Transparent stoneware glaze problem.

[marek]

Firstly, a Hello to everyone on this forum. After years of lurking, gonna get stuck in with my first post.

I recently switched from an electric kiln to reduction w' gas, both stoneware to cone 10. I'm developing a new body, slip and glazes for it and struggling with a strange problem with colourless gloss transparent. This is, oddly, the only glaze to give me problems. Celadon, Iron Red, Tenmoku, Shino all fine. This was never a problem in the electric kiln. My aim is glossy transparent (ideally, a simple lime glaze) to show up white slip, and make a stoneware body look darker where glazed.  All the transparents I've tried turn out very glossy, most fit well BUT they develop this strange and ugly pale bluey-green opacity and obscures the slip.  I have seen this at cone 9, 10 and 11 and with many different recipes and on many different bodies, so I don't think maturity or the body are the problem. I suspect the firing but I could be wrong. I fire in a tiny downdraught insulating brick kiln, previously bisqued ware with propane: neutral to 880C, hold 1h in strong reduction, light reduction to peak (cone 10), slow cool in neutral to 750C (for Iron Reds, not shown here). The tile here is representative of dozens of different recipes and line blends tried on dozens of bodies. White slip on left, thick glaze layer on top, thin layer on bottom.  Fired upright in a special rack. This particular glaze has a little iron in it (hence the green grey blush) but all the glazes do this iron or no, ash or no. This body has high iron and iron filings for fleck, but all the bodies do this, even the light stoneware.

[neilestrick]

Can you post a couple of the glaze recipes?

[marek]

Sure. I tried dozens but here are rhe most promising:  Leach 4:3:2:1 Potash 'spar, Quartz, Whiting, Kaolin. The same with 5% and 10% washed mixed hardwood ash. And the same replacing half of the Potash spar with Soda spar, then all with Soda 'spar. My former main oxidation glaze (superb in oxidation) 30 Neph Sy, 35 Quartz, 15 Whiting, 5 Talc, 5 Zinc Oxide, 10 Hyplas (low iron) ball clay. Then two variants of it replacing Zinc Ox with Barium Carb then Strontium Carb. Korean lime glaze (which I back calculated in software from a published glaze analysis of Buncheong ware)  12 Potash 'spar, 20 Soda 'spar, 25 Quartz, 17 Whiting, 10 Dolomite, 16 Kaolin and Nigel Wood's Song celdaon (with the RIO left out)  25 Potash Spar, 20 Quartz, 27 Wollastonite, 12.5 Kaolin, 12.5 Ball Clay (I used Hyplas) and 3 Talc.

[Dick White]

Unlike oxidation, in redux there is no such thing as a white stoneware under a transparent clear glaze. In reduction, iron turns black, green, or blue depending on what other chemistries might be present. A white stoneware will have some iron contamination of the clay. This remains unnoticeable during an oxidation firing, but a reduction firing will bring it out. The iron in the clay will cause the white stoneware to turn grey during body reduction, and the glaze as it matures will flux some of the body surface and draw the impurities up into the glaze, causing it to discolor. The only way to avoid this is use a clean porcelain body or at least a clean porcelain slip that you've tweaked until it fits the body properly.

[Magnolia Mud Research]

There are clear glazes that will not turn green/blue/yellow when used on stoneware; such a glaze will not be found in the traditional calcium based recipe.

I have a colleague that uses an AMACO low-fire clear glaze for his cone 10 clear.  YES it does work just fine at cone 10 reduction, and it does not run off the pot when applied.  I have used it on several projects, but my colleague uses that glaze as his base cone 10 glaze.   We also have a studio clear that is clear on stoneware which is not a whiting based recipe.   I have not used it but will look up the recipe when I go back to school on Tuesday.   A wash of soda ash and/or sodium borate (aka Twenty Mule Team borax)  will produce a clear coat if applied to bisque ware without making a celadon color.  

The color observed through a clear glaze  will be controlled by the color of the underlying surface.  Most stoneware fired in reduction will achieve its color from the reaction of the clay body with the atmosphere of the kiln before the glaze begins to form a melt.   Many stoneware, and some porcelains, have a grayish tint that is fired in prior to glaze melt.

During some testing 10 years ago I found that straight gerstley borate would make a clear glaze when fired to cone 10.   You might start some testing of gerstley borate glazes to see if there is a concoction that will meet your requirements.   

LT

[marek]

Thanks for the helpful replies.

I don't use porcelain or white stoneware but a dark iron rich body decorated with kaolin based white slip. The effect I am shooting for is Korean Buncheong - the body is anything from grey to dark brown/bordeaux where unglazed and grey to nearly black where glazed but decorated with kaolin based white slip with no hint of green or blue where the shiny glaze is over the white slip. I posted a picture of what I mean (not my work, Kang Hyo Lee's). There is virtually no discolouration over the slip (you can't really see it in the pic but I've seen pieces of his), yet the body is reduced. It doesn't matter if the glaze picks up a little iron over the body as the darkness of the body would obscure any tinge of colour, but it should not look very blue or green over the slip.

Thanks LT, I'd be very interested in trying out your recipe in my next firing run as well as a few Gerstley borate recipes. I'll be a while as I need to order new kiln shelves but I'll post the results!

I also wonder if my problem is caused by devitrification at the glaze body interface. The theoretical fix is a crash cool from peak to 1000C. Might try that too.

[Mark C.]

This glaze works great on Porcelain-I cannot say about high iron stoneware -I have used it for many many decades

Clear cone 10 glaze

HT-51

Potash Feldspar-27

I use Kingman but you can use Custar or another potash feldspar

Ball Clay -19.5

whiting -19.5

Silica (325mesh)-34

Mix Thin apply thin and rub out any runs or bubbles when dry with fingers-glaze fast when dipping or pouring

try to get a uniform coating-sraying works well.

I have used this over cobalt drawings for 40 years

very clear at a hot cone 10.

[Magnolia Mud Research]

Marek,

you might try a coat of straight sodium silicate syrup as a cover glaze.  When fired the sodium silicate will melt into the clay body at a very early temperature.  I don't know why I overlooked that approach earlier.

 

[marek]

LT,

Interesting idea - I guess the sodium will provide extra flux at the glaze body interface. I presume you apply a thin layer on bisque beneath the glaze? 

[Magnolia Mud Research]

I was thinking that the  item had all the slips (or engobes) applied and bisqued;  and the sodium silicate would be the medium applied to the bisqued ware.   

My experience with this approach was ~10 years back when I was working on carbon trapping chemistry.   I fired several  bisqued cone 10 buff stoneware vertical test pieces using only a mixture of sodium silicate and graphite as the glaze medium.  The sodium silicate syrup soaked into the pores of the bisqued tile.  The graphite burned out leaving a clear glaze;  thin applications looked liked bare clay with a shine ; thick application was just a semi-gloss clear.   Neither had carbon trapping so I moved on.  (Actually the experiment proved my hypothesis).  

I now routinely use soda ash applied to unglazed  bisque ware to create a faint shine on the texture of the surfaces.  When I treat the wet surface of a pot with a kaolin or ball clay before expanding the pot the expanded surface develops a crackled texture.   After the bisque the kaolin and/or ball clay is whiter than the stoneware substrate.  I apply a thin sprayed (spray bottle)  coat of soda ash solution to the bisqued ware.   The fired item will have a faint shine where the soda ash was applied.  The amount of shine is related to the amount I sprayed in an area.    I am working to develop contrasts between various areas on the pot simultaneous with not changing the tactile characteristics of the surface.   

Until now I had forgotten about using straight sodium silicate syrup as a glaze medium for bisqued ware.    It is now on the experimental agenda for February.  

LT