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Hi, 

I wonder if someone can say if this is crawling or not? I'll post pictures in a minute. It works fine on some pots but not others. It gives little bare patches and also some bubbly bits occasionally. 

I'm thinking of increasing the frit or maybe thinning glaze a bit...

Thanks.

 

            Blue
wollastonite           9
Frit 3134           17
China clay           22
quartz           13
soda feldspar           30
talc           9
        0 £0.00  
        0 £0.00  
             
cobalt oxide       0 £0.00 0.2
Zirc Silicate           7

 

 

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Do raw glaze/single fire? Looks like outgassing from the claybody causing pinholes, the 3134 in the recipe will be an early melter, if the glaze seals over the body before the gasses have had time to escape (either during a bisque or in the earlier stages of a glaze firing) the gasses will blow through the glaze and cause the kinds of craters and pinholes like in your photos. If you do bisque fire post your firing schedule, might need slowing down.

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10 minutes ago, liambesaw said:

Pinholing, high viscosity melt, probably due to the magnesium content.  With a more fluid melt the bubbles may heal themselves, but I've found the manganese they use in those speckled bodies gasses off a LOT and late in the firing too.  Need a glaze that's less viscous in my opinion

A confusion of related ideas is presented in please explain these bubbles http://www.potters.org/subject35206.htm

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If the glaze is a stiff melt with a high surface tension at the firing temperature, I would look for application issues.  
My Shino glaze will do this (often deliberately used as a decorative effect) if applied thick on dry bisqued surfaces.   Also with celadons and other thick glaze slurries.  These glazes have high melt viscosity and high surface tension.   

higher firing temperature may help, but the increase in temperature to lower viscosity (and surface tension) may larger than you want to use.   From my glass work, I would not expect longer time at same temperature will be effective.  
LT

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Hmm top temp could be an issue. I usually only get up to around 1210C which I know is low for cone 7, even though cones are well melted. I did hold for a half hour at top temp in this firing, and downfire to 1100. I don't fire super slow - around 8 hours so not sure why top temp is low.

I bisque fire in a larger gas kiln, its about 12 hour bisque with 3 hours between 700C and 900C.

 

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9 hours ago, tomhumf said:

Hmm top temp could be an issue. I usually only get up to around 1210C which I know is low for cone 7

Hmm, do you use cones everywhere  or just fire to temp? UMF for this glaze looks fine and if I picked your materials correctly you should have plenty of boron to melt at cone 6 so in theory no need for more fritt.  It has a decent  flux ratio, and when fully fired appears it should be a gloss ............. thinking......... have you ever fired this progressively on test tiles  to get an idea what cone range it performs best at?

F83460A4-1848-4392-B485-3C67845793A8.png

Edited by Bill Kielb
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20 hours ago, Bill Kielb said:

Looks under fired actually with some pinholes. Traditional crawling below. Will need to put this into a glaze calc to form further opinions which won’t be for a bit actually.

95FC0706-BE61-47A0-B983-E862127376A9.jpeg

Hi Bill. I've just had this happen with my test glaze pieces. Do you know what's the fix for glaze crawling is? 

Thanks N 

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@Naomi
Source is all over it! A common reason If it’s a glaze that usually works, too thick of an application is often the cause and glaze that has dried out and gotten too thick in the bucket (for dipping glazes) is often the cause of a too thick application.

Often after dipping you will see very fine cracks as the glaze dries and shrinks which is a tip off the glaze application is too thick and likely will crawl.

Edited by Bill Kielb
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On 11/24/2020 at 11:58 PM, tomhumf said:

 

Hi, 

I wonder if someone can say if this is crawling or not? I'll post pictures in a minute. It works fine on some pots but not others. 

Is it the same position of kiln which gives problematic pots?

Are the pots which fail glazed last therefore drier because your wiping clean moisture is gone?

End of bucket so glazed thicker if dipping?

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Bill - I do fire to cones and temperature, it was only hitting around 1210C with cone 7s down before.

I've done two firings since posting this message. Both times I've got kiln up higher in temp - around 1230C and held for 30mins. The cones are very melted, it might be nearer cone 8 now. 

All pots so far have been problem free, glaze is a bit shinier, so it must just have been underfiring before I think. 

I do prefer the more Matt look. I may try increasing clay, reducing quartz with new firing style. I'm not sure if that might bring this problem back though.

Edited by tomhumf
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@tomhumf “I do prefer the more Matt look”

Maybe this will help, increasing quartz will generally make it glossier ........ to a point. Increasing clay will increase quartz and alumina together and will generally push the melting point higher. In a glaze that melts fully you should be able to go from true matte to gloss just by increasing the quartz from a si:al ratio of approximately 5:1 (matte)  to prox 9:1 (gloss) with around 7:1 highest gloss.

If I entered your recipe correctly its fired appearance is a matte but at 7:1 should be fairly glossy. It has enough boron to melt at cone 6 so next question is are there colorants added that are refractory or maybe a high iron claybody in oxidation which is refractory and of course you fire down so that can matte up a glaze nicely. As well.

I am not a fan of holding at top temperature since it basically adds another cone or two of heatwork (just another rabbit hole to chase) so unless you have a reason such as preserving color, holds at the top can cause pinhole issues.  Folks preserving color will often fire to a lower temp by a cone or so with a hold to still get the required heatwork. So 5/15 is a common one, fire to cone 5, hold 15-20 minutes, you get cone six without having to hit the cone six temperature helping preserve color of some underglazes and stains.

Just a personal preference but I like to establish a glaze that fires per composition, (matte as matte, gloss as gloss) nothing fancy, straight firing to cone, then dial up the look and texture with simple additions or subtractions like quartz and or down firing to get a specific look.

The clear glaze in the picture below fully melts at cone 5 and is a true matte. It was designed to melt  readily to go over heavy underglaze that can be refractory. It fires matte, straight away (no holds) because it’s SI:Al ratio is 5:1 or less. Just progressively adding silica, this glaze will go to a high gloss if I like. In other words it performs as its chemistry indicates which makes it much easier to tune in a particular direction.

Not sure that helps, but might give you something to try or contemplate.

A35895CB-41AE-4B09-AD6D-59F59C5F69AB.jpeg

Edited by Bill Kielb
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