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Glaze Problem on slipcast items


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#1 Thrown_In_Stone

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Posted 14 March 2013 - 10:24 AM

Hi everyone,

I have a problem which is driving me absolutely insane.

This problem has only just started popping up in the last few months. I am using earthenware clay and earthenware glazes, but the glaze just doesn't take correctly on 60% of the pieces. I am using a comercial claybody I have tried with a commercial white glaze and a glaze from a recipe and both give the same effect.

For this particular project I am using the clay to make casting slip (Which I have done in the past without issue).

I have been using this clay and glaze for years and never had a problem. I'm not doing anything different at all.

I bisque to 1075ºc and glaze to 1060ºc

I wash each piece of bisque to ensure dust/dirtfree surface before glazing, allowing it to dry for a few days in a warm spot. I also use surgical gloves to ensure I don't get finger grease on the pots before glazing.

I attach a photo of what's happening.

Any help would be much appreciated. Thanks.

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#2 Thrown_In_Stone

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Posted 14 March 2013 - 11:09 AM

I should note also, the insides of the cups come out perfect. It's just the outsides where the problem is occurring.

#3 perkolator

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Posted 14 March 2013 - 11:25 AM

are you using gloves when you unload your bisk, as well as when you wash them? (assuming washing is taking place days/weeks afterward) as for your washing - what are you using? it's possible that you're transferring skin oils to the pieces when unloading, and just using water to rinse isn't going to cut through this. i would guess it's something like this that's happening since the inside is ok, where you're most likely not grabbing the vessels from. try addressing this first to eliminate this possibility before moving on to other diagnosis

#4 Chris Campbell

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Posted 14 March 2013 - 11:39 AM

Well, if you have not changed your process then it might be your supplier has changed something in their products.
It could be as simple as changing sources. Call them and they might tell you if anything in their composition has changed.

I would suspect the reason it does not happen on the inside is because the surface is concave, so the glaze does not have to stretch to fit.

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#5 Thrown_In_Stone

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Posted 14 March 2013 - 11:56 AM

are you using gloves when you unload your bisk, as well as when you wash them? (assuming washing is taking place days/weeks afterward) as for your washing - what are you using? it's possible that you're transferring skin oils to the pieces when unloading, and just using water to rinse isn't going to cut through this. i would guess it's something like this that's happening since the inside is ok, where you're most likely not grabbing the vessels from. try addressing this first to eliminate this possibility before moving on to other diagnosis


I never used to until I got this issue. Since then I have used gloves every step of the way but it doesn't seem to change anything.

#6 Thrown_In_Stone

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Posted 14 March 2013 - 11:57 AM

Well, if you have not changed your process then it might be your supplier has changed something in their products.
It could be as simple as changing sources. Call them and they might tell you if anything in their composition has changed.

I would suspect the reason it does not happen on the inside is because the surface is concave, so the glaze does not have to stretch to fit.


This is why I changed to a glaze I mix up from a recipe and I'm still getting the problem. :(

It makes no sense at all.

#7 neilestrick

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Posted 14 March 2013 - 02:09 PM

Is a certain percentage of you clay body recycled clay? Do you ever add anything to the mix to stiffen it up like ball clay, etc. When I was a tech for a clay supplier I used to run into problems with low fire bodies where the customers always just added ball clay when recycling, and eventually the clay body formula was so out of whack that they had glaze problems. It was usually shivering, though, not crawling.

What brand clay are you using? Not to bad mouth anyone, but I also found that a couple brands of low fire clays sometimes didn't jive with other brands of glaze.

Since the outsides are the problem, another possibility is that there is something soluble in the water or the glaze that is precipitating out and causing the issue. Maybe a seasonal change in your water supply?
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#8 Thrown_In_Stone

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Posted 14 March 2013 - 06:51 PM

Is a certain percentage of you clay body recycled clay? Do you ever add anything to the mix to stiffen it up like ball clay, etc. When I was a tech for a clay supplier I used to run into problems with low fire bodies where the customers always just added ball clay when recycling, and eventually the clay body formula was so out of whack that they had glaze problems. It was usually shivering, though, not crawling.

What brand clay are you using? Not to bad mouth anyone, but I also found that a couple brands of low fire clays sometimes didn't jive with other brands of glaze.

Since the outsides are the problem, another possibility is that there is something soluble in the water or the glaze that is precipitating out and causing the issue. Maybe a seasonal change in your water supply?


Thanks for your reply.

The clay isn't recycled no, pretty much straight out of the bag, dried, and ground up and mixed with water and sodium silicate to deflocculate, sieved and into a mould. It's by Valentine Clays.

I have noticed that about 70% of the problem pots have the problem on the side facing the elements, could there be a problem with some kind of over-firing?

#9 jo4550

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Posted 14 March 2013 - 08:36 PM

I should note also, the insides of the cups come out perfect. It's just the outsides where the problem is occurring.


Several questions spring to mind here. How are you glazing? Are you glazing the inside and outside separately and in what order. If so what is the drying time between the inside and the outside? Are you brushing on the glaze? Are you spraying the glaze?

I ask these questions as it seems to me that you may have a problem with crawling (glaze pulling away from the surface during the firing). I have experienced this when dipping glazes on thinly constructed work. The bisque becomes saturated when more glaze has been added to a surface. Bubbles or blisters appear on the unfired glaze during drying. These can be smoothed back by rubbing. However in the majority of cases the glaze crawls during the firing because of the lack of adhesion of the glaze to the surface.

With brush on glazes this can also occur when subsequent layers can lift off the first layer be it ever so slightly.

With sprayed glazes this can also happen when too thick a layer is applied in one hit rather than several thinner layers.

One way or another the bisque can becomes saturated. Saturated areas do not have good adhesion with the glaze.
With reference to your statement "about 70% of the problem pots have the problem on the side facing the elements" it may be that the side facing the elements initially heats up at a more rapid rate than the rest of the pot and trapped moisture causes further problems with poor glaze adhesion.

I note also that your bisque temperature is higher than your glaze temperature. Could this be contributing to the poor adhesion?

Regards
Johanna

#10 neilestrick

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Posted 14 March 2013 - 09:21 PM

I wouldn't think that overheating is causing that problem with the glaze. Crawling isn't usually associated with over-firing. If your bisque is running hot, though, it could cause glaze adhesion problems. Have you checked the accuracy of your bisque with cones?
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#11 AtomicAxe

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Posted 14 March 2013 - 09:47 PM

How much sodium silicate are you using? is your mold transferring impurities or have a build up of sodium in the mold? when the glaze pops off, does it leave a powdery surface behind? are you letting the clay defloculate for 24 hours? or just straight pouring after sift mix? do you sift and let sit? or do you mix again before pouring?

#12 Thrown_In_Stone

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Posted 15 March 2013 - 06:50 AM

How much sodium silicate are you using? is your mold transferring impurities or have a build up of sodium in the mold? when the glaze pops off, does it leave a powdery surface behind? are you letting the clay defloculate for 24 hours? or just straight pouring after sift mix? do you sift and let sit? or do you mix again before pouring?


I add the sodium silicate in stages over a few days to get the correct consistency, mix with a drill mixer and then let it sit before putting it through a sieve, it's then usually the next day when I use it. I stir it up every time I use it and put it through a larger sieve (a kitchen one) into my pouring jug just in case there are any lumps.

I can't imagine there is a layer of sodium on the mould because it does it even on new ones.

The surface where the glaze crawls isn't powdery, it's slightly shiny, kind of like a very very very thin layer of transparent glaze.

#13 Thrown_In_Stone

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Posted 15 March 2013 - 06:54 AM

I wouldn't think that overheating is causing that problem with the glaze. Crawling isn't usually associated with over-firing. If your bisque is running hot, though, it could cause glaze adhesion problems. Have you checked the accuracy of your bisque with cones?


Thanks for your reply.

I use the same kiln for bisque and glaze (obviously separate firings). I don't think the bisque is getting too hot, as it's only 15ºc more than the glaze firing. I haven't tested it, perhaps I should get some cones. I'm getting a new kiln in the next couple of months anyway.

#14 Thrown_In_Stone

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Posted 15 March 2013 - 07:02 AM


I should note also, the insides of the cups come out perfect. It's just the outsides where the problem is occurring.


Several questions spring to mind here. How are you glazing? Are you glazing the inside and outside separately and in what order. If so what is the drying time between the inside and the outside? Are you brushing on the glaze? Are you spraying the glaze?

I ask these questions as it seems to me that you may have a problem with crawling (glaze pulling away from the surface during the firing). I have experienced this when dipping glazes on thinly constructed work. The bisque becomes saturated when more glaze has been added to a surface. Bubbles or blisters appear on the unfired glaze during drying. These can be smoothed back by rubbing. However in the majority of cases the glaze crawls during the firing because of the lack of adhesion of the glaze to the surface.

With brush on glazes this can also occur when subsequent layers can lift off the first layer be it ever so slightly.

With sprayed glazes this can also happen when too thick a layer is applied in one hit rather than several thinner layers.

One way or another the bisque can becomes saturated. Saturated areas do not have good adhesion with the glaze.
With reference to your statement "about 70% of the problem pots have the problem on the side facing the elements" it may be that the side facing the elements initially heats up at a more rapid rate than the rest of the pot and trapped moisture causes further problems with poor glaze adhesion.

I note also that your bisque temperature is higher than your glaze temperature. Could this be contributing to the poor adhesion?

Regards
Johanna


Thanks for your reply. Some good things for me to test out here.

At the moment I'm dipping them in one. Do you think adding a small amount of medium to the glaze will help adhesion.

I'm going to try dip the outside then pour the inside to see if that fixes things (letting them dry between).

With regards to firing schedules, I was taught to bisque higher than glaze for earthenware to make sure the impurities and carbon is all burnt out and more won't come out at the lower glaze temp.

I wonder if there is still water in the bisque after glazing despite leaving it a day or two to dry. Perhaps I should slow down the initial ramp of the glaze fire; I suppose this could explain why it mostly happens next to the elements because this is where it's getting hot first and is where the water is coming out and dislodging the glaze?




#15 neilestrick

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Posted 15 March 2013 - 08:19 AM

The surface where the glaze crawls isn't powdery, it's slightly shiny, kind of like a very very very thin layer of transparent glaze.


This makes me think there may be something soluble in either the slip or glaze that is coming to the surface. Maybe try a different water supply with a small batch, bottled water, etc.
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#16 Thrown_In_Stone

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Posted 15 March 2013 - 09:09 AM


The surface where the glaze crawls isn't powdery, it's slightly shiny, kind of like a very very very thin layer of transparent glaze.


This makes me think there may be something soluble in either the slip or glaze that is coming to the surface. Maybe try a different water supply with a small batch, bottled water, etc.




Can epsom salts affect anything?

#17 AtomicAxe

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Posted 15 March 2013 - 11:44 AM

When firing your glazed pieces, do you fast do you try to fire you work? If the surface of the pieces were the problem you would not have a fluxed surface on your pieces ... what you may be encountering is a shrinkage issue in firing. A final soak of when you reach temp could help with this. The addition of sodium silicates to formulated clays shouldn't affect the clay body shrinkage rate as all it does is let you use less water to obtain solution and you would need a LOT of epson salts in the clay in order for it to flux over the surface, and the major issue you would encounter would be fuming of salts to cause major pinholing and pitting ... which you aren't encountering. plus that much epson salt would be a flux in your clay body and would start to turn your claybody into a clay puddle.

try defloculating your glaze so it uses less water to hold in thin suspension, and try to heat soak your ware at final temp ... could fix your issue. won't know what else to try until you try atleast the heat soak and see if that works.

#18 neilestrick

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Posted 15 March 2013 - 12:32 PM



The surface where the glaze crawls isn't powdery, it's slightly shiny, kind of like a very very very thin layer of transparent glaze.


This makes me think there may be something soluble in either the slip or glaze that is coming to the surface. Maybe try a different water supply with a small batch, bottled water, etc.




Can epsom salts affect anything?


Not likely.
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#19 Thrown_In_Stone

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Posted 16 March 2013 - 06:10 AM

When firing your glazed pieces, do you fast do you try to fire you work? If the surface of the pieces were the problem you would not have a fluxed surface on your pieces ... what you may be encountering is a shrinkage issue in firing. A final soak of when you reach temp could help with this. The addition of sodium silicates to formulated clays shouldn't affect the clay body shrinkage rate as all it does is let you use less water to obtain solution and you would need a LOT of epson salts in the clay in order for it to flux over the surface, and the major issue you would encounter would be fuming of salts to cause major pinholing and pitting ... which you aren't encountering. plus that much epson salt would be a flux in your clay body and would start to turn your claybody into a clay puddle.

try defloculating your glaze so it uses less water to hold in thin suspension, and try to heat soak your ware at final temp ... could fix your issue. won't know what else to try until you try atleast the heat soak and see if that works.


The epson salts are in the glaze not the clay.

#20 Thrown_In_Stone

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Posted 16 March 2013 - 06:23 AM

When firing your glazed pieces, do you fast do you try to fire you work? If the surface of the pieces were the problem you would not have a fluxed surface on your pieces ... what you may be encountering is a shrinkage issue in firing. A final soak of when you reach temp could help with this. The addition of sodium silicates to formulated clays shouldn't affect the clay body shrinkage rate as all it does is let you use less water to obtain solution and you would need a LOT of epson salts in the clay in order for it to flux over the surface, and the major issue you would encounter would be fuming of salts to cause major pinholing and pitting ... which you aren't encountering. plus that much epson salt would be a flux in your clay body and would start to turn your claybody into a clay puddle.

try defloculating your glaze so it uses less water to hold in thin suspension, and try to heat soak your ware at final temp ... could fix your issue. won't know what else to try until you try atleast the heat soak and see if that works.


What kind of soak time would you put in?

Would you reduce the final temp to compensate slightly?




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