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Joseph F

Wood Ash Glaze and it's durability

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So after that wonderful discussion about soda ash over the past few days. I wanted to get another discussion going about a sort of similar topic. Wood Ash Glazes.

I have a few cups that are wood fired with no glazes on them besides the ash build up. They form really beautiful yellows that are crystalized inside the cup and I drink out of them a lot. 

What is the difference between a wood ash glaze that is mixed up to contain fluxes vs a wood ash glaze that is naturally occurring from the actual wood firing?

For example, I have been tinkering with wood ash and various fluxes, clay, and other ingredients, then forming it into a slurry paste and brushing it on tiles. I am getting some really nice results that I am very happy with, but I was curious what the real difference is between wood ash glaze and the wood firing wood ash? 

I have read many places that wood ash glazes made at cone 6 are not food safe... ( i dislike this word, "food safe"). I think what people might mean is that the surface isn't durable maybe?

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Wood ash is primarily calcium, and from my experience, natural wood ash is quite durable. However, it rarely coats the inside of a piece where it really takes the brunt of abuse in regular use. That's what liner glazes are for. A formulated glaze that uses wood ash in it can be just a durable as any other glaze, providing it's formulated properly. The ash is just a source of calcium (plus some trace minerals), so it's not really any different than using whiting or dolomite or wollastonite from a formulation standpoint. Different types of ash melt differently, since the mineral makeup of each tree is unique, so some testing and tweaking will be needed, but that's true of any glaze. Also, if you don't wash the ash, you also need to take into account the solubles that will contribute to the melt, but again, it's just a matter of formulating it well. I have not heard about cone 6 ash glazes not being food safe. However if they were referring to being fired in a wood burning kiln, then they are correct, because the ash won't melt until cone 8.

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Joseph 

I suggest you look carefully at the elemental analysis of various woods and straw fuels — especially levels of  silicon ,alkali, and alkaline elements.  Search in the fuel combustion discipline not the pottery discipline because the fuel domain has a better grip on the chemistry of the entire combustion process.  

 

Meanwhile l’ll try to put some coherent thoughts together after class tomorrow.

 

LT

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I should have been clear when I was saying I have read that people say it isn't food safe. I meant random people on forums and other clay boards. People will ask for a cone 6 wood ash glaze then someone else will post a recipe but say it isn't food safe. I went around plugging in those recipes in and they all are skirting borders of stable glaze, but I didn't see anything not safe about it, again besides maybe its durability, but I don't think that is a factor of safety.

The rest of both of your post makes sense and is what I expected. Wood ash just forms a chemical mixture of some calcium ingredients. It is the variety that unwashed wood ash provides that I enjoy so much using it to replace the other calcium parts in a glaze. It is rather spectacular. 

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5 minutes ago, neilestrick said:

The runny, rivulet type ash glazes don't fit into limit formulas. They're off the charts in calcium, really low in silica, but they work, and they typically make surfaces that are hard and durable.

This is what I have found with the most recent tiles I pulled out, although its still early. They have been soaking in test cups to see how their durability are. So far I haven't noticed anything out of the norm. I don't understand why they can be outside of limits so badly and still form such a strong surface. Is it because of the high levels of calcium creating a strong durable surface?

I always test durability on everything that I am interested in using, so I was surprised when I thought these tiles would lose surface and degrade quickly, but so far they haven't.

Edited by Joseph F

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I think Neil has pretty much covered it.   I have had a few ashes tested (mostly eucalyptus) and indeed they are predominantly calcium, with a bit potassium, magnesium and sodium thrown in.  However, there are a few species plant ash which are NOT primarily calcium.  Some have lots of silica (eg palm ash), others have lots of magnesium, others are even super loaded with potassium.

Also, what oxides you get from the ash depends a lot on what part of the tree was burned to make it.  heartwood is different from bark which is different from seeds.   Some woodfirers swear by lots of bark as fuel, particularly at the reducing stage.

From a practical perspective, most ashes have a significant loss on ignition.  Not as much as whiting, but not that much less either.  About a 1/3 of the material goes up the chimney.  You wont see that in a wood firing of course, except that hard cored woodfirers seem to need to fire for days and days and days to get noteworthy ash buildup on their pots.  Our relatively short wood firings (25 to 30 hours) seem only to generate a thin ash sheen where the ash hits...not unlike, say, the sheen you get in a soda or salt firing :lol: 

I have played around with ash as a smaller or greater formal glaze ingredient, and if you treat it like whiting and incorporate it into a clear glaze, you get, well, pretty much a clear glaze...with a  few impurities, specks, etc..  It can be nice if you don't expect too much.  A matter of degree I would say.

I have also tried using ash as a body flux (5% weight if I recall), which is not a good idea.  Throwing such a clay provides a free, hyper-powerful exfoliation experience for your hands, which you will be feeling for some days.  Also smells kinda like lime when you use for olfactorial abatement in the outhouse...

Finally,  while looking further into the soda ash story I ran across this interesting article regarding what kind of ash products can be found in wood combustion devices (kilns!) at different temperatures using different types of woods.  May be of interest.

https://www.fpl.fs.fed.us/documnts/pdf1993/misra93a.pdf

 

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