Raku Troubleshooting Clay + Glaze

[Laurène Ashley]

i all, 

 

I've recently set up a raku kiln and have been experimenting with different clay bodies.  Someone requested on this forum that I try Laguna's B-mix with grog.  I was looking for a nice creamy white clay to glaze with a clear..  I like the clay, but have had a few - not all - pieces came out with a few hairline cracks that were happening during reduction.  I've attached a few photos.  This past weekend, I fired around 10 different pieces with varying thicknesses, and had only about 3 cracks, so I'm not thinking it's the clay.  

Could I be doing something wrong in the reduction? The pieces were also bisque fired at cone 04.

 

For the frit - I used 90% Ferro frit + 10% kaolin (another suggestion from the forum), but had a really difficult time brushing it on.  I watered it down thinking it was just too thick, but it was still brushing on really thick, and impossible to brush on a second coat.  I've used a frit before, and know they are not the easiest to brush on.  The previous teacher I had used a Frit + kaolin at the same percentages, but I don't remember it being so difficult to apply.  I tried smoothing out the bumps, but when my pieces came out you could really see the uneven brush strokes, making the surface bumpy.   As you can see I got some nice crackles, but some pieces gave me hardly any crackles and a lot of black dots.  

 

Any advice on frit application?  Marcia are you around?? 

 

 

I'm still new to doing this on my own, and am trying to self teach myself as I go along.  This forum is a great help!  I appreciate any help!!  Thanks!!!!! 

[rakukuku]

Well it was probably me that recommended Laguna b mix with grog since I use it all the time. A friend of mine has good luck using laguna danish white with sand. She had cracking with the b mix with grog.

 

clay planet makes a raku/sculpture clay that I don't have much experience with. 

 

Heat your tongs up a bit before grabbing a piece. 

 

Flat pieces seem to crack less if fired on edge rather than laid flat. 

 

I let clear crackle pieces cool for a bit before removing them - like 30 seconds or a minute. I mist them lightly with a bit of water before reduction to encourage crackling.

 

For glaze, the 80% gherstly borate and 20% nepheline syenite works well and mason stains or oxides can be added to it. 

 

Be careful not to fire too fast until you are past quartz inversion phase. I raku to cone 06 which is pretty common.

 

Some artists apply raku glaze and then let them sit overnight before firing so that there is no moisture in the piece that might cause cracking. 

 

Hope this helps.   Rakuku

[Laurène Ashley]

Well it was probably me that recommended Laguna b mix with grog since I use it all the time. A friend of mine has good luck using laguna danish white with sand. She had cracking with the b mix with grog.

 

clay planet makes a raku/sculpture clay that I don't have much experience with. 

 

Heat your tongs up a bit before grabbing a piece. 

 

Flat pieces seem to crack less if fired on edge rather than laid flat. 

 

I let clear crackle pieces cool for a bit before removing them - like 30 seconds or a minute. I mist them lightly with a bit of water before reduction to encourage crackling.

 

For glaze, the 80% gherstly borate and 20% nepheline syenite works well and mason stains or oxides can be added to it. 

 

Be careful not to fire too fast until you are past quartz inversion phase. I raku to cone 06 which is pretty common.

 

Some artists apply raku glaze and then let them sit overnight before firing so that there is no moisture in the piece that might cause cracking. 

 

Hope this helps.   Rakuku

Hi rakuku, 

 

I've treid misting with water, and didn't see much of a difference in the crackle.  I've learned to put them in right away, so I'll try leaving them out for a bit.  I fired slightly less than cone 06, 1800 to be exact.   I don't understand what you mean by firing past the quartz inversion phase... sorry I'm still a newbie!  Also maybe I fired too soon and the glaze wasn't dry completely. 

 

I'd like to try adding stains or oxides.  Would you have any photos of the gerstly borate and nepheline syentie with oxides added to it??  How much do you add?

 

Thanks so much!

Laurène 

[Callie Beller Diesel]

Did you sieve your kaolin/ frit mixture to get rid of the lumps and make sure the mix is evenly distributed? You describe your glaze as being lumpy, and if that's the case, the clay isn't distributed in the frit enough to do what it's supposed to.

 

In terms of your cracking, some of it looks like it might be your forming technique. It's kind of hard to tell this from photos, but is the cross section, especially at the bottom pretty even, or is it thicker than the rim? Was it slab built with a foot ring attached, or is it wheel thrown and trimmed and altered? It looks to me like that rim is cooling faster than the middle.

 

The thermal shock of raku means you have to really know what your clay will handle because they're all a bit different. If you have a clay like b-mix, the regular stuff doesn't take too well to thermal shock. It's kind of picky at the best of times. It means you have to be pretty even in your forming if you're going to abuse it.

 

 

As for quartz inversion, this is a simplification, but good enough for our purposes. It's a physical phase change, where silica in the clay changes from alpha to beta quartz on the way up in temperature, and from beta to alpha quartz on the way down. The reason this is important, is because it expands and contracts, respectively, and if it does so at different rates in a given piece (a thicker bottom vs a thin rim for example), it causes cracking in the piece. Quartz inversion happens at 573 *C, or 1063*F, and you should ease any kiln through that temperature range for about 50 degrees on either side of that point. If the pieces you're making are uneven, easing them through quartz inversion slowly, so that the thick parts of the piece have a chance to catch up with the thinner bits will solve many an issue.

[Laurène Ashley]

Hi Diesel, Thanks so much for your response.  I did run my frit through a sieve, the glaze itself wasn't lumpy, it was hard to apply resulting in uneven surfaces.   

Both of the pieces in the photos were slab at around 1/4".  For the bowl, the rim did cool faster, I remember when it was drying, but isn't this normal?  It's also been very hot in my Austin, TX studio... meaning things are drying too fast, or unevenly.  How can I prevent this??

 

Thanks for the quartz inversion explanation.  I took almost 3 years of raku, and was never taught this.  I actually realize that I wasn't taught a lot because it was a "hobby" class.  Now I'm trying to go at it on my own and learning that I have a ways to go before I fully understand!  

 

My kiln is gas fired.  Should I slow down the flame before and after 1063°??  

 

Laurène

[Callie Beller Diesel]

Hey, don't be down on the Raku as a starting point: I got hooked on clay at 16 firing a Fibrefrax barel kiln with a tiger torch!

Having seen your Instagram today, I feel like I should advise you: Raku glazes aren't durable over time. They're too soft, and they tend to be soluble. It's not a suitable process for foodsafe wares. I don't know if your instructor would have advised you of this or not.

 

Drying pieces fast is fine, industry does it all the time: but drying unevenly isn't. Plates are tricky: put a strip of plastic around the rim to slow your drying down, or if you can, flip the piece upside down on a clean ware board or bat once it's firm enough so the bottom of the plate is more exposed to air. Slow the rim down so the bottom has a chance to catch up.

Minuscule cracks that can occur while drying sometimes don't become visible until the piece is fired. You can hear them if you flick the rim after the piece is bisqued, though. If a piece is cracked, it'll thunk, if it isn't, it should ring a bit.

 

With regards to the brushability of the frit, I'd actually be inclined to use Gerstley Borate instead. Its cheaper, and it brushes nicely all by itself, without having to add extra clay.

 

Regarding firing speed, when heating the piece, you want to slow the firing down starting 50 degrees below quartz inversion, ease it gently through the next 100 degrees (so to 50 degrees above quartz inversion), and then go as you please. When you cool things through quartz inversion, the best thing to do in your situation is to not take the piece out of the reduction bin until it's properly cooled.

[neilestrick]

If you use the 80GB/20NephSye glaze, it will brush okay on it's own, assuming you use real Gerstley. Any other glaze will require the addition of gum solution to make it brushable. Make the gum solution by mixing 2 tablespoons CMC Gum to a gallon of water. Also add 1/4 teaspoon copper carbonate as a preservative, or the gum will get eaten up by bacteria. Let it sit overnight, then blend with a stick blender. When you mix your glaze, use the gum solution for about 1/3 of the water. If you've already got a bunch of glaze you need to make brushable, double the amount of CMC in the gallon of water. It will make a gel that you can then add to your already mixed glaze.

 

Crackle glazes need to be on thick in order to get good big crackles.

[bciskepottery]

The hairline cracks may come from the rims/edges being too thin (versus the rest of the ware) or, perhaps, over-handling at leather-hard/bone dry stages or picking up an item by/near the edge/rim.  Likely, they are there after bisque firing, but too faint to be noticed.  The glaze firing just brings out the crack more and makes it visible. 

[Laurène Ashley]

Hey, don't be down on the Raku as a starting point: I got hooked on clay at 16 firing a Fibrefrax barel kiln with a tiger torch!

Having seen your Instagram today, I feel like I should advise you: Raku glazes aren't durable over time. They're too soft, and they tend to be soluble. It's not a suitable process for foodsafe wares. I don't know if your instructor would have advised you of this or not.

 

Drying pieces fast is fine, industry does it all the time: but drying unevenly isn't. Plates are tricky: put a strip of plastic around the rim to slow your drying down, or if you can, flip the piece upside down on a clean ware board or bat once it's firm enough so the bottom of the plate is more exposed to air. Slow the rim down so the bottom has a chance to catch up.

Minuscule cracks that can occur while drying sometimes don't become visible until the piece is fired. You can hear them if you flick the rim after the piece is bisqued, though. If a piece is cracked, it'll thunk, if it isn't, it should ring a bit.

 

With regards to the brushability of the frit, I'd actually be inclined to use Gerstley Borate instead. Its cheaper, and it brushes nicely all by itself, without having to add extra clay.

 

Regarding firing speed, when heating the piece, you want to slow the firing down starting 50 degrees below quartz inversion, ease it gently through the next 100 degrees (so to 50 degrees above quartz inversion), and then go as you please. When you cool things through quartz inversion, the best thing to do in your situation is to not take the piece out of the reduction bin until it's properly cooled.

Yes, I do know that raku is not for food.  My instructor did tell us this, however still served us tea in her raku tea cups.... I also see quite a few ceramic artists make raku tea bowls... so does that make raku ok for tea?  

 

I'll try covering the rims in plastic next time-  good tip.  

 

I tried gerstly borate, nepheline syenite combo, but the crackles were much smaller and closer together, making the pieces too grey.  However I've recently been told I need to put about 4 coats to get bigger crackles.   I learned using a frit + kaolin, but it was a frit in France (frit 1254) and couldn't find the exact equivalent here, so someone told me to try 3124 + kaolin.  

 

Thanks for clearing up the quartz inversion.. very interesting and I will try it next time! 

[Laurène Ashley]

If you use the 80GB/20NephSye glaze, it will brush okay on it's own, assuming you use real Gerstley. Any other glaze will require the addition of gum solution to make it brushable. Make the gum solution by mixing 2 tablespoons CMC Gum to a gallon of water. Also add 1/4 teaspoon copper carbonate as a preservative, or the gum will get eaten up by bacteria. Let it sit overnight, then blend with a stick blender. When you mix your glaze, use the gum solution for about 1/3 of the water. If you've already got a bunch of glaze you need to make brushable, double the amount of CMC in the gallon of water. It will make a gel that you can then add to your already mixed glaze.

 

Crackle glazes need to be on thick in order to get good big crackles.

  Sounds complicated.  I'll try thicker layers of gerstly/neph sye... 

[Callie Beller Diesel]

Raku tea bowls are indeed a valuable and desirable item in the Japanese tea ceremony, but Japanese Raku bears only a vague, passing resemblance to North American Raku. And Americans have a very different idea than the Japanese about what makes good tableware. You're not too likely to poison anyone with frit 3124, Gerstley Borate or Neph Sye, but it just isn't that durable. It's likely too porous and the cups will weep, and the glaze will dissolve over time in the dishwasher, or discolour permanently with use.

John Baymore could tell you much more about the history and aesthetics.

[rakukuku]

Hi, if you look at my gallery, some of the pieces are raku, The big carrot with bunnies is all raku.  I use mason stains in the clear crackle until they look about right. mix thoroughly and try to apply evenly as color will vary with thickness.  Also I often use commercial low fire glazes for reds and oranges etc on the animal figures. you can put the clear over it or even mix it with the clear. The turquoise crackle is a recipient called "Gary's Green" from Gary Fergusson which you should be able to find on line somewhere. Very close to the clear recipe but has about 5% copper oxide.

 

Quartz inversion is a molecular change that occurs in firing somewhere around 900 or 1000 degrees. Its thought its best to fire slowly through this phase to prevent cracking. Or so I am told.    GL    Rakuku

[Laurène Ashley]

Very cute rakuku!  I love your little snails and your chickens!  

[douglas]

Are you letting your pots dry before firing? If your pot is still wet from glazing you need to heat the kiln slowly so that water is not boiling from inside the pot. I use B-mix and other clay bodies, and have not noticed pieces cracking more often using B-mix, so it is more likely glaze fit, or perhaps the glazing and firing before the pieces dry out.

[preeta]

which Bmix with Grog are you using? ^10? i've read raku specialists use ^10 clay because it does  not crack that much. 

 

i would experiment with thickness. i sometimes attend a raku group. and a large surface body as a plate repeatedly cracks if not splits in half. thinness as bsicke said. yet the masks in the shape of plates but more curves and thicker have never cracked.

 

i would read up on raku. some forms are better suited for raku. 

 

your experience illustrates why raku should not have been called raku in the US. 

[Laurène Ashley]

your experience illustrates why raku should not have been called raku in the US. 

 

 

 

 

your experience illustrates why raku should not have been called raku in the US. 

 

What does this mean?  I didn't learn Raku in the US, and am seeing that there are very different standards here. 

 

I'm using the B-mix cone 5.  Followed someones advice as I'm still looking for a clay comparable to what I was using before. 

[Callie Beller Diesel]

I did a paper on this back in the day.

 

Western "raku" originated when Paul Soldner, after reading accounts of a Japanese firing method used to create tea wares, decided to replicate the process himself in a class with his students. The account told of how a maker of roof tiles had an order that needed to be filled in a hurry. He used a coarse, refractory clay fired to a low temperature that he pulled from the kiln while still hot, and quenched them in water to increase the turnaround time. Someone noticed that the surface results were interesting, and decided it would look good on tea wares for the Zen tea ceremony. Over the long years, low-melting, lead glazes were added, things like crazing glazes in a very limited pallete, undulating rims and a perfectly imperfect, quiet earthy aesthetic known as "wabi" became highly coveted by tea masters.

 

Sounded pretty good as a fast and dirty pottery lesson to our intrepid Mr. Soldner. So to replicate this, he used a stoneware with a bunch of grog wedged in to make some simple pots, knocked together a simple glaze that would melt in an impromptu kiln built in the college quad, got some welding tongs to pull the wares out of the kiln with, and ventured forth with his class on a fine fall afternoon.

The firing went reasonably smoothly, and when the pots came out of the kiln.....they seemed to be missing something. The "rich, quiet surface" (whatever that meant) weren't appearing.

(Clays available in Japan are very geologically different than those found in North America, and even Europe. They do different things, but no one was taking this into account at the time.)

Seized by inspiration, Soldner decided to roll some of the hot pots in a nearby pile of dry leaves in order to get some more visual interest going, and western raku was born.

 

There are a lot more rules, and fewer variations allowed within traditional Japanese Raku wares. The traditional colours are red, black and white, they don't use reduction bins filled with stuff that burns, and lead is still commonly used in the glazes. (The level of wiping and cleaning and care given to a tea bowl in the ceremony ensures the user, who is immediately consuming one cup of tea, isn't exposed to any meaningful amount of harmful material.) As far as I know, Raku pottery (which is now a hereditary family title) is only tea ware, no other decorative pieces. (If Baymore-San wishes to kindly correct my errors,I'd appreciate it.)

 

Western raku is pretty much wide open. We tend to use boron as a primary flux, we use all kinds of weird and wonderful things to add to the reduction bin, metallic flashing from copper is pretty common, and because we leave cups of tea lying around with a complete lack of ceremony or Zen, it's not generally considered to be a food-safe method of making pottery. It's used in mostly decorative pieces of one kind or another. It's a very immediate method of working, and so it can make for a good introduction to pottery.

 

I have massive holes in my understanding of how European raku has evolved, or how it's practiced, and if Marcia or Evelyine want to chime in, they are your go-to people.

[douglas]

Lots of good advice already posted. I'd recommend bisque firing to 06 instead of 04. There is little benefit to bisque firing at the higher temperature, and the risks of the pot cracking from thermal shock increase the more vitrified the clay gets. 04 Bisque is a little more vitrified and sturdy than 06, but neither is going to stand up to heavy use.

 

I use the same crackle recipe but dip the pots instead of brushing. Give that a try to avoid seeing brush strokes.