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tinypieces

Not sure why the glaze did what it did

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If I could figure out how to attach the correct size photo you could see the what I’m gonna describe...

Duncan Clear Satin (SN351) glaze is not running into all the incised lines after the recommended number of coats and fired to cone 06.. It looked completely covered. This has now happened a couple of times and I’m not sure what’s going wrong.

 The clay is Standard 528. The  design was inscised then bisque fired to 06. It was then stained using Amaco velvet underglaze (V361)...allowed to dry before applying the Duncan clear glaze as per mfg.  

I’ve used this technique for a long time with other clay bodies and this never happens, no problems. Any idea what’s going on? Is it the clay?  The glaze? My technique? The firing? I could use some help. (And how to upload the right kind and size image).

 

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Yes difficult without a picture so hopefully you can resize one and post it. If a windows operating system right click on the picture, say send to and choose reduce picture size and send it to yourself. You should be able to post after all that.

with respect to the underglaze amaco  V361 is black which if applied thick or continuous ends up being fairly refractory. I actually had to increase the boron in my clear glazes to melt effectively over this color especially when thick.

not sure that helps but I have been working on this issue for months now and have found the color black and medium blue to be quite refractory when applied thick or over very large areas.

good luck with your picture.

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If you put  underglaze on too thick or use multiple layers and don't allow each layer to dry before putting the next layer on the underglaze can lift from the pot. Looks like this is happening on the spoke shaped lines and the short lines, is this what you are referring to? I've only ever had one underglaze shiver but that's another possibility. 

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I've had a couple underglazes shiver, or otherwise come off the ware.  However, I don't know if it was an issue with the underglaze, or possibly a contaminated surface. 

It's very rare that I've had it happen.

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Ahh,

this is lowfire clear to 06 so it should melt over anything. Your picture really looks like the glaze shivered off in several areas but maybe more revealing is the minor amount of black underglaze scattered around these areas. The glaze and some of the underglaze appears to have been ejected locally before this melted which likely means the underglaze popped off early in the firing.  I would love to see a close up (in focus) of a couple of these spots. If you have a tripod or very steady hand and good lighting you likely could zoom in with your phone and get some detail. Of course the whole mail it to yourself thing might be necessary. In the picture below, the areas circled may be most revealing.

Inked2026398686_glazegonewrong.jpg.0d5ba4d3ad37f5465a183d8125e47d4d_LI.jpg

Edited by Bill Kielb

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It looks like the glaze pulled some of the underglaze with it.  I've had the black amaco velvet underglazes act almost like a wax resist when it's on too thick.  It's like it causes the glaze to crawl away from it.  I haven't had it actually pull bits of the underglaze with it though.  

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Since it looks like you're using the underglaze as texture, have you tried using stained slip or engobe?  I use a flocculated slip in a bulb syringe for raised designs and it works fantastic.  Fishsauce slip with 5% mason best black isn't quite as black as that, but with 10% I bet it would be!  (And a lot cheaper than underglaze)

Edited by liambesaw

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Thank you all so much. I do not have any knowledge or experience with the chemistry of clay and glazes so I appreciate  what you're sharing!

Benzine, I wonder what you mean by contaminated... the clay? the underglaze? the clear glaze?

Bill Kielb,  I've taken a few more close ups. They may not be the same exact spots you indicated on your photo. I'm not sure you can see much more and I can't seem to get any closer.

Neil, the edges of the bare spots are definitely not sharp, they are more rounded.

liambsaw, youare speaking a foreign language! I sorry, I don't think I know what a stained slip is or an engobe and you completely lost me on the flocculated and Fishsauce slip,  haha.  Although I am really curious it being a lot cheaper than underglaze. Too, I prefer to incise the lines vs raised.

890476879_glazegonewrong4.JPG.f1597ee5236f147c694460c8762c379d.JPG

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Thank you Min for your comments but I don't think the points you raised are the source of the problem. I'm pretty sure the underglaze was not applied too thick  because I like using it like a stain and water it down a bit. Watching it wash down the drain is too expensive.  So I'm confident that the UG wasn't too thick. And this piece, dried for at least a week before I glazed it so that is not likely the problem either.

Edited by tinypieces
put this comment in the wrong place

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Looking at these pictures it looks like the underglaze was ejected from the location. At each of these sites I can see residual underglaze adjacent to the decoration so something made this pop off prior to the melt.

My guess at this point is trapped moisture or air that when heated, literally made these spots delaminate and land nearby in the base glaze.  

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1 hour ago, Bill Kielb said:

Looking at these pictures it looks like the underglaze was ejected from the location. At each of these sites I can see residual underglaze adjacent to the decoration so something made this pop off prior to the melt.

My guess at this point is trapped moisture or air that when heated, literally made these spots delaminate and land nearby in the base glaze.  

Thanks. Since this has happened before in separate firings, what would you suggest I do differently to avoid this from happening again? I have another piece glazed just like this waiting to be fired so I guess I'll just cross my fingers for now.

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2 hours ago, tinypieces said:

I don't think I know what a stained slip is or an engobe and you completely lost me on the flocculated and Fishsauce slip

slips are basically clay bodies (like what you work with) in a suspension of water. I.e. thinned down, kind of like reclaim. Fishsauce is just a recipe, just like there are different clay bodies. A slip is designed to go onto leather hard or wetter/softer works; engobes are slips that are designed to go onto bone dry, or bisqueware. A stained slip is just a slip with color(ants) mixed into it. The best black Liam mentioned is a "color" mixed by the Mason Company, who makes Mason Stains.

Flocculated is a little more complex, but essentially a flocculant is a material which causes the particles to clump together, like a FLOCK of birds. A deflocculant causes the particles to repel each other. Not sure about Liam's flocculated slips, but I use a deflocculated slip, which basically makes the water wetter, which means it will shrink less, which provides a number of benefits. A flocculated slip would be one that would benefit in the attaching of handles; a vinegar(flocculant) slip will help increase the bond between attachments.

Its much cheaper than underglazes because YOU would be mixing the materials together, instead of paying a company to basically do the same thing. Its not rocket science, kind of like baking a cake. Does require some precautions regarding the dust and the dangers of working with raw materials (and colorants more specifically), but easily done. It also means you can mix "custom" colors that a manufacturer may not offer.

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On 2/19/2019 at 11:29 AM, neilestrick said:

Is it shivering or crawling? Are the edges of the bare spots sharp like it flaked off (shivering), or rounded like it pulled away (crawling)?

Hi Neil, I don't know if you saw my response to your post because I didn't put it here. To answer your questions... the edges are not sharp, they are rounded.

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3 minutes ago, hitchmss said:

slips are basically clay bodies (like what you work with) in a suspension of water. I.e. thinned down, kind of like reclaim. Fishsauce is just a recipe, just like there are different clay bodies. A slip is designed to go onto leather hard or wetter/softer works; engobes are slips that are designed to go onto bone dry, or bisqueware. A stained slip is just a slip with color(ants) mixed into it. The best black Liam mentioned is a "color" mixed by the Mason Company, who makes Mason Stains.

Flocculated is a little more complex, but essentially a flocculant is a material which causes the particles to clump together, like a FLOCK of birds. A deflocculant causes the particles to repel each other. Not sure about Liam's flocculated slips, but I use a deflocculated slip, which basically makes the water wetter, which means it will shrink less, which provides a number of benefits. A flocculated slip would be one that would benefit in the attaching of handles; a vinegar(flocculant) slip will help increase the bond between attachments.

Its much cheaper than underglazes because YOU would be mixing the materials together, instead of paying a company to basically do the same thing. Its not rocket science, kind of like baking a cake. Does require some precautions regarding the dust and the dangers of working with raw materials (and colorants more specifically), but easily done. It also means you can mix "custom" colors that a manufacturer may not offer.

I flock mine for raised texture, when it comes out of the bulb syringe it stays put.  I use deflocced for painting.  

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@tinypieces if you want you can use the @ symbol in front of a forum member's name and it will allow you to "link" or notify them that you are directing a comment towards them.

I regards to whether or not its shivering or crawling, even if the edges are rounded, leaning towards a crawling issue, it might not be as easy as that to deduce. It could be shivering off (the opposite of crawling) and just melting into a rounded shape in the clear glaze you fired. Because its a "flat" surface, it would easily melt into the clear glaze and give the appearance of crawling.

Id think that if you were having a "fit" issue (shivering/crawling), we'd see the same result almost everywhere. Whereas its sporadic; Im wondering if you are applying numerous coats of this underglaze, and you have thinned it down, that there may be TOO much water in your second/third coats, which are loosening(clay "ridges" become saturated with water, which decreases the "suck" that porous bisque has to wet glazes) the physical bond the first layer has with your clay. Then, either when the clear glaze is applied over top, you arent seeing the underglaze being "brushed" around, or it is peeling away, like crawling (even though its technically not a "fit" issue) in the firing, and melting into your clear.

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@hitchmiss thanks for the replying tip.

I like that you think I apply 2-3 coats of underglaze! Maybe I should but I don't. Mainly because I don't think I need to and I definitely don't want to see underglaze uneccesarily going down the drain... but I do not. It's usually only 1 coat. Can you water down underglaze too much? Like maybe there wasn't enough pigment laid down in that line and it poofed out?

It's curious because I use these very same techniques, underglazes and clear glazes on two other clay bodies (Standard 103 and 105) without anything like this happening Why do you think it happens with just this clay body?

 

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24 minutes ago, tinypieces said:

Thanks. Since this has happened before in separate firings, what would you suggest I do differently to avoid this from happening again? I have another piece glazed just like this waiting to be fired so I guess I'll just cross my fingers for now.

Since this is all speculation I would start ruling out things. My first would be thoroughly drying these with a really good preheat just below 200 degrees just to be sure that trapped moisture regardless of how minuscule did not cause this to burst early on leaving particles in the immediate area.  After the preheat,  I think I would stop the kiln and inspect. You could always throw these in an oven for an hour or two at about 175 degees. If you are brave there is the microwave which if cycled in small  steps would reveal the moisture as localized heating.

My feeling is, this visual evidence is significant. In my lab I would put them in an evacuation chamber and pull the atmosphere down to 30 microns which would eliminate all possible moisture.  You are not gonna do that. My next thought is (already dry) I would put it in front of a fan, scan it with my infrared camera and see if there are obvious spots that have cooled. Very efffective way to detect moisture evaporating. Again, you are not likely gonna do that.

my last thought would be to take a sample one and gently go around it with a torch to see if I could make a spot or two eject. As you can tell I am seeking the real answer. Most folks would likely do the long preheat and hope that it is indicative for future firings.

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Based on the close-up photo, those edges look pretty crisp to me. In crawling, the glaze pulls away from the clay and globs up around the edges, and I'm not seeing that. I usually equate shivering with the glaze coming off after the firing, but in this case it looks like it popped off earlier in the firing, because it appears to have melted into the glaze, which wouldn't happen if it was popping off after the glaze had hardened. As Bill said, it may be a moisture thing. How long do let the underglaze dry before applying the clear glaze? Do you let the pieces dry completely before putting them in the kiln for the glaze firing?

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I'ld go back to basics, try the underglaze alone, no glaze, and bisque fire it on.  Run 2 tests simultaneously, underglaze on green ware and on bisque. Bisque fire them both with no glaze and see if it stays attached to the clay. After bisquing try tapping the underglaze with a knife handle, see if any flakes off. Put the underglaze on with 3 thicknesses, 1 coat, 2 and 3. 

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