Rice ash glaze recipe adjustment

[reneetje]

Hi there,

I'm new on this forum and quite new to ceramics and glazes in general so please excuse my lack of knowledge...

I've been making ash glazes using this Katherine Pleydell-Bouverie nuka glaze recipe;

Mixed woodash 33.0
Feldspar 33.0
Kaolin 16.5
Ball Clay 16.5

On a recent trip to China I aquired some rice straw ash and I was really excited to turn this into a glaze. From what I read rice straw ash should be very high in silica too like other (mixed) wood ashes so I used the same recipe. The results came out rather different then expected or other wood ash based glazes I made using the same recipe. It looks like the glaze didn't melt or run at all. Please see photos for your reference. It seems like there is a lack of silica and fluxed present in the glaze. I want to do some line testing by adding silica in 10% increments and another one adding flux like gersley borate but perhaps somebody here sees something I'm missing or has other ideas.  The other thing that could be possible is that I misunderstood in China and I've acquired rice HULL ash instead of rice straw ash.

I'm firing in an electric kiln on 1240 degrees and have gotton desirible effects with mixed wood ash glazes based on the same recipe and firing.

Thank you for any suggestions or advice.

 

[C.Banks]

I was curious and found this clayart post. 

I would suspect rice hull rather than rice straw ash as well - or a mistake in the mixing.

A blend with gerstley borate is a good thought. I'd try 5, 10, 15, 20. 5% might not be enough and 20 sounds like a lot but smaller increments will help narrow the number.

This is a good place for ceramic adventures. Goodness knows we've all had our share of ??? moments.

[Tyler Miller]

Wood ash is generally a source of flux, unwashed woodash a mixed of soluble potash and soda ash, washed woodash is mostly calcium carbonate.  There is generally very little silica (with exceptions).

Rice straw ash is high in silica (over 50% I think), like bamboo ash and other grasses—wheat straw ash can be silica rich too.

Rice hull ash is almost entirely silica.

[reneetje]

@C.Banks Hi and thank you for your response. Going to read the clayart post and will definitely try doing line tests with 5% increments of gerstley borate!

@Tyler Miller That's what I thought - rice straw ash high in silica-, but judging the glaze mixture (in the photos) I thought the glaze seems to be lacking silica, or do you expect it's just a good flux that's missing?

[Tyler Miller]

9 minutes ago, reneetje said:

@C.Banks

@Tyler Miller That's what I thought - rice straw ash high in silica-, but judging the glaze mixture (in the photos) I thought the glaze seems to be lacking silica, or do you expect it's just a good flux that's missing?

You’re missing flux.  While I wouldn’t put too much weight on this comparison, the rice straw ash probably has more in common with the feldspar than it does with washed mixed hardwood ash.

[hitchmss]

It seems contradictory, but when you have a runny glaze, adding silica slows/stops your runs.

A feldspar/flux is a material which aids in lowering the melting point of silica. Think of it like a ratio; in theory, if it takes 1 part flux, to melt 1 part silica fully, then if you have 1 part flux, and 2 parts silica, it wont melt. Just because your surface isnt glassy doesnt mean there isnt enough silica present, it may just mean you dont have enough flux to make the silica melt.

No experience with rice ash myself, but if its high in silica you likely just dont have enough feldspar. Try increasing your feldspar in 2-3% increments, while you decrease your ash content in the same 2-3% increments. A line blend of maybe 4-7 combinations will likely yield a good result. What feldspar are you using?

I also assume you mean 1240 C, and not F, right?

[reneetje]

@hitchmss Thank you so much for your reply as well.  Your explanation of the flux and silica ratio is very helpfull! 

I'm using kali feldspar which I believe is the most commonly used feldspar in The Netherlands - where I'm based. And yes, you are right to assume I was talking about 1240 Celcius and not Farenheit! Should have stated this more clearly.

Will go ahead with the two different suggested line tests (adding flux in 5 % increments and increasing feldspar & decreasing ash). Thank you!

[C.Banks]

Uncertainty in results drives me a bit around the bend at times.

If you have rice straw ash that you are certain of you can help to eliminate some uncertainty by melting a small amount of known material alongside what may be misidentified material.

Melt tests are useful for comparing all sorts of stuff.

 

 

[reneetje]

Thank you @C.Banks - really helpful again! I don't have rice straw ash that I'm certain of but can see if can get some through somebody else.