Wood ash glaze conundrum

[Sam D]

Hello all,

I use a homemade wood ash glaze with ash I collect from my pellet stove. I vacuum it out and collect throughout the winter. I use the Leech wood ash recipe which calls for:

40% mixed wood ash, 40% BPS potash feldspar, and 20% ball clay.

Now, I use:

40% mixed wood ash, unwashed, and ultimately contaminated with other dust and stuff from the vacuum. 40% custer feldspar, and 20% ball clay.

So I'm already a little off recipe, but the problem I'm having is after I dip a bisqued piece in the glaze, or spray it on something, it dries slowly, always cracks apart, and sometimes flakes right off. If the glaze applies thinly this cracking is reduced. (so maybe that's the problem just make it thinner?) However once fired it's beautiful, it all holds together and shows no signs of crawling, nor does it run or have any other defects. This has always helped me overlook the bisque problems because it fires so well, but it's still an inconvenience and something I feel could be fixed. I recently tried adding 2% bentonite and it seems to help a bit but not enough for me to feel it's resolved. I think there is also a flocculant problem because after mixing and wetting the dry glaze mix, sieving a bunch of times, it will thicken over time and I have to add a lot more water to keep it the right consistency. I know this messes with the gravity of the glaze I just don't know why or how. The recipe also recommends 4% whiting to help it melt more but that's not a problem I'm having so I haven't added any (also because I don't have any). 

So I'm hoping one of the many great minds on this forum will have some insight into how to help this glaze behave a little better. I'll include some photos to show it glazed bisque, and fully fired. Thanks all!

Edit: having trouble adding photos, the computer says the files are too large so i'll keep at that.

Edited May 22 by Sam D

[neilestrick]

I would try substituting some or all of the ball clay with kaolin for starters, and adding a little bentonite. If that doesn't do the trick, try washing the ash at least a couple of times. Unwashed wood wash has a lot of soluble material in it that affects flocculation. Washing will probably affect the melt a bit, so it may require a recipe adjustment. FYI, unwashed ash is very caustic, so it's a good idea to wear gloves while dipping and definitely wear a respirator when spraying. Washing it will make it less caustic, so that's a bonus.

[Callie Beller Diesel]

+1 for the clay being the problem, not the ash. Or at least less the ash. Glazes with clay in them also shrink when they dry, not just your pots. 
Another idea would be to calcine part or all of the ball clay if you don’t want to change any materials. You will have to do a little math to keep the proportions the same, because the calcined clay will weigh less than the raw, but will be supplying the same amount of oxides. 

The low math version of doing this involves weighing the clay you want to calcine, fire it and add it to your glaze bucket. eg, if your recipe has 20% ball clay and you want to calcine half of it, you’d weigh 10% to be fired first and the other 10% as usual. 
 

If you want to figure it out by math, you’ll have to look up the specific LOI (loss on ignition) of the brand you’re using and reduce the weight of the calcinend portion by that amount. 

[Sam D]

35 minutes ago, neilestrick said:

I would try substituting some or all of the ball clay with kaolin for starters, and adding a little bentonite. If that doesn't do the trick, try washing the ash at least a couple of times. Unwashed wood wash has a lot of soluble material in it that affects flocculation. Washing will probably affect the melt a bit, so it may require a recipe adjustment. FYI, unwashed ash is very caustic, so it's a good idea to wear gloves while dipping and definitely wear a respirator when spraying. Washing it will make it less caustic, so that's a bonus.

I'll try a small batch subbing epk for the ball clay, see how that goes, and if that doesn't react well I'll try half ball and half epk perhaps. I've gone back and forth on whether or not to wash the ash but I really like the color I'm getting now, but could still be worth experimenting. I learned the caustic lesson the first time I mixed this glaze, lost some layers on my hands for sure! Not fun. Thanks so much for the advice!

[Sam D]

2 minutes ago, Callie Beller Diesel said:

+1 for the clay being the problem, not the ash. Or at least less the ash. Glazes with clay in them also shrink when they dry, not just your pots. 
Another idea would be to calcine part or all of the ball clay if you don’t want to change any materials. You will have to do a little math to keep the proportions the same, because the calcined clay will weigh less than the raw, but will be supplying the same amount of oxides. 

The low math version of doing this involves weighing the clay you want to calcine, fire it and add it to your glaze bucket. eg, if your recipe has 20% ball clay and you want to calcine half of it, you’d weigh 10% to be fired first and the other 10% as usual. 
 

If you want to figure it out by math, you’ll have to look up the specific LOI (loss on ignition) of the brand you’re using and reduce the weight of the calcinend portion by that amount. 

I was just reading about calcining materials but only read about alberta slip, I didn't know I could calcine the ball clay. So I would measure out the same amount, but I shouldn't calcine the whole batch? I'll read what you posted before asking too many more questions 😁 Thank you!

[Callie Beller Diesel]

I can use some real numbers as examples so it’s maybe more clear. 
If you want 1kg (1000g) of your glaze and the ball clay is 20% of the recipe, there’s 200g total of uncalcined ball clay.  
The low math version involves weighing out half your ball clay (100g) and calcining it, and adding it to your bucket with the other 100g of uncalcined ball clay. 
 

The more math version:

I don’t know what you’re using for ball clay, so I’ll use OM 4 as an example. According to Digitalfire, the loi for that material is 9.3%. 
So assuming you had already calcined some ball clay and just needed to weigh it out, you’d use 100 g of regular ball clay and 90.7 g of the calcined stuff for that same 1kg batch using adjusted materials. 
 

 

[Sam D]

13 minutes ago, Callie Beller Diesel said:

I can use some real numbers as examples so it’s maybe more clear. 
If you want 1kg (1000g) of your glaze and the ball clay is 20% of the recipe, there’s 200g total of uncalcined ball clay.  
The low math version involves weighing out half your ball clay (100g) and calcining it, and adding it to your bucket with the other 100g of uncalcined ball clay. 
 

The more math version:

I don’t know what you’re using for ball clay, so I’ll use OM 4 as an example. According to Digitalfire, the loi for that material is 9.3%. 
So assuming you had already calcined some ball clay and just needed to weigh it out, you’d use 100 g of regular ball clay and 90.7 g of the calcined stuff for that same 1kg batch using adjusted materials. 
 

 

Excellent, and yes I'm using OM4. I'm going to calcine 1000g of it so I have some extra, and don't waste so much propane heating up my kiln. I've see different temps to bring it to, from 1000-1500f. What do you recommend, seems around 1450?

[Dick White]

26 minutes ago, Sam D said:

Excellent, and yes I'm using OM4. I'm going to calcine 1000g of it so I have some extra, and don't waste so much propane heating up my kiln. I've see different temps to bring it to, from 1000-1500f. What do you recommend, seems around 1450?

To calcine clay (ball or kaolin), just put a big bisqued bowl of it in your next bisque firing. Unlike some other materials that might sinter if calcined to too high of a temperature, clay is refractory and can withstand bisque temperatures, so just use your next bisque firing for calcining it rather than using your propane for firing a single item.

[Min]

Could also try adding a bit of CMC gum solution to the glaze.

[Sam D]

Update!

Made a test batch of 1/2 ball clay and 1/2 epk and it's working great thus far as far as glaze cracking and coming off is concerned. Actually seems to be acting like a normal glaze. Also in the process of calcining some ball clay so am excited to try that as well. Will be able to fire both soon and see what happens! Thanks everyone, I'll repost once I've fired the new batches.

[Magnolia Mud Research]

Sam:

If I were doing what you are trying to do: I would do the following:
1. Use all of the dry ashes and the water that is use to suspend the solids in the glaze slurry.  I would also add some strong clear vinegar to convert the potassium and sodium caustics to netural that becomes dissolved into the glaze slurry. I would also measure the amount I use so that I would know the amount to use next time.   
2. Add about 4 grams of sugar for ever 100 grams of dry ash that is used for the glaze. The sugar will help keep the calcium in the dry ash from converting the glaze slurry to a solid lumps of concrete after setting for a day or so. If the slurry goes solid over night I would know I did not add enough sugar. The 4% is based on the assumption that the calcium compound is about half of the total weight of the ash. If the calcium were less than half I would use less sugar, and if more than half, use more sugar.   

My reasoning:  Loosing the potassium and sodium components is a waste; both helps melting of the glaze (think American Shino).  Calcium is a main ingredient for concrete and sugar slows setting of concretes; I learned that from a university professor that studied concretes; a bucket of such glaze with sugar survived half a year before I got bored with it and switched to a different base glaze.  

It is the potassium and sodium that creates the great coatings in wood fired ware along with the other elements released from the buring wood that also get cared to the surfaces of the wood fired wares.  If done right, using wood ash as a applied glaze one can almost make pots in a non-wood fired kiln that will appear to have been wood fired. 

LT
 

[Kelly in AK]

I can’t add more to this excellent discussion in technical terms, but offer some general principles that could be “useful ways to think” about your materials. Ball clay shrinks a lot. It’s great for keeping things in suspension while also giving you the alumina and silica the glaze needs. It has good green strength, so helps dry glaze be less powdery. Kaolin is the star for alumina and provides plenty of silica, among its other good qualities. It has lower green strength, but shrinks less, and will still aid in suspension. It’s more expensive, but that’s truly a drop in the bucket compared to your time. The reason bentonite is used in a glaze is to help with suspension, that doesn’t seem like a problem here. Both the ball clay and the bentonite hold on to water and will make the glaze dry slower. CMC may, by its sheer “gluey” force, may keep the glaze from peeling apart, but it will add to the time it takes your glaze to dry. 

My instinct when you said you added bentonite (to a glaze with 20% ball clay) was that it would make the problem worse because it’s only adding to the shrinkage issue. That’s my sense of the root of your problem. I think kaolin in general is a good direction. Calcined kaolin is the off-the-shelf workhorse for reducing wet to dry shrinkage. 

 

[PeterH]

17 hours ago, Magnolia Mud Research said:

2. Add about 4 grams of sugar for ever 100 grams of dry ash that is used for the glaze. The sugar will help keep the calcium in the dry ash from converting the glaze slurry to a solid lumps of concrete after setting for a day or so. If the slurry goes solid over night I would know I did not add enough sugar. The 4% is based on the assumption that the calcium compound is about half of the total weight of the ash. If the calcium were less than half I would use less sugar, and if more than half, use more sugar.   

 

A thread testing the effect of sugar versus some other additives in a related context.

 

[Sam D]

On 5/22/2024 at 4:38 PM, Magnolia Mud Research said:

Sam:

If I were doing what you are trying to do: I would do the following:
1. Use all of the dry ashes and the water that is use to suspend the solids in the glaze slurry.  I would also add some strong clear vinegar to convert the potassium and sodium caustics to netural that becomes dissolved into the glaze slurry. I would also measure the amount I use so that I would know the amount to use next time.   
2. Add about 4 grams of sugar for ever 100 grams of dry ash that is used for the glaze. The sugar will help keep the calcium in the dry ash from converting the glaze slurry to a solid lumps of concrete after setting for a day or so. If the slurry goes solid over night I would know I did not add enough sugar. The 4% is based on the assumption that the calcium compound is about half of the total weight of the ash. If the calcium were less than half I would use less sugar, and if more than half, use more sugar.   

My reasoning:  Loosing the potassium and sodium components is a waste; both helps melting of the glaze (think American Shino).  Calcium is a main ingredient for concrete and sugar slows setting of concretes; I learned that from a university professor that studied concretes; a bucket of such glaze with sugar survived half a year before I got bored with it and switched to a different base glaze.  

It is the potassium and sodium that creates the great coatings in wood fired ware along with the other elements released from the buring wood that also get cared to the surfaces of the wood fired wares.  If done right, using wood ash as a applied glaze one can almost make pots in a non-wood fired kiln that will appear to have been wood fired. 

LT
 

That's really interesting and sounds worth exploring. I'll have to test this mixture out and see how it does.

[Sam D]

On 5/22/2024 at 10:52 PM, Kelly in AK said:

I can’t add more to this excellent discussion in technical terms, but offer some general principles that could be “useful ways to think” about your materials. Ball clay shrinks a lot. It’s great for keeping things in suspension while also giving you the alumina and silica the glaze needs. It has good green strength, so helps dry glaze be less powdery. Kaolin is the star for alumina and provides plenty of silica, among its other good qualities. It has lower green strength, but shrinks less, and will still aid in suspension. It’s more expensive, but that’s truly a drop in the bucket compared to your time. The reason bentonite is used in a glaze is to help with suspension, that doesn’t seem like a problem here. Both the ball clay and the bentonite hold on to water and will make the glaze dry slower. CMC may, by its sheer “gluey” force, may keep the glaze from peeling apart, but it will add to the time it takes your glaze to dry. 

My instinct when you said you added bentonite (to a glaze with 20% ball clay) was that it would make the problem worse because it’s only adding to the shrinkage issue. That’s my sense of the root of your problem. I think kaolin in general is a good direction. Calcined kaolin is the off-the-shelf workhorse for reducing wet to dry shrinkage. 

 

I think you're right about the bentonite, it was one of those things where right after I added it I thought, "hmm, probably not going to help with what I'm trying to do." Then plowed right ahead anyway. Don't think I'll use it in the next batch.

[Sam D]

The glaze applied easily and came out beautifully. Thanks so much for everyones input it was really invaluable. I have some shino questions coming up so stay tuned!!

 

Edited May 29 by Sam D