High percentage ash glazes in oxidation

[Northen Pots]

Hi everyone,

I'm currently on my 4th or 5th round of testing some high % ash glazes in my electric kiln and seem to have hit a bit of a wall and could do with some input 

I'm trying to achieve the runny, rivulet look of high fire ash glazes and every time I get close another issue pops up. This time round I fired to cone 6 with a drop and hold taking it to a cone 7.

The glaze has melted well, it shows some rivulet effect and pooling, but has run off of the rims , despite dipping the rims 3 times to try and avoid this, and most of the glazes are exhibiting these nasty white patches which appear to be patches of micro bubbles, but they seem to appear at random and don't correspond to thickness application or clay body and seem to crop up at random. The last batch was fired with a longer soak at top temp taking it to a hard ^8 and those tests didn't have any white patches so I think that's a surface tension issue.

The last pic is a test of the same glaze with the longer soak and no bubbles, but still running off of the lip.

Does anyone have any experience with any of these issues or this type of glaze who might have some advice?

the recipe is as follows, if you see any glaring issues please let me know as I'm building my recipes through trial and error rather than any formal glaze knowledge or software.

UNWASHED WOOD ASH    56.4

KAOLIN   18.8

SILICA   13.5

FRIT (3124)  11.3

I've heard that high calcium glazes exhibit the runny ash look so I'm tempted to substitute some of the woodash for whiting but I can't imagine that would help the blistering. 

Ready to pull my hair out so any suggestions are welcome  

 

[Min]

What process are you using to dial this glaze in? Line blend or ?

There aren't a lot of cone 6 ash recipes published, one that has been around for a while is Frasca Ash ^6. Don't know if you've come across it, link here if not. Note how it's really high in ash, as is your recipe but it also includes whiting. It also uses ball clay instead of kaolin, this could be helpful as ball clays are "stickier" so if you have a soft glaze laydown using ball instead of kaolin will help. Regarding the rough rims, often people will glaze a pot with a base glaze then apply the ash glaze to the upper areas of the pot and allow it to run down the pot. Having a stable base layer glaze will help it from running off the rim. If you try that recipe or adjust yours to something similar it's still very much dependent on the chemistry of the ash you are using and that is going to be different with every batch of ash. This is where doing a line blend could be very useful.

I've seen cone 10 ash glaze recipes with under 20% ash, they are using a larger proportion of whiting plus some dolomite to get the rivulets / stringers effect. Even taking an existing glaze you have and doing a progression blend of it plus a blend of ash + whiting should get you some ash effects.

 

 

[Northen Pots]

Thanks for your reply Min, some good points in there.

I've tried ball clay and kaolin and felt like the ball didn't provide enough alumina and that was causing problems with the resultant glass not being stable, although I don't know if that's actually the case. 

Ive been line blending silica and the frit to try and balance melt and stability of the glaze as I'm trying to make this resistant to leaching.

My original test of 75 ash 25 ball clay melted and rivuleted well at cone 8 but leached terribly, hence adding silica to form a better glass then adding frit to maintain the melt accordingly.

I will have a look at that recipe and try some of your suggestions.  Thanks very much Min.

[Northen Pots]

Min - that Frasca Ash recipe is similar to mine - am I correct in my understanding that whiting will have both a fluxing and rivulet effect I were to sub it for the frit in my recipe? 

[Min]

6 hours ago, Northen Pots said:

Min - that Frasca Ash recipe is similar to mine - am I correct in my understanding that whiting will have both a fluxing and rivulet effect I were to sub it for the frit in my recipe? 

If you take out the frit I would add some feldspar, preferably a potash feldspar like custer. If it comes out too matte then add a titch of frit also. Experiment with the thickness of the glaze also.

6 hours ago, Northen Pots said:

as I'm trying to make this resistant to leaching.

Ash glazes are typically used on the outside of pots and a liner glaze used inside. Ash glazes are typically low in silica and alumina.

Have a look at the article by Harry Spring in this link, might be useful.

[Callie Beller Diesel]

How would the boron in the frit be affecting this melt?

[Min]

As a glass former.