Any idea why the glaze is moving like this?

I mixed a large quantity of this glaze early year and I have been using it since. In the past Ididn't have a problem with this batch or previous batches The only difference I can think of is when I used my sitter kiln I couldn't slow down the cooling as well as I can with my controller kiln. About an 1-1/2 to 2 hours longer on the later

One confusing aspect is with the last firing only two very small pots of 20 pots (various sizes) with this glaze were effected.


A few firings ago 1 large bowl of many had this problem.
Three different clay bodies, Standard's 112, 225 and 240

In these photos I'm using 240, If you look closely the bottom of the pot is still white. In the area where the glaze has crawled away from the pot you'll see brown specks and brown edges. Its my thinking that is from the red iron oxide and the rutile.

Oxide Formula
CaO     0.517
MgO    0.261
K2O     0.018
Na2O   0.203
P2O5   0.001
TiO2     0.162
Al2O3   0.340
B2O3    0.337
SiO2    3.682
CoO     0.044
Fe2O3  0.055

Fired to ^6

Any input would be appreciated

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Your oxide formula doesn't tell us about your raw materials. Glazes with high clay content shrink and crack and are somewhat de-adhered while drying. Then at temperature, if the melting glaze has a high surface tension, it tries to ball together, pulling away from clay surfaces with weaker bonding than the force of this surface tension. The solution in this case is to replace raw clay with calcined clay to reduce drying shrinkage. The other classic causes of crawling are skin oils or excessive clay dust on the bisqueware, causing poor adhesion of wet glaze to the bisque body. 

Opposite in effect from surface tension are the fuming/wetting properties of a particular glaze. You see this in glazes that will bridge, fill and seal cracks in the clay body and heal over surface defects, or even sealing up over wax resist designs that you wanted to remain unglazed.

I'm seasonably sure its not skin oils or resist, and I do wipe the pots with a damp sponge before dipping the pot into glaze.

Glaze Base

Ferro Frit 3134............. 29.000
Silica...................... 26.000
EP Kaolin................... 22.000
Minspar 200................. 9.000
Pioneer 2661 Talc........... 10.000
Wollastonite................ 4.000
Plus Colorants

Do you think maybe 5grams of Calcined EPK and 17 of EPK is enough of a change?

Olive Green Celadon shares many features of your glaze - a runny glaze with too much magnesium.

I think the solution is to replace half the magnesium with calcium - or apply it over a calcium glaze with no magnesium.

.

1.) Many of the ingredients of the glaze (3134, Talc and Wollastonite) become liquid at a low temperature (around Cone 016)  while the rest of the ingredients are still solid;

2.) These glazes, and two of the low temperature melters, have a high percentage of magnesium which is used to create crawl glazes due to the high surface tension as George mentioned.

3.) These glazes have a very high Silica to Alumina ratio (about 11 to 1) - with means they're very runny, even after everything is melted;

You've noticed Iron binds readily to clay leaving behind the coloration, but virtually nothing else from the glaze.

You can see how thick the glaze is where it still remains - because that's where the missing glaze went.

If you apply a gaze prone to crawling thickly enough, it can be as thick as the surface tension wants to be, so no crawling - but that turns out to be thicker than you think, as on the bottom of the pot, and not that attractive.

Thanks Norm I'll give those suggestions a try.

BTW, sorry it took so long to return and view this message.

Wiping with a sponge can cause crawling if the sponge has picked up any dirt/clay, If you are in a dry area (not humid but arid) you can quickly rinse the pot under running water but your glazing technique has to adapt to this. An air gun is better than a sponge or rinse off when they come out of the kiln, let dry and cover loosely with plastic to keep clean.

RS said:

I'm seasonably sure its not skin oils or resist, and I do wipe the pots with a damp sponge before dipping the pot into glaze.

Glaze Base

Ferro Frit 3134............. 29.000
Silica...................... 26.000
EP Kaolin................... 22.000
Minspar 200................. 9.000
Pioneer 2661 Talc........... 10.000
Wollastonite................ 4.000
Plus Colorants

Do you think maybe 5grams of Calcined EPK and 17 of EPK is enough of a change?

Sorry, also for a recipe like this I made up a second batch with the entire EPK calcined and then mixed the two but I would not do that unless you were having more serious problems (you said only two small pots). You could though for instance just mix up a quarter size batch with all calcined EPK; mix the two and see if that helped. 

Donna Kat said:

Wiping with a sponge can cause crawling if the sponge has picked up any dirt/clay, If you are in a dry area (not humid but arid) you can quickly rinse the pot under running water but your glazing technique has to adapt to this. An air gun is better than a sponge or rinse off when they come out of the kiln, let dry and cover loosely with plastic to keep clean.

RS said:

I'm seasonably sure its not skin oils or resist, and I do wipe the pots with a damp sponge before dipping the pot into glaze.

Glaze Base

Ferro Frit 3134............. 29.000
Silica...................... 26.000
EP Kaolin................... 22.000
Minspar 200................. 9.000
Pioneer 2661 Talc........... 10.000
Wollastonite................ 4.000
Plus Colorants

Do you think maybe 5grams of Calcined EPK and 17 of EPK is enough of a change?

Donna

In the past I have tried wetting the pots like you mentioned, to humid around for it to work well here but thanks for the thought.

I decided to experiment with a different base, while the resulting glaze is really nice it wasn't what I was looking for ( I plan on adding it to my list of glazes I like and use)

After playing around and trying to figure out what went wrong with the batch I'm beginning to think that somehow it got contaminated. I'm going to mix up 500g of fresh glaze to see if this is the case

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