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White Glaze Issues.


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#1 High Bridge Pottery

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Posted 21 July 2014 - 09:40 AM

I am having a few issues with my kiln.

 

Two bowls, same glaze. Left bowl on the middle shelf and right bowl on the bottom shelf. Only put a cone on the middle shelf as I forgot the bottom one. Got to cone 9 in the middle, I was aiming for cone 10 but firing and me have a few issues to sort out.

 

The bottom one looks under fired on half but has this really nice white speckle look on half. If I remember right, all the parts away from the elements of the kiln have this white, the half nearer the elements look under fired. The whole thing might just be under fired. I have never really understood that much what I am looking for that makes it the right kind of fired. Is my kiln much hotter in the middle than around the edge? Even the bowls on the middle shelf have this half and half look. I don't know if it is glaze or kiln related. Probably both.

 

The one from the middle is a lot more even but maybe still under fired. Got the nice speckle but a much more subtle white. There was only a small shelf after the middle which didn't cover any of the bowls.

 

Here is the glaze recipe, I have been messing around with the specific gravity so this was maybe a little thin for the glaze which is why they are not very white. I may have messed up the recipe a bit when trying to change it for the better.

 

I think I need to do a kiln full of glaze tests before glazing any more of my work.

 

Soda Feldspar = 10

Cornish Stone = 30

Quartz              = 10

Dolomite White= 10.5

Whiting             = 10.5

China Clay        = 25

Tin Oxide          = 2.7

Zinc Oxide         = 4

 

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#2 Chris Campbell

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Posted 21 July 2014 - 09:56 AM

Electric kilns fire this way ... Hotter in the middle, cooler top and bottom.
One way to even it out is to do a hold at the end of the firing ... I do about a ten minute hold and get pretty even temps top to bottom.
I also do controlled cooling because this also improves results.
Another option is to use the facts to your advantage by placing certain glazes in different areas to get the look you want. Wood firers do this all the time. Witness cones on every shelf will help you learn exactly how your kiln is firing.
Basically, you are learning about your kiln and your glaze ... and how cool that you have this interesting glaze to play with.

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#3 High Bridge Pottery

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Posted 21 July 2014 - 11:02 AM

I would like to do a controlled cool but that would require a new controller and also more variables to control.

 

It is an interesting glaze but every time it comes out different so not very consistent! I think today I am going to go back to basics and work on developing a similar glaze that fires to cone 8. Need to do some proper absorption tests and shrink tests with the clay too.

 

Firing schedule was 5 hours to 1100degc and then 4 hours to 1280degc with a 15min hold to try and even things out.



#4 Wyndham

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Posted 21 July 2014 - 11:56 AM

I'm thinking to widen the firing range a small bit

you might try to tweak the glaze by taking  your Cornish stone down to 25-27 and upping the soda spar to13 to 15 to get a smoother melt with out movement, just a starting point for testing

Wyndham



#5 neilestrick

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Posted 21 July 2014 - 02:46 PM

Any particular reason you're firing to cone 10 in an electric kiln? Your elements will last twice as long at cone 6. And since you're not reducing you're not getting any benefit from firing that hot.


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#6 High Bridge Pottery

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Posted 21 July 2014 - 04:11 PM

Not really, besides my own satisfaction in getting the pots really hot and that is how they fired when I was learning. I am really thinking of moving down to cone 6.

 

Could I move down to cone 8 or is there a reason why most people go cone 6? I never realised there was no benefit firing electric in this way.



#7 Chris Campbell

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Posted 21 July 2014 - 05:14 PM

I think those numbers have become common through usage in schools, studios and large potteries. Because they are so common, there are a lot of clays and glazes formulated around those temperatures. It keeps everything a lot easier if clays and glazes fit at set points.

In a teaching studio you will be limited to a strict set of firing temps in order to keep the sanity of the firing crew intact. But in your own kiln at home ... it is a great learning curve to see what your clays and glazes can and cannot do.

If you are not doing functional work you can really experiment with clay bodies to find the texture or color you want by firing it at all kinds of temps. I have a clay that is a lovely color at a lower temp then it is designed for so that's what I fire it to. You can use high firing clays at lower temps for a nice open body in raku and pit firings. High firing clays also don't have to always hit the top of their range. The only way to find this out is to take some time to get to know your clay and your kiln ... Test tiles and drip catchers mandatory.

But ... if you are doing functional work you cannot wing it ... You have to test to make sure your wares are properly fired.

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#8 jrgpots

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Posted 21 July 2014 - 06:09 PM

I'm thinking to widen the firing range a small bit

you might try to tweak the glaze by taking  your Cornish stone down to 25-27 and upping the soda spar to13 to 15 to get a smoother melt with out movement, just a starting point for testing

Wyndham

I agree here.  You may want to add Ferro 3124 or 3134 frit to widen the melting range in your glaze. Start at 10% and do a line test in increments of 1-2%.  Ferro3134 would give the glaze a little more fluidity compared to 3124, if you wanted the glaze to be a little runny.

 

Jed



#9 neilestrick

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Posted 21 July 2014 - 06:12 PM

Not really, besides my own satisfaction in getting the pots really hot and that is how they fired when I was learning. I am really thinking of moving down to cone 6.

 

Could I move down to cone 8 or is there a reason why most people go cone 6? I never realised there was no benefit firing electric in this way.

 

At cone 8 your clay will be underfired or overfired. Dropping down to cone 6 isn't so difficult. Some cone 10 glazes can get there with less than 5% boron frit added. And since you're already firing in oxidation you shouldn't get any big color shifts.


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#10 High Bridge Pottery

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Posted 21 July 2014 - 06:32 PM

The three clays that I have bought say 1220-1280. Well one even says School Buff (1120°-1160°C Earthenware)(1220°-1280°C Stoneware) Does that not mean they can be used between 6-10?

 

Thank you for the tips on the glaze, I don't have any frits right now but I will try adjusting the cornish stone and soda spar. Made a lot of test tiles today and some little bowls to melt some raw ingredients in to see what they look like. Hopefully get a few test done with this glaze as I really like it, it needs to be more consistent though...

 

I think you are right Chris, that was all they seemed to do at college and in the studio I went to. I keep trying to come up with ideas for sculptural work but at the moment I am loving functional stuff. Lots more work needed on my glazes !

 

I am really into my dull/matt whites with coloured slip underneath and the orange / toasty look of the clay body. Not sure if this is the body or because of the soda spar. I remember reading somewhere it can cause this flashing.



#11 Min

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Posted 21 July 2014 - 07:36 PM

If I was to tinker with your white recipe to help the melt I would do a triaxial blend. In one corner of the triaxial I would decrease the kaolin by 5. In 2nd corner I would increase Cornwall by 3, in the last corner I would add 6 silica. (the silica is low in this glaze) Tall test tiles with lots of room for glaze runs and/or fired on waste pieces.

 

The look you are going for is very nice, I really like the toasty colour of your clay where the white glaze breaks.



#12 oldlady

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Posted 22 July 2014 - 05:13 PM

since you have cornwall stone, you might want to try a second glaze recipe.  i have made this and it is very smooth and satiny.  the clay i use is white so though the recipe calls it a transparent glaze, my test shows as white.  in my next firing, i plan to test it over several underglazes and with colorants.  the original tests i did in 2011 were limited to only three variations.

 

Cornwall Transparent  ^6 or ^8   (this is written on the original recipe as given to me, i fire at cone 6)

 

cornwall stone            82

whiting                        12

colemanite/gerstley      2

nephaline syanite         4

 

 

when you add tin or something to make it white you should have a great glaze that is simple.  it could be great on your clay.

 

hope it helps.


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