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Dear all

Hope this is the correct place for such a question…

Recently I started to use for the first time some colored glazes (before I’ve always used uderglazes+clear glaze, cone 07. Never a problem). I've bought some commercial ready-to-use liquid glazes (cone 08-07 ) by an Italian producer (Colorobbia, line HSC). I know that those glazes are intended primarily for hobbyists and schools, so they should be quite forgiving, and is pretty viscous so its certainly added with CMC or something else to be applied with a brush... I did some tests with small  objects like beads and pendants for necklaces but unfortunately some colors give always some defect... basically they “crawl” in some zones and make some pin hole...! I know that it could be due to excessive thickness of glaze, but otherwise for some colors it happen also with thin layers and others (if the thickness is smaller) have an inhomogeneous texture, they do not spread well… this is pretty odd because some other colors have never given problems… being all part of the same line I suppose that the basic composition is always the same, and only change the stain.  There’s certainly something that I’m doing wrong but sincerely I don’t know what… !!!

Somebody could have any  idea?

IMG_20171201_122743p.jpg

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Since the crawling is happening in small spots and not over the entire piece, I would guess that the pieces were not entirely clean...almost like there is some sort of material acting as a resist in those small spots...possibly hand oils. You might try wearing latex gloves while handling the pieces out of the bisque firing and while glazing to see what happens.

JohnnyK

 

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Yes, this was also my scuspect but the fact that this is happening only with some colors (in particular that darn pink!) and not others is pretty puzzling... moreover I washed with water (I think carefully) and dryed in the oven all the stuff before glazing. This also because I've previously sandpaper all the surfaces and I was scared that some dust could remain in the pieces... Could be that some glazes are more sensitive to greases or dust even if they "belong" to the same line?

Andrea

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I recommend that after being sure that no finger oils (which cannot be removed with just water, add dish soap which dissolves oil)) or dust remain, test the problem glazes by brushing different areas with one, two and three coats of glaze. It could just be that the application is too thick on such small pieces. 

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Rae:

The % of flux in a cone 6 body is much lower than cone 04/06: and even less in cone 10. While in most cases crawling usually occurs because of dust or oil, and or too thick of an application: in some cases it is a failure to fuse. The clay/ glaze interface is both a chemical and mechanical bond. The lack of flux in a clay body can on some occasions cause crawling. Question of curiosity,  because clay gets over looked. As in this example: you can see some of the clay pulled into the glaze, and some of the glaze pulled into the clay. If the clay lacks the required level of flux, or fired well under formulated cone value: this mechanical bond does not form.

gallery_73441_1250_32790.jpg

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@andros are the glaze you are using the one at the top of this page?  To me it looks like you have it on much thicker than the tiles on that link.  Think I would try some test tiles with some embossed stamping then thin, med and thick applications and see how they turn out, watering down the glaze if necessary. Thinking you should get a better idea of thickness if you can see the glaze pooling like on the test tiles from the manufacturer.

Also, when you said "I've previously sandpaper all the surfaces" can you avoid doing that and just use a damp sponge instead? Much safer for your lungs and those around you.

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Wow! How many comments and questions! Thanck you very much! So, I try to answer... unfortunately busy days plus big time zone difference doesn't allow me to do it in time...
 

16 hours ago, glazenerd said:

What is the cone rating of the clay body? 

The body is fired at cone 07 (approx, 980°C). According to the producer the clay should be fired at cone 07 - 05 1/2 (approximatelly). 980°C is at the bottom of range but with clear glaze it always worked well...

13 hours ago, neilestrick said:

I think they need to be fired hotter, like cone 05 or 04. I think they're getting to their bubbling stage but not getting hot enough to smooth out completely. When glazes cool in their bubble stage, they often crawl.

May be... but the producer delear  a firing range of cone 08-07... Colorobbia is a big company that produces mainly for industries, I can't immagine that it misses the firing range of it's glazes...  Am I too confident?

15 hours ago, Rae Reich said:

Are you thinking that the glaze shrinks more than the clay?

Actually I have no idea... but with a shrinking problem I think that the problem would be some crazing, it is not so? This problem looks to me that happen when the glaze is still molten...

13 hours ago, oldlady said:

andros, on the pink one and the flat, square turquoise one, i notice a VERY thick application.  maybe it is just the photo.

The pink it's definitely thick, yes... But sincerely i didn't manage to spread it properly with brush with inferior thickness, so the texture appears dishomogeneous, and the crawling still happens! Even if reduced...

7 hours ago, Min said:

@andros are the glaze you are using the one at the top of this page?  To me it looks like you have it on much thicker than the tiles on that link.  Think I would try some test tiles with some embossed stamping then thin, med and thick applications and see how they turn out, watering down the glaze if necessary. Thinking you should get a better idea of thickness if you can see the glaze pooling like on the test tiles from the manufacturer.

Also, when you said "I've previously sandpaper all the surfaces" can you avoid doing that and just use a damp sponge instead? Much safer for your lungs and those around you.

The glaze is just that.

All this discussion (thank you for this!!) bring me to think at multiple solutions:


to take more care about dust and oil and avoid as much as possible to sandpaper;


to try to adjust the consistency of the glazes in order to give a thinner and more homogeneous layer with brush.

Anyway... I use this glazes also for "cuerda seca" technic... and I have the problem of crowling  also with it.  In this technic thick layers of glaze are pretty unavoidable and is one of the distintive characteristic!

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what cone are you bisque firing to? have you washed the pieces with sponge before glazing, just to put a little moisture in the piece to slow glaze absorption? Do the pieces react differently in different placements in the kiln?

 

best,

Pres

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Both bisque and glaze are fired at  at cone 07 (approx, 980°C), with the same firing program. 980°C should be a temperature suitable both for clay (according to the producer the clay should be fired at cone 07 - 05 1/2) and glaze... Actually I washed the pieces (in order to remove dust) but not just before glazing... in the case of cuerda seca I use squeezable bottles  to apply the glaze, and excessive the absorption looked to be not an issue... I didn't do systematic test in order to see if pieces react differently in different placements in the kiln, but my impression is that this doesn't happen...

Anyway, just some minutes ago I thought to one fact that could be decisive: I'm forced to use somebody else kiln to fire my stuff, and the owner of kiln has difficulty to attend all the firing program, and doesn't feel confortable to leave the kiln not attended, so almost always switch off the kiln as it reach the desired temperature... so it doesn't have  an holding period and has a uncontrolled cooling... May be this can cause such a problem? I think it's likely...

Andrea

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Washing just before glazing will help ease glaze application, and keep from being overly thick. I have found with brushing that this can be a problem. When I was not able to wash a piece because of delicate areas, I would use s mister to spray the piece with water first, let set for a few minutes and glaze. You may find this helps.

 

At the same time, cooling cycles and hold at peak are important to the glaze maturation. I would think you will get more pin holing that crawling, but both could happen.

 

Humble thoughts,

best,

Pres

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On ‎14‎/‎12‎/‎2017 at 9:38 PM, glazenerd said:

Rae:

The % of flux in a cone 6 body is much lower than cone 04/06: and even less in cone 10. While in most cases crawling usually occurs because of dust or oil, and or too thick of an application: in some cases it is a failure to fuse. The clay/ glaze interface is both a chemical and mechanical bond. The lack of flux in a clay body can on some occasions cause crawling. Question of curiosity,  because clay gets over looked. As in this example: you can see some of the clay pulled into the glaze, and some of the glaze pulled into the clay. If the clay lacks the required level of flux, or fired well under formulated cone value: this mechanical bond does not form.

gallery_73441_1250_32790.jpg

Does this mean that I possibly should bisque fire at higher temp. of fire the glaze or both (at highr temp.)?

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Bisq lower, so the clay remains more open and porous. Glaze fire to cone 04 to start; from reading you are firing to cone 06?  The type of work you are making requires that you do not get the glaze excessively fluid: the danger of running into other colors goes up as it gets more fluid.

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dearest nerd,   i guess i will remain confused.  i am rarely confused but it has been known to happen.:wacko:  it has been a severely trying day but i lived through it thus far and have hopes to reach midnight without further harm.

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14 hours ago, glazenerd said:

Bisq lower, so the clay remains more open and porous. Glaze fire to cone 04 to start; from reading you are firing to cone 06?  The type of work you are making requires that you do not get the glaze excessively fluid: the danger of running into other colors goes up as it gets more fluid.

I bisque at cone 07 that is the minimum firing temp suggested by clay supplier... Then I fire the glaze at cone 07 as well... in this case this temperature is the maximum suggested, but actually I never faced a problem of running, but rather the  problem of crawling... both with cuerda seca and "normal" glazing. Anyway one test I can do (together with apply a thinner layer of glaze and take more care about oil and dust) could be to fire the glaze at cone 09-08... if successful with this temperature I could use also some other amazing glazes that now I need to fire aside...

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