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Danielle Caron

Transparent glaze over dark clay

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Hi,
I am looking for a transparent glaze that won't cloud the surface of a Plainsman M-390 or M-332 clay fired at cone 6.  It could be manufactured or a recipe. A glossy or satin finish, although I read that the more mat it is, the riskier it is to get a cloudy finish as well.
I ruined many weeks of efforts with the  homemade transparent glaze available at my workshop/class last week.
All the pieces came out  with a pinkish-beige tone just good enough for the trash can. 
I then read in John Britt's book (mid-range glazes) that clear glazes on dark stoneware bodies can appear gray when fired due to the underlying iron which affects the color, which I am trying to avoid from now on.
Many thanks in advance for your help.

DC

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Hey Danielle! Welcome to the forum.

Getting a clear glaze that works nicely over M390 is a PITA, but it can be done. It's not just the glaze you have to address, it's your firing cycle and your bisque as well.  I find the best results if you fire to get the clay body itself mature, and then many of the glaze flaws resolve.

I bisque to 04, and use the Plainsman clear stoneware glaze, which you can buy as dry mix, but the recipe is on digitalfire here.  I like this one because it survives thermal shock testing, and it's a very durable glaze that holds up over time on this clay body. I mix it to a SG of 1.28-1.3, and flocculate the glaze with some Epsom salt soloution, as it does settle out rapidly. Apply thinly.  I put a small cone 7 in the sitter, and after the kiln shuts off I cool to about cone 5/bright yellow and soak the kiln for 20 minutes. (I obviously don't have a digital controller to work with). My cone packs show this results in a solid cone 7, with 8 getting soft. Using this method, you get minimal glaze defects, but drips will show. You will likely have to do some fine tuning in your own kiln of this firing cycle.

I haven't needed a plain clear in a while, but I was working on some tests of adding small quantities of zinc to the base glaze to be able to increase the thickness somewhat while retaining the glaze clarity. This will of course, alter some underglazes and Mason stains that are used underneath this glaze, but if you're using oxides, this may not be a concern. 

 

 

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6 minutes ago, Callie Beller Diesel said:

Hey Danielle! Welcome to the forum.

Getting a clear glaze that works nicely over M390 is a PITA, but it can be done. It's not just the glaze you have to address, it's your firing cycle and your bisque as well.  I find the best results if you fire to get the clay body itself mature, and then many of the glaze flaws resolve.

I bisque to 04, and use the Plainsman clear stoneware glaze, which you can buy as dry mix, but the recipe is on digitalfire here.  I like this one because it survives thermal shock testing, and it's a very durable glaze that holds up over time on this clay body. I mix it to a SG of 1.28-1.3, and flocculate the glaze with some Epsom salt soloution, as it does settle out rapidly. Apply thinly.  I put a small cone 7 in the sitter, and after the kiln shuts off I cool to about cone 5/bright yellow and soak the kiln for 20 minutes. (I obviously don't have a digital controller to work with). My cone packs show this results in a solid cone 7, with 8 getting soft. Using this method, you get minimal glaze defects, but drips will show. You will likely have to do some fine tuning in your own kiln of this firing cycle.

I haven't needed a plain clear in a while, but I was working on some tests of adding small quantities of zinc to the base glaze to be able to increase the thickness somewhat while retaining the glaze clarity. This will of course, alter some underglazes and Mason stains that are used underneath this glaze, but if you're using oxides, this may not be a concern. 

 

 

 

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Hi Callie,

Thanks a million for your detailed reply. The only kiln I have access to is the school one and they only fire at cone 6. I don't know the details of their firing cycle. Will have to enquire. I will show them your reply. Wondering if firing at cone 6 could work as well with this particular glaze?

DC

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@Danielle Caron, if the transparent glaze isn't going over slips or underglazes then this glaze might do the trick. I used it over a dark Laguna body and even though the glaze is amber coloured over white / light bodies it appears clear over dark bodies. It was okay with fast cooling when I used it.  There are some images of it over Plainsman claybodies in that link.

edit: I really would suggest you make up some test tiles plus some really quick and simple mini pots and try out glazes on those before committing work to them. (hindsight is 20/20 isn't it?)

Edited by Min

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Yes. I found this one earlier (GA6-A), unfortunately it is going over a white or lightly colored slip. Interesting for future use, though.
So far, the only one I find that seems to work is the G3806C (from Digital Fire). Definitely will do several test tiles and mini pots before going further. 

DC

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https://glazy.org/recipes/20547

 

Try this glaze. Derek Au has tested it on a brown stoneware and there is a picture in the link. He has tested many cone 6 clears and most of them cloud on a dark body. The determining factor from my point of view seems to be the boron level. This glaze has 2x the amount of boron necessary for cone 6, which will diminish the glaze's durability long term. If you want to see the other glazes he has posted click on his name, his tests are a great resource for us all. 

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12 minutes ago, tinbucket said:

https://glazy.org/recipes/20547

 

Try this glaze. Derek Au has tested it on a brown stoneware and there is a picture in the link. He has tested many cone 6 clears and most of them cloud on a dark body. The determining factor from my point of view seems to be the boron level. This glaze has 2x the amount of boron necessary for cone 6, which will diminish the glaze's durability long term. If you want to see the other glazes he has posted click on his name, his tests are a great resource for us all. 

That brown test tile is still full of bubbles, might be better mixed thinner.

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10 hours ago, liambesaw said:

That brown test tile is still full of bubbles, might be better mixed thinner.

I believe what you are seeing is the grog/sand in the clay. If you look in the lower left of the tile you can see bubbles where the glaze has dripped/pooled. Yes, I agree, thinner mixing and application is better.

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@tinbucket The result seems quite interesting. Just left with the difficulties to find the right materials here in Canada (Gillespie borate  & Jackson Ball clay). I assume that Gillespie borate could be replaced with Gerstley borate, but Jackson ball clay: this would require some reseach. Thanks. DC

Edited by Danielle Caron

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