Crawling of clear glaze over underglaze

[GestaltArtworks]

First time poster here.... have been experiencing some glaze/underglaze defects and wonder if anyone has any ideas as to the reason why...

I'm using speedball underglazes and Georgie's No More Tears clear over their Trillum porcelain. The glaze crawling pulls the underglaze with it, exposing the clay body. It has happened with a few colors and more commonly on the edges/lips of plates and bowls, but closed forms also have been affected. The effect is like an archipelago of multiple small spots, a few millimetres (~1/8") diameter, usually.

Has anyone else had this issue?

[GestaltArtworks]

4 minutes ago, GestaltArtworks said:

First time poster here.... have been experiencing some glaze/underglaze defects and wonder if anyone has any ideas as to the reason why...

I'm using speedball underglazes and Georgie's No More Tears clear over their Trillum porcelain. The glaze crawling pulls the underglaze with it, exposing the clay body. It has happened with a few colors and more commonly on the edges/lips of plates and bowls, but closed forms also have been affected. The effect is like an archipelago of multiple small spots, a few millimetres (~1/8") diameter, usually.

Has anyone else had this issue?

Here's a pic!

[neilestrick]

Chances are the underglaze is too thick.

- What color underglaze is it happening to?

- When are you applying the underglaze? Leather hard, bone dry, bisque?

- How many coats, and are you thinning out the underglaze before applying?

- How are you applying the glaze?

[GestaltArtworks]

It's happening to a few different colors, mostly black and teal in this last kiln full.

I keep my underglazes thinned with Darvan 7 & water to the same consistency, and typically apply 3 coats at the hard leatherhard stage with brushes, before sgraffito carving. The puzzling thing is, it's happening randomly yet - like on this bowl - it's just in some places around the rim only, not the whole rim, not inside, not outside (underglaze/glaze is everywhere except the foot). 

On pots that I've left underglaze decoration unglazed, this has never happened, so I had guessed that it had more to do with the glaze/underglaze combo, as this glaze/clay fit is perfect and I've not seen any defects otherwise. Also I wondered if it might have been too close to the kiln element, but the clay technician who fires for me didn't think so, and didn't have a clear idea of what might be causing this.

 

[Min]

1 hour ago, GestaltArtworks said:

keep my underglazes thinned with Darvan 7 & water to the same consistency, and typically apply 3 coats

Why the Darvan and not just water? Darvan is going to slow down the drying time. I have found that each coat needs to be dry before putting on the next.

[Babs]

It looks too thick, Some underglaze colours I have to add frit to so glaze performs as expected. If only on rims it may be that your rims are drier than rest of pot and underglaze doesn't attach , notnoticed until glaze firing often.

[Piedmont Pottery]

Are you bisque firing after the underglaze and before the transparent, or are you single firing?  If you are bisque firing first, how well adhered does the underglaze appear to be before applying the transparent glaze?

[neilestrick]

The Darvan is not necessary, just thin them out with water.

Speedball black will definitely crawl if too thick.

How are you applying the underglaze with the brush? Just brushing it on like painting a wall, or applying it while it spins on the wheel? It's really easy to get it on too thick at the lip if you're not careful, as the brush will wrap around the lip and kind of scrape extra underglaze off the brush.

[GestaltArtworks]

On 10/5/2023 at 10:11 PM, Min said:

Why the Darvan and not just water? Darvan is going to slow down the drying time. I have found that each coat needs to be dry before putting on the next.

Some colors are grainy and need  Darvan periodically to apply smoothly. Yes I'm diligent about applying only on dry previous coats 

[GestaltArtworks]

On 10/6/2023 at 8:51 AM, neilestrick said:

The Darvan is not necessary, just thin them out with water.

Speedball black will definitely crawl if too thick.

How are you applying the underglaze with the brush? Just brushing it on like painting a wall, or applying it while it spins on the wheel? It's really easy to get it on too thick at the lip if you're not careful, as the brush will wrap around the lip and kind of scrape extra underglaze off the brush.

Actually I forgot that on this particular piece it is Spectrum black; all the other colors are Speedball though, and still a couple colors like I mentioned. 

Application is with small brushes for each painted shape.

I've had this situation happen that you mention - especially around lips - and am diligent about making sure there are no blobs like this, the chances are they will break/flake off.

[GestaltArtworks]

On 10/6/2023 at 7:52 AM, Piedmont Pottery said:

Are you bisque firing after the underglaze and before the transparent, or are you single firing?  If you are bisque firing first, how well adhered does the underglaze appear to be before applying the transparent glaze?

Yes bisque then separate glaze firing. I did have to do a bit of touch up of the underglaze on one spot along the rim before glazing, so perhaps this is the answer... underglaze not well adhered! What are the conditions for best attachment/adherence? I'm guessing NOT bone dry as Babs alludes to? I spritz my pieces to maintain some body moisture throughout painting and carving, so painting for me is never on bone dry...

[GestaltArtworks]

On 10/6/2023 at 12:28 AM, Babs said:

It looks too thick, Some underglaze colours I have to add frit to so glaze performs as expected. If only on rims it may be that your rims are drier than rest of pot and underglaze doesn't attach , notnoticed until glaze firing often.

This never occurred to me before.... What are the best conditions to ensure underglaze attaches? And which colours do you add frit to?

[Min]

2 hours ago, GestaltArtworks said:

Some colors are grainy and need  Darvan periodically to apply smoothly.

 

2 hours ago, GestaltArtworks said:

Actually I forgot that on this particular piece it is Spectrum black

I've used a lot of Spectrum underglazes (plus some Speedball, Crysanthos and SPs ones). I've had some lumpy ones that I've needed to sieve but never they have never been grainy. 

The black on the bowl is Spectrum (blue is too). 3 coats of thinned down u.g. with water. Spectrum can be even thicker than Speedball, I add water to make a consistency that can drip off the the tip of the brush, wipe it against the edge of the container then brush it on. I would try thinning a test amount of yours down even more than you have been with just water and do some test tiles with it. See how many coats of diluted u.g. get you the coverage you need.

[GestaltArtworks]

2 hours ago, Min said:

I've had some lumpy ones that I've needed to sieve but never they have never been grainy. 

The black on the bowl is Spectrum (blue is too). 3 coats of thinned down u.g. with water. Spectrum can be even thicker than Speedball, I add water to make a consistency that can drip off the the tip of the brush, wipe it against the edge of the container then brush it on. I would try thinning a test amount of yours down even more than you have been with just water and do some test tiles with it. See how many coats of diluted u.g. get you the coverage you need.

 

Speedball yellow, orange, amethyst and carmine have been grainy for me. Spectrum I love for the smoothness!

I've done what you've suggested with test tiles; all mine have 1, 2 and 3 coats of u.g., with my chosen gloss and matte glazes, so I know which colors need more or fewer layers depending on glaze plan. I apply at the consistency of heavy cream, and am very careful with drying between layers and not applying too thickly. I feel like it's maybe not a 'too thick' issue since most of this 10 cu. ft kiln load wasn't affected, but absolutely I could be wrong so will watch this more closely in the future.  Thanks for taking the time to give me feedback!

[Babs]

If you are touching up underglaze after the bisque then if not dry as rest of pot, those areas withh take glaze differently.

I underglaze just after trimming, spritzing the entire pot ,inside and outfor an even acceptance of the u.g.

I add frit to a couple of blues and a black. Not knowing the products you are using, I don't know if this is relevant. When I don't add the frit , the glaze doesn't "meld" with the underglaze, it "crumples" and crawls.

Rims are tricky for the moisture though, and grabbing more than their share of u.g. therefore.

[GestaltArtworks]

40 minutes ago, Babs said:

If you are touching up underglaze after the bisque then if not dry as rest of pot, those areas withh take glaze differently.

I underglaze just after trimming, spritzing the entire pot ,inside and outfor an even acceptance of the u.g.

I add frit to a couple of blues and a black. Not knowing the products you are using, I don't know if this is relevant. When I don't add the frit , the glaze doesn't "meld" with the underglaze, it "crumples" and crawls.

Rims are tricky for the moisture though, and grabbing more than their share of u.g. therefore.

As this issue is mostly on rims, I think it might be a moisture unevenness problem. Thanks for bringing this up. I'll watch the rims!

[Rae Reich]

I’ve thought that my overglaze has crawled sometimes because I rubbed or disturbed the underglaze while green, so that a surface gets a bit less smooth where rubbed and, later, the glaze doesn’t adhere in the same way. Alternatively, the fired underglaze could have been touched before the overglaze. 

[GestaltArtworks]

On 10/8/2023 at 8:11 PM, Rae Reich said:

I’ve thought that my overglaze has crawled sometimes because I rubbed or disturbed the underglaze while green, so that a surface gets a bit less smooth where rubbed and, later, the glaze doesn’t adhere in the same way. Alternatively, the fired underglaze could have been touched before the overglaze. 

Do you mean the interaction of oils/sweat from bare hands touching before glaze is applied? I always wear gloves when handling bisqueware - after wet sanding - to avoid contaminating the surface.

[Rae Reich]

Yes, oils on hands are sometimes responsible for the problem. Do you also use gloves to unload the bisque?

If you are wet sanding your bisque ware, it could be that you have not completely removed sanding dust from the surfaces. As long as the pieces are wet, you might as well scrub sanded surfaces thoroughly with a brush or green scrubber and rinse well.

Edited October 11, 2023 by Rae Reich

[GestaltArtworks]

On 10/10/2023 at 7:59 PM, Rae Reich said:

Yes, oils on hands are sometimes responsible for the problem. Do you also use gloves to unload the bisque?

If you are wet sanding your bisque ware, it could be that you have not completely removed sanding dust from the surfaces. As long as the pieces are wet, you might as well scrub sanded surfaces thoroughly with a brush or green scrubber and rinse well.

I don't unload the bisque (I fire at a community centre so the technicians load/unload) but do rinse after wet sanding and after that it's glove only handling until after they are glazed. I've had scrubbing damage the underglaze in the past so i only use a sponge now, but wet sand in one sink basin & rinse in the second basin.