asumming by 'bat' you mean a kiln shelf?? If you are not glazing the work you don't need it. Kiln wash simply absorbs glaze drips and makes them easier to clean off.

 

 

Thanks for answering, Chris Campbell. Yeh, I meant kiln shelf. I had a feeling that I didn't need to use the kiln wash, so I'm happy to know I can stop. Yay, no more going outside and sanding white powder onto the lawn.

 

If I wipe the kiln wash on my bat with my hand, a cloud of powder puffs into the air. Is that normal, or did I do something wrong when I mixed it?

 

You probably applied it too thickly.

 

 

Ah, I bet that was the problem. My kiln wash was very thick.

 

Regarding grog strengthening or weakening the clay, I read on this forum that grog weakens clay. But then an experienced potter told me it strengthens it.

 

I'm guessing it weakens it, as nobody corrected the person on this forum.

[Chris Campbell]

"I'm guessing it weakens it, as nobody corrected the person on this forum."

NO ... that is definitely not what it means when no one corrects a person on the forum!!

It just means no one corrected or commented.

 

Your question is huge and its one of those whose only answer is "It all depends ... " .... which clay, which grog, what for ??? If your clay has no strength and just flops and collapses on the wheel then grog is your friend. If you clay shrinks too much grog could be your friend. Grog increases particle size and does good things for an otherwise balky clay. The answer is nowhere near yes/no.

 

I am going to recommend a book that is my personal 'go to' book for clay ... not saying it is the only, or the best, or whatever ... just that it is one I use most often. Potters Council Members can even get a discount.

 

Clay - A Studio Handbook by Vince Pitelka

http://ceramicartsda...tudio-handbook/

 

 

I count Vince as a friend, but that is not why I recommend it ... Vince is picky, particular, opinionated and precise. He seldom sees grey when the answer should be black and white.

If Vince says this is what happens and this is why ... you can bet money on it. He's the guy you want writing the book.

There are a lot of others whose info is spot on but unfortunately they have not written books yet ( John?? Marcia?? ) ... hmmm, probably don't want to give up three years of their lives to do so.

[Claybot]

NO ... that is definitely not what it means when no one corrects a person on the forum!!

It just means no one corrected or commented.

 

Haha, I love the bolded red "no."

 

I followed your link; that book looks great. Bookmarked for purchase.

 

Regarding my grog question, after accidentally clinking my pots together a few times, I discovered that my whitish earthenware pots chip too easily. So I switched to terracotta earthenware after having heard that it was the toughest, most chip-resistant low-fire clay. It does seem tougher, but I haven't clinked any pots together lately to find out.

 

Then I read here that having no grog makes fired terracotta more chip-resistant than having grog.

 

Then an experienced potter contradicted that info. She said that grog makes ceramic tougher, like straw in bricks.

 

So...I don't know what to think. Maybe grog makes no difference?

[bciskepottery]

Check out point #1 . . .

 

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 Lincoln

[Claybot]

Awesome! That's just what I was hoping for, but a thousands times better.

 

Thank you!

[Mark McCombs]

Great information.

[Chris Campbell]

Great info with such unexpected results. If any of you get a chance to attend a workshop with Pete, grab it and go. He has the unique quality of following up on all the things he wonders about and finding out what happens if ... A really great teacher.