Troubleshooting Peeling And Cracking Glaze (Amateur)

[skessel1]

Please help!!  I have been using all commercial bisque and commercial underglazes (duncan and mayco) with duncan pure brilliance dipping glaze for several months now with mostly good success.  This latest piece is troubling and I am not experienced enough to know what the problem is or how to fix it.  Please see the attached photos.  The black lettering is not "attached" to the body of the piece in a few areas and there seems to be tiny clear bubbles laying on the surface of only the black lettering.  Also, the colored glaze is cracked in places, but only near the lettering...

 

A few other side notes that may be helpful to know.  

 

#1--I attached a picture of the pyrometric cones used during this firing.  The curved tip indicates that the kiln fired hotter than cone 06, correct?  Is this what caused the problem?  If so, how do I correct it?

 

#2--The Pure Brilliance Dipping glaze is supposed to have a viscosity of 19-24.  I was getting a 16-17.  I'm pretty sure I accidentally introduced water to the glaze when wiping down the sides of the container with a sponge.  I will try to siphon some of the water out, but could this lower viscosity level be the problem.  

 

#3--I had several other pieces firing at the same time and all of them turned out okay.  Could the problem have been the thickness of the black underglaze applied?  

 

I am truly hoping one of you can help me, but I am also interested in other resources for troubleshooting, so I don't have to bother people.  I would welcome recommendations of all kinds--books, websites, etc...

[Callie Beller Diesel]

Hey, welcome!

 

1) Your cone looks just fine. Sometimes they bend a bit weird like that crooked tip. If it's a bit over fired, it would be much flatter than that. It's probably not your temperature.

 

2) I have no direct personal experience with that specific glaze, but given that the cracking is happening around the underglaze parts, I think you are right to suspect the underglaze.

 

3) Yes, definitely. I've observed the same about that Mayco underglaze. I found that if I left more drying time between underglazing and dipping, it helped. Not overnight or anything, just go for lunch after you decorate and before you dip. And going too heavy handed was likely to cause it as well.

 

Throwing it out to other under glazers: does it help to apply underglaze to leather hard or green? Does bisquing help?

[Joseph Fireborn]

How quickly are you putting these pieces into the kiln after you apply the glaze? When I first started I put my pots into the kiln with the glaze still damp I had this effect happen sometimes. 

[skessel1]

Thanks so much for the responses!

 

I'm glad to know my kiln temperature is okay!  That's a relief.

 

--There was around a week in between completing the underglaze and dipping in the clear glaze, so I think I was okay there, but maybe I didn't let the glaze dry long enough before firing, as Joseph suggested??  I did my first dip around 10, let it dry about an hour, did my second dip around 11 (just to cover the un-dipped parts, not a full second dip, but you guys probably knew that's what I meant) and started the kiln around 2:30.  It was definitely not completely dry, but to get it completely dry, it seems to take forever, like 24 hours at least...  is that normal?  How long should I be waiting between dipping and loading the kiln?

 

Thanks again!

[Min]

Hmmm, I actually think you did over fire. For self supporting cones the tip should come down to the top of the wedge shaped base. By having the tip touching the shelf for self supporting cones it is over fired. I don't think that is what caused the underglaze issue though. Is it just me or does this look like the underglaze is shivering? I would try bisque firing some tests with just the underglaze and see if you get the same problem. I'm not sure I am reading your post right or not, you are putting the black underglaze overtop of the glazes? The yellow and orange are glazes, not underglazes?

 

edit: you can see the finished angle to self supporting cones in this Orton video 

[bciskepottery]

Is the black underglaze made by the same company as the other underglazes? Is it possible your brush was contaminated? And, it could be thickness of the black line of underglaze.

 

Some black underglazes use cobalt -- which can be refractory and not take the glaze over it. If you have more than one black (one from Duncan, one from Mayco) you might have one with cobalt and one without.

 

Maybe try putting your underglaze on greenware, bisqueing, then glazing to see if there is a difference.

[skessel1]

Hi Min,

 

To answer your question, all of the colors, orange, yellow and black are underglazes.  After those were applied and dried, I dipped them in clear glaze.  Now...  If I did overfire...  How do I fix that issue or where do I go to learn about how to fix it?  I have an electric kiln and just use the automatic settings (I press cone 06, then slow, med, or fast, then a hold amount (I've been using 5 minutes)).

 

Hi bciskepottery,

 

You have me wondering about the Mayco underglaze...  it was the black Mayco liner that was used.  I just went to their website and this is what it said: "Designer Liner is not a dimensional product. If applied too thick or heavy, Designer Liner will crack and possibly pop off."  I think this may have been the problem.  Do you agree?  It was not used as it should have been as a liner, but was put on too thick/heavy.

 

Thanks again!!

[bciskepottery]

Yep. Think you found your answer.

[Min]

+1 for what Bruce said.

 

If you want the kiln to fire cooler then check your manual for instructions for doing a thermocouple offset, basically you are just adjusting the temp the thermocouple is reading. Sometimes small kilns or kilns with new elements run a bit hot. Before doing that double check how far the thermocouple(s) are projecting into the kiln and that the cones are not too close to elements.

[MatthewV]

Throwing it out to other under glazers: does it help to apply underglaze to leather hard or green? Does bisquing help?

 

Leather hard is my first choice with underglazes. They apply like acrylic paints at this stage. They stay fluid a little longer and allow mixing of colors. It is also possible to accent features with carving.

 

On bisqueware they dry faster. This makes doing solid colors easier but I personally find it harder to get an even coat. I have been known to give them a 2nd bisque firing; it burns out the gum and makes the glaze application easier and most consistent.

Working on greenware is dangerous. They dry as fast as bisque but also can easily break or damage the piece.

 

I don't do many pieces with underglazes so when I do putting forth the extra time and kiln space is worthwhile to make the finished product as nice as possible.

[Celia UK]

I do sympathise with you as I had a similar issue just before Christmas.

 

I used the amaco underglazes on Earthstone ES 5 clay and bisqued to 1000oC. Everything looked fine. Photo 1. I then dip glazed in transparent, leadless glaze and fired to 1200oC (digital program). Cones 3,4,5 were all completely over (I'm using cones to see what the kiln is actually doing cone-wise) and the cone 6 minibar in the sitter was almost at 90o, I estimate the kiln achieved cone 6 or possibly a bit more.

 

The interior had a base layer of white underglaze (applied at leather hard stage), when this was dry I applied all the paper triangles to create the pattern, and then applied coloured underglazes on top. (2/3 coats of thinned underglaze, for a solid effect, as per the instructions.)

 

The white base areas are ok - just a few pinholes that I could live with. But over the black, orange, red etc. the glaze is milky, with the black also being rough to the touch. On a small area of yellow, to which I had added a tiny amount of tangerine stain to change the tone, the glaze has crawled into a bubbly clump. Photo 2.

 

The outside was not underglazed with the base white - just a few small areas of coloured triangles. The glaze over the clay is as I expect - a nice gloss. One small area of pattern on the outside isn't as bad as the rest of the bowl - the colours show through an almost clear glaze (tiny bit milky). Photo 3.

 

Other pieces dipped in the same glaze were fine, so I guess the firing temperature was ok?

 

Looking closely at the bowl, I'd guess the glaze on the outside is generally too thick - even though this really was a quick dip in a thinned out glaze (so I thought ). Would that account for the milkiness?

Why would the stain addition on the yellow triangles make such a difference to how it accepted the glaze?

Just how thin should it be? I've thinned down my bucket significantly following advice.

 

As a bit of a generalisation I would say my experience is that underglazes don't seem to be as 'absorbent' to glaze as the clay - as I've had glaze crawl over it in the past - is that possible? Does it just need much thinner glaze over?

 

Any further thoughts/advice so that I don't make the same mistake again?

 

Many thanks in anticipation and Happy New Year to you all!

Celia

[Babs]

Looks like what Bruce said, it's pretty 3D compared to the other underglazes, I thought I was looking at a trailed glaze which was a bit thick? I've had this happen when I've used underglaze as an onglaze but then I was pushing it a bit, I do get away with some U.glazes used in this way, but the blues need to have frits added to let them melt into the glaze.

[Joseph Fireborn]

Celia, I loved that bowl. I feel your pain!

[Babs]

I do sympathise with you as I had a similar issue just before Christmas.

 

I used the amaco underglazes on Earthstone ES 5 clay and bisqued to 1000oC. Everything looked fine. Photo 1. I then dip glazed in transparent, leadless glaze and fired to 1200oC (digital program). Cones 3,4,5 were all completely over (I'm using cones to see what the kiln is actually doing cone-wise) and the cone 6 minibar in the sitter was almost at 90o, I estimate the kiln achieved cone 6 or possibly a bit more.

 

The interior had a base layer of white underglaze (applied at leather hard stage), when this was dry I applied all the paper triangles to create the pattern, and then applied coloured underglazes on top. (2/3 coats of thinned underglaze, for a solid effect, as per the instructions.)

 

The white base areas are ok - just a few pinholes that I could live with. But over the black, orange, red etc. the glaze is milky, with the black also being rough to the touch. On a small area of yellow, to which I had added a tiny amount of tangerine stain to change the tone, the glaze has crawled into a bubbly clump. Photo 2.

 

The outside was not underglazed with the base white - just a few small areas of coloured triangles. The glaze over the clay is as I expect - a nice gloss. One small area of pattern on the outside isn't as bad as the rest of the bowl - the colours show through an almost clear glaze (tiny bit milky). Photo 3.

 

Other pieces dipped in the same glaze were fine, so I guess the firing temperature was ok?

 

Looking closely at the bowl, I'd guess the glaze on the outside is generally too thick - even though this really was a quick dip in a thinned out glaze (so I thought ). Would that account for the milkiness?

Why would the stain addition on the yellow triangles make such a difference to how it accepted the glaze?

Just how thin should it be? I've thinned down my bucket significantly following advice.

 

As a bit of a generalisation I would say my experience is that underglazes don't seem to be as 'absorbent' to glaze as the clay - as I've had glaze crawl over it in the past - is that possible? Does it just need much thinner glaze over?

 

Any further thoughts/advice so that I don't make the same mistake again?

 

Many thanks in anticipation and Happy New Year to you all!

Celiaimage.jpgimage.jpgimage.jpg

YOu may need to add frit to your underglaze, Bruce mentioned the refractory nature of some colourants above. Cavey Fire wrote something about what she adds to her underglazes, think she used GB Frit but if you search her posts perhaps you'd find the details. I have a blue and an orange which just do not melt into the glaze unless I frit them

[Celia UK]

Thanks Babs - so if I'm working from small, 2oz pots of underglaze, how much would I add? And what! I do have various glaze ingredients (mostly bought as job lots from retiring potters, so I'm not altogether au fait with what they all do!) so am sure frits are among them.

If I added some to an underglaze and it wasn't actually needed, what would happen - might it make it too runny!

 

Would you have any idea why a tiny amount of tangerine stain added to yellow u/g caused the glaze to bubble up on top? I just don't know enough about the chemistry here?

[Chilly]

We had peeling underglaze at the centre a while back.  

 

I think the only answer that came up was perhaps the pot had not always been fully stirred before use, and by the time it got near the bottom, it was lacking essential ingredients.  (Think of mixing up a fruit cake and leaving it to sit for a while, then scooping from the top to make a cake - little to no fruit as it has sunk.  By the time you scoop up the last it's almost all fruit and no cake.)

[Babs]

Don't know the chemistry either, just observations and thinking...  I think stains are oxides that have been first fired then ground up.  From your statement I guess the stain is very refractory, or nothing has been added to  enhance a melt at the temp your working at. or upsets the balance of the underglaze which has been formulated to do so ,so that the mix is more refractory or may not allow the glaze to penetrate and interface with the clay body.

 

It could make it run on vertical surfaces I guess, ie too much frit. I'm talking about. If you have time  you could test I think from memory people use 1/3rd frit, 1/3 kaolin and 1/3 stain. Are you firing to c6 as the frit you choose would be significant as well. Frit 3134 for lower temps.

YOur pots are delicious so testing would be the way to go.

I had honey bees run down the sides of mugs in a bleary mess from not testing. I test now!

Good luck.

[kangarabbit]

I saw your post, and hoped somebody would answer with salvation for your problem, because I am experiencing something very similar.  I think I might make things worse, but will add my two pennyworth to the discussion. I have recently started to experiment with decorating using coloured slips on leatherhard clay, which I've always avoided because I feel more confident working on bisque, but wanted a new direction.  I have tried using underglazes on bisque in the distant past, but found them difficult to apply smoothly and patchy when glazed - some colours, especially red seem to suck up the glaze and turn out matte or patchy. I didn't try black, partly because it was horribly messy and I hated the feel of it. (Wimpy reason I know).  Also, underglazes are horribly expensive in the UK, so I found myself being really mean and fiddly with them instead of loose and free. So I abandoned underglazes, and for ages used oxide washes on textured bisque with no glaze.

 

I know your problem asks about underglazes rather than slips, but there seems to be a similarity in the actual end problem you have with the black, so I hope this isn't too irrelevant. Obviously slips and your bisque don't mix (unless you use a vitreous slip/engobe), but as your black and my black seem to be giving us fits...well perhaps this might be of interest.

 

Back to the slip. I couldn't find a commercial slip that actually fired a proper black, so made up small amounts using various recipes from books, the internet - even contacted a guy whose black and white slipware work is wonderful (http://www.markdallyceramics.co.uk )  He was very helpful and shared the supplier of his black stain. (Sneyd Ceramics).

 

The supplier kindly sent me 3 different free samples of black stain, and I have made up small amounts of probably 6 additional different mixes, all red clay based, with various 'these make black' oxides in different proportions. I also used Mason Stain 'Best Black'.  So pots of black slip all over the place.

 

On test tiles, under a leadless glaze, at around 1060C, most of the blacks were dark brown at best. One of the free samples was 'almost' black - even though I used twice the percentage that Mark recommended. The best 'real' blacks were the ones I mixed myself from recipes using oxides.  They looked fine until glazed, but THEN - the exact same thing happened as with your black underglaze. The black looks as if it has minutely tiny bubbles all over it, so looks milky and has a rough surface.  Absolutely hideous and a lot of work wasted.  It happened without cobalt, and with cobalt, so I don't think that's it. It happened when I fired at 1060C, it happened when I fired at 1140C - the clear glaze I used had a wide temp range. I thought I might be over firing or underfiring, although I stayed within range of the glaze.  Like you, I don't always have the knowledge to be able to look at something wrong and know what caused it. I tried soaking at top temp ( as advised by Mark) - didn't work.  Tried not soaking - didn't work either.

 

All the other slips I used were absolutely fine at both temps, with or without soaking.  I make white with dried ground up porcelain and have had no issues with cracking or shrinkage (beginner's luck maybe). Purchased coloured slips - no problem. Porcelain slip coloured with stains - no problem.  It's only the wretched BLACK!!

 

So...I can use the 'almost black' - but no, it's dark brown really.  I could use the 'just about black' - but the stain is REALLY expensive, and the problem with cobalt I have experienced is not that it makes the glaze bubble or go milky, but it does bleed blue sometimes into the adjoining white. I could keep trying to find out why my own mixed blacks are doing what they are doing under the glaze -  but I may go mad.

 

I am now wondering if it has something to do with the oxides reacting with the leadless glaze.  The slip made with the Sneyd stain samples fired smooth, but only one (as I say,containing cobalt, so expensive) was an acceptable 'just about black'.  Maybe it's a case of 'you get what you pay for' for me, as I don't seem to be able to fix the problem with my own mixes. Even though they allegedly work for those people who's recipes I used. Most of the recipes come from 'Techniques Using Slip' by John Mathieson, and I'm following them to the letter. Are they not telling something?

 

I know it's frowned upon, but I am about to test some clear leaded glazes - using lead sesquilicate which is said to be safer.  I have a feeling it isn't going to make any difference, but hey ho...if that doesn't work, I give up. 

[Min]

 I have tried using underglazes on bisque in the distant past, but found them difficult to apply smoothly and patchy when glazed - some colours, especially red seem to suck up the glaze and turn out matte or patchy. I didn't try black, partly because it was horribly messy and I hated the feel of it. (Wimpy reason I know).  Also, underglazes are horribly expensive in the UK, so I found myself being really mean and fiddly with them instead of loose and free. So I abandoned underglazes, and for ages used oxide washes on textured bisque with no glaze.

 

 

Underglazes are pricey where I live too but I think the cost of them is made up for by the benefits they provide. I use mostly Spectrum brand, a pint of the black here costs approx 6 pounds less than what Bath Potters Supply sells it for in England. (http://www.bathpotters.co.uk/spectrum-black-515-underglaze-500-series/p2621) It's very concentrated,I dilute it to approx 1 part ug to 2-3 parts water. I fire to ^6 but they work at lowfire also. The pot below is ^6 porcelain, no cobalt blurring the edges, no pins or pits or other problems with the glaze over the ug. The rim is brushed on (couple coats), the lines are slip trailed. Don't know if all this is redundant and you have already tried the Spectrum ug but thought I would pass it along.

 

Sorry for the lousy pot pic, it's just the cats bowl that I have at home right now with the Spectrum 515 black.

[Celia UK]

Ann - I do usually give them a stir but can't swear I did so with the black on this occasion. One more thing to consider.

 

Thanks Kangarabbit - a very comprehensive post! Sounds as if you've tried the lot! I have to agree that your expensive options seem to have given the best results. I love the decoration style. Did you use resist for the third one? The Sneyd stain there looks great.

I haven't got back into the studi since Christmas and New Year yet - perhaps today's the day?

 

Babs - I'm going to do a couple of tests with the underglazes I've got and also try making a small amount myself using your recipe. (have to check my 'ingredients' box for the frit!)

 

Min - your Spectrum black looks good - have made a note of that one.

 

Thanks all - onward and upward!

[Babs]

Think the ruuning honey bees were more a bad choice of clear glaze and thick application. SO test on the vertical as well as flat...

[drmyrtle]

Just to add another perspective, a friend does elaborate underglaze painting on porcelain bisque. Hours of work, with dilute underglaze (often black) in up to three to four layers. Four light layers is definitely pushing towards too much. Anyway, she had two years of "ruining everything" with the bubbly effect you show, because she was dipping her clear overglaze. When she shifted to a very light spray application of a clear that she tested (5-20 clear) she reliably got good results.

 

So, two things: your black trails are too thick, and your overglaze is dipped, not sprayed.

 

Where's Guinea on this topic?