Glaze Problem on slipcast items

[Thrown_In_Stone]

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.

[Thrown_In_Stone]

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

[perkolator]

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

[Chris Campbell]

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.

[Thrown_In_Stone]

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.

[Thrown_In_Stone]

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.

[neilestrick]

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?

[Thrown_In_Stone]

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?

[jo4550]

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

[neilestrick]

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?

[AtomicAxe]

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?

[Thrown_In_Stone]

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.

[Thrown_In_Stone]

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.

[Thrown_In_Stone]

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?

 

 

 

[neilestrick]

 

 

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.

[Thrown_In_Stone]

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?

[AtomicAxe]

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.

[neilestrick]

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.

[Thrown_In_Stone]

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.

[Thrown_In_Stone]

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?

[etched]

I was having a similar problem a few months back. I found out that the glaze that I was using a store bought one had one of the components changed. It was no longer available here in Australia and was now from an overseas supplier and it was slightly different. This glaze was now crawling if you used the recommended 2 coats. When you only used one coat it was fine.

 

could this be similar to what is happening to you?

[AtomicAxe]

Epson salts in the glaze won't affect this. The amount needed to cause changes is about 200% more than the glaze itself since most will burn off before it even decides to turn to a flux.

 

To be honest, I highly doubt even soaking it will affect the glaze, since i just spent about 15 minutes staring at the photo ... do you have glaze splatters on the kiln shelf? since the sections are missing, the surface of the clay is glazed thinly and there are no hard ridges, (i.e. if it chipped off in cooling it would be hard edged and would indicate shivering) ... I can almost assure you that your glaze is not firing slowly enough to let the glaze properly deal with gasses so it just pops out as it melts. Inside wouldn't be affected since it's a little more protected from external heat from your elements. Try to fire slower. as this happens when the glaze melts and doesn't have a chance to slowly outgas. Which is the moste likely problem since it looks like it has extremely thick edges around the missing glaze spots. or it happens beforehand you're glaze even gets to properly stick ... at which point ... i would suggest a different glaze ... you will have to experiment with ramping speeds at how slow and when you need to ramp. Sorry man, without seeing you fire, or knowing every inch of your process after you put that glaze on there ... it's scientific method time, chart what you do, and when you do it, then try to change variables. also try to use the slip to make some non-poured items and glaze those to see if it does the same thing as well as thicker pieces.

 

Personally though, I would probably change both the clay and the glaze to eliminate both variables since something has changed and not for the good. The cost associated with testing will probably not be worth trying to fix the problem unless you want to do it on the side so eventually you can either go back to it or have options for merch. since the problem is probably not the clay, I would use that for items not cast if you do change your slip cast ware process.

[Thrown_In_Stone]

Epson salts in the glaze won't affect this. The amount needed to cause changes is about 200% more than the glaze itself since most will burn off before it even decides to turn to a flux.

 

To be honest, I highly doubt even soaking it will affect the glaze, since i just spent about 15 minutes staring at the photo ... do you have glaze splatters on the kiln shelf? since the sections are missing, the surface of the clay is glazed thinly and there are no hard ridges, (i.e. if it chipped off in cooling it would be hard edged and would indicate shivering) ... I can almost assure you that your glaze is not firing slowly enough to let the glaze properly deal with gasses so it just pops out as it melts. Inside wouldn't be affected since it's a little more protected from external heat from your elements. Try to fire slower. as this happens when the glaze melts and doesn't have a chance to slowly outgas. Which is the moste likely problem since it looks like it has extremely thick edges around the missing glaze spots. or it happens beforehand you're glaze even gets to properly stick ... at which point ... i would suggest a different glaze ... you will have to experiment with ramping speeds at how slow and when you need to ramp. Sorry man, without seeing you fire, or knowing every inch of your process after you put that glaze on there ... it's scientific method time, chart what you do, and when you do it, then try to change variables. also try to use the slip to make some non-poured items and glaze those to see if it does the same thing as well as thicker pieces.

 

Personally though, I would probably change both the clay and the glaze to eliminate both variables since something has changed and not for the good. The cost associated with testing will probably not be worth trying to fix the problem unless you want to do it on the side so eventually you can either go back to it or have options for merch. since the problem is probably not the clay, I would use that for items not cast if you do change your slip cast ware process.

 

 

 

 

 

Thanks for your input. No, there's no glaze chips on the shelf at the end of the firing. I put another kiln load on yesterday, I'll see how this one goes.

 

My normal firing schedule is 3:30 to 500ºc then full power to 1060ºc. The firing normally takes around 9 hours.

 

The firing from last night I had it on a ramp of 4:45 to 500ºc then full power to 1060ºc with a hold of 2min. I increased the first ramp to ensure it was slower whilst any remaining water was pushed out from the glaze incase this was dislodging it. I guess for what you describe you mean slow the top end.

 

On my next firing should I slow it down by an extra hour?, ie something like this: 5:00 to 500ºc then 5:00 1060ºc?

 

I've never used a ramp at the top end, I hope the kiln can cope with it. It seems to go slowly when it gets above about 900. I don't want it to suddenly switch off with an error that it can't heat fast enough. I guess I'll not know until I try.