Flashing Slip

[preeta]

Curious what is in the kaolin that makes for instance Helmers kaolin flash better than other kaolin in an atmosphere kiln?

 

Our local claymaker adds helmer kaolin to their wood fire body to encourage color! If I were to make a slip out of that and apply it to another claybody would it behave the same? Meaning would the slip and claybody flash in a similar fashion.

[Marcia Selsor]

I have used Helmar Kaolin directly onto clay in a dry form and as an addition to slips for wood firing. 

I can't say it would be the same as what your local clay maker is creating because I have no idea what that is.Here are some for several firings in the past 2 years.

 

http://community.ceramicartsdaily.org/gallery/image/9238-selsorrlccwoodfire2017bottle-copy/

 

http://community.ceramicartsdaily.org/gallery/album/1176-nceca-pre-conference-wood-firings/

 

 

Marcia

[bciskepottery]

clc_flashing_slips.pdf

 

Not all kaolins are the same . . . each carries various other minerals in them besides the primary mineral of kaolin.  So, depending on the "impurities" contained, along with other slip additions, each will produce different effects in the wood kiln. The effects also depend on the type of wood being burned -- in addition to heating the kiln, your wood will determine the ash that moves through the kiln. How they react will also depend on the clay body being used -- stoneware vs porcelain. 

 

You really didn't think this would be easy, did you? 

 

 

 

 

[glazenerd]

Sometimes Preeta, it is not what is in the clay, but what is not in the clay. In this case, the way Wendt processes his clay, removes all the soluble salts. Which means the clay just acts as a base for the reactions of other elements to take place on.

Hey, I am getting good at this: I answered in less than a paragraph.

 

Usual and customary greetings, etc. etc.

Nerd

[GEP]

To answer your specific question, yes you can make a slip out of the clay body from your local supplier. It would have the same color response, combined with the color of the claybody underneath that is peeking through the slip, depending on how you applied the slip and how thickly.

 

In the context of wood firing, where there are so many out of control variables, the specific type of kaolin being used in this claybody doesn't really matter. Lots of wood firers will mix stoneware and porcelain together, and this claybody is basically the same thing. Stoneware fired in reduction will be somewhere on the brown spectrum. The results can be too dark or muddy for some. Adding white to the equation allows a wider spectrum of pretty browns and oranges. This is not about chemistry, it is about color theory.

 

Edit to add: Not to imply that regular stonewares fired in reduction are undesirable. They can be incredibly beautiful. Adding slips to your work, or mixing white into your stoneware, is a personal choice only. 

[preeta]

Lol no B. I've been pouring over ash and the different kinds and types. Hard wood, soft wood, grasses, etc. I've wondered if I can use BBQ ash since it's from charcoal which was once wood.

 

This is why ceramics is right up my alley. My daughter has branded me a 'stalker for knowledge' which is quite an irritating quality for a mom to have.

 

And then I came upon those who don't like using flashing slips.

 

It's exciting to read about all the different lengths of firings.

 

Marcia the pictures were very helpful. Were they a 2 day firing or longer.

[Tyler Miller]

Alumina is the key to the flashing. Not common knowledge, but that's the stuff. Lee Love's gorgeous toasty shinos are based on this. He adds alumina to increase the effect. Properly it's soda, silica, and alumina, but the former two are usually supplied in wood fire.

[preeta]

Tom that was very helpful. For once though I was looking for a very scientific answer. But the idea what is not there blows my mind.

 

Aaah Mea! That's helpful too. I was going to mix clays to see what happens since I recall in my first ceramics class the prof saying he mixed his red and white clays.

 

I would imagine them if it's color theory then I should not wedge too much. I have 4 different categories of color.

 

I hope one day I am able to take a wood firing class from mark lancet at a community college two hours away since he mixes his own claybody.

[Guest JBaymore]

Alumina combined with soda and ALSO potassium.  Potassium gives a redder fire color to high alumina glazes and clays and slips.  It is just a bit redder rendition than soda effects.  Some folks use pearl ash instead of soda ash in mixing such slips....or a combination of both. 

 

Of course the volatiles that are in the wood ash and get driven off at high temps that cause the "flashing" (not the ash buildups and stuff like goma) are potassium and soda compounds.  So give it some high alumina stuff to "work on" and voila'.

 

True "flashing slips" go on THIN and most go on bisque ware.  Consistency is typically mixed about like skim milk.

 

best,

 

..................john

[GEP]

I would imagine them if it's color theory then I should not wedge too much. I have 4 different categories of color.

 

Wedge a little to see the two different colors, or wedge a lot to create a smooth third color. One is not better than the other. Again it's a personal choice only. 

[Marcia Selsor]

Preeta,

Here are some flashing slip recipes from Offcenter (Jim Sandefur) who is no longer on this forum.

 

Posted 09 July 2013 - 07:34 PM

 Here are a few I have but  never use:

 

Basic Flashing Slip

Neph Sy ... 30

Kaolin ... 70

 

Johnston Flashing Slip

Avery or other Kaolin ... 50

Neph Sy ... 50

 

Flashing Slip #3

Neph Sy ... 35.4

Helmar Kaolin ... 39.8

Alumina Hydrate ... 4.4

Titanium Dioxide ... 2.7

Soda Ash ... 17.7

 

Helmar Flashing Slip

Helmar Kaolin ... 50

EPK ... 20

Neph Sy ... 20

Silica ... 10

 

Jim

 

[neilestrick]

Alumina combined with soda and ALSO potassium.  Potassium gives a redder fire color to high alumina glazes and clays and slips.  It is just a bit redder rendition than soda effects.  Some folks use pearl ash instead of soda ash in mixing such slips....or a combination of both. 

 

I went to grad school at Utah State U. with Jason Hess, who now teaches at Northern Arizona U., and he did his grad thesis on wood firing with cottonwood, which is plentiful out west. It's nasty wood to cut and split, and difficult to fire with because it doesn't produce as much heat as other woods, especially harder woods, but it produces a lot of ash, which is why folks don't like to burn it in their fireplaces. He found that colors didn't develop well like with other woods. Cottonwood produced a lot of what he described as 'varnished concrete'- gray and shiny. After having the ash tested, he found that it was very high in potassium due to the makeup of the soil in the area. He was able to reproduce the varnished concrete effect with pearl ash. Eventually he figured out that good color could be obtained in cottonwood firings if you used clay bodies that were high in iron, like +4%, similar to those bodies used for reduction cooling. Reduction cooled cottonwood firings are gorgeous, BTW.

 

Just thought you might find it interesting.

 

As Tyler said, high alumina, low silica. The key is to keep it from glassing over too much. And you need a little iron impurity. Really good white porcelains don't do well in wood firings. They just go glossy white. Slightly gray porcelains can flash really well, assuming they don't get too much vapor and glass over. Usually you get one side glossy, one side flashed.

[preeta]

Oh this is so much fun. Sooo much to learn. I've also been emailing Richard Hotchkiss and learning sooo much. Clay folks are sooo generous like all of you unlike most other mediums.

 

There are so many more questions this is creating.

 

I love slip. Most of my experiments are with slip. Thanks John. Dick said they don't use slip on bisqueware. So I will be discovering without slip philosophy. I've tried the buncheong style of dipping in slip (forgot the term) with an extra fire which taught me about thickness of slip.

 

Tyler that was exactly what I was looking for.

 

Thanks Marcia for the recipes. I found some from the kazegama site. I am just going to use my simple slip. I'm trying to not go all over the place - because I always try a whole bunch. I want to focus on clay body, And organic form this time. There's so much information out there. I'm trying to be more specific about short fires.

 

You all are my go to people where woodfire is concerned. My new prof does not really know much of woodfire. But we will be learning and doing soda fire o my new alternatefiring class.

 

So exciting.

 

Do the same principals apply to soda firing (I think it's gas) about flashing and alumina and color? How Fire changes surface?

[glazenerd]

Preeta:

 

I am not going to comment on wood firing, what I know about it would not cover the back of a match book. Clay/s however, I do know about.

Kaolins typically have 48% silica, and 37% alumina. So the flashing recipes all use kaolin for that reason: high alumina. So adding alumina would not be necessary unless you are mixing in other ingredients that would lower the Si/AL level of the typical slips shown above. Again, not being familiar with wood firing, I did notice some comments about iron and possibly magnesium helping the flashing effect. You could experiment with the basic recipe of 70% kaolin and 30% Nep SY: to

 

60% helmer

10% red art    (which has 7% iron and a PCE around 30)

30% nep sy.   

 

a "grey" colored porcelain would have FHC, or other type of ball clay with that color. Or the color could come from bentonite or other bentones that are high in magnesium.  So adding 2-3% bentonite would be another way of adding magnesium. So using clays that are high in iron or magnesium might produce some alternatives to color.  Others who do wood firings could probably expound on this more than I can. I am suggesting strictly from a chemistry point of view. May not be feasible in a wood firing.. but then again...

 

Nerd

[jrgpots]

Marcia and others, I miss off center. It was our loss when he left.

 

Jed

[jrgpots]

Has anyone used manzanita wood? I know it is off topic a bit,,,,sorry. I was just curious.

 

Jed

[Guest JBaymore]

Has anyone used manzanita wood? I know it is off topic a bit,,,,sorry. I was just curious.

 

Jed

 

Never heard of it!  Tell us more.

 

best,

 

.....................john

[Tyler Miller]

Has anyone used manzanita wood? I know it is off topic a bit,,,,sorry. I was just curious.

Jed

I knew a bladesmith who used manzanita wood for a while instead of charcoal/coal to prove a point. Tai Goo--drifts around the Arizona, New Mexico, California area. Really high btu stuff.

 

This ash analysis might help you make up your mind. https://www.ecn.nl/phyllis2/Biomass/View/267

[jrgpots]

We used to burn it for outdoor fire pits.  It can bend the cast iron grates because of its heat output.  It grows in CA, AZ, UT and NM at about 3,500 to 4,500 ft elevation.  In CA is can grow up to 20 ft as a dense tree that has slick red bark and light green leaves.   When it burns, it leaves almost no ash.

 

Jed