10 replies to this topic

[GavJ]

I am looking for a liner glaze that fits the following characteristis, if possible:

 

1) Made out of realistically locally gathered or processed materials. I'm willing to go way out of my way to painstakingly make lots of ingredients, such as potash or other ash products or limestone (which is "whiting," yes? I have a local inexhaustible source) or vegetable concoctions, or whatnot, and I have a decent source of local earthenware clay. I live in Iowa, and I have not yet located any local FIRE clays, sadly. I do also have a local source of pyrite, which I am able to process into sulfuric acid + a type of iron ore myself (although not pure iron), if that helps.

 

2) Reasonably food safe. I understand that's a very controversial/meaningless phrase. I don't mean anything very strict, just a liberal interpretation of the term, as in "not obviously full of SUPER toxic things, and not obviously likely to deeply crack or craze all over the place and be impossible to wipe down."

 

3) Able to fire to the above specifications at around cone 1-2ish OR lower. I'm using small charcoal or wood fired homemade kilns and can't get much higher than that yet.

 

I don't care at all about the color or opacity or whether it drips interestingly, or anything like that. I just want a lining for earthenwares that will let me (carefully) use them as actual functional pieces. For example, for cold drinks at a minimum. *Ideally* some ability to handle reasonable temperature changes (it's okay if it has to be pre-warmed first or whatnot. It doesn't need to be super convenient, just usable at all). My clay body has already proven to be quite thermally shock resistant itself, for a local clay.

[Wyndham]

I'd try about 30-40 % local red clay and the rest fireplace ash. Wet Screen the red clay with an 80 mesh screen , add some screened ash, make some test tiles and try it out. From this you can see what adjustments are needed.

Anytime you use ash in water, you'll be making an alkali solution that can burn you hand, use dish washing gloves.

This is just a starting point, from there look to some of the books on ash glazes.

Wyndham

[GavJ]

So just unwashed ash then, yes? Or would already leached potash be better? Or even calcined pearlash? I have quite a lot of all of this on hand already as I'm also working on glass making.

 

I.e., is the lye helpful for the glaze, or just a contaminant? Since it sounds basically like just the recipe for normal soda glass with clay instead of sand. Also, thinking from that point of view, would 5-10% calcium carbonate help it be more waterproof at the end, similar to soda glass? I do have access to both local chalk and limestone.

 

(Yes I will look for a book too =)

[Wyndham]

Instead of loading you up with a lot of variation, it's most likely better to start simple, get results and add to or change ingredients, then fire again

The other is to make a line test 10/90 20/80 /30/70 percent of the 2 ingredients and go from there. Most of your time will be testing until you find a promising path to refine your test.

You will likely go through hundreds of test tiles to find what you like.

Wyndham

[JBaymore]

Cone 1-2 is tough to get things fluxed down without going to some commercial materials.  At that temp calcium oxide is just becoming really active.... so wood ash and limestone is of limited use as the primary source of mainly CaO flux .  Pearl ash is soluble,..... high levels will cause reall issues with the body as it is absorbed and does not all come back to the surface on evaporation of the water...... but for that temp range your main fluxes will tend to be K2O and Na2O......... without going to using the lower melting glass former (less durable and "food safe") B2O3 for some of the silica. 

 

Lots of testing is going to be the answer working this way.  The old potter methods work for getting er' done..... but it takes time.

 

best,

 

......................john

[GavJ]

Oh, I wish I had seen the last post here earlier, but yes went ahead and checked out some books and discovered the same thing stated immediately above: ash glazes appear to be almost exclusively cone 9-10, which I cannot achieve.

 

So basically, you're saying I have to just use pure potash as my "raw" ingredient... wonderful. So instead of 0.5% of wood I get to use, like, 0.005% of wood. *sigh* But that's okay.

 

So just to double check, the process would be:

1) Burn some stuff (using charts to try and guess best local material for high sodium or potassium concentrations)

2) soak the ashes for a long time, mixing occasionally, and retain the solution. Maybe a couple of times.

3) evaporate the water off, getting crystals (carbonate) <- edit: that would probably be dumb, when they're just going right back into water again. Could just use the solution FOR the slaking. Although would have to check to make sure potassium carbonate doesn't itself react with quicklime itself in an undesired way if so, since it would be exposed to unslaked lime if done this way.

4) Mix the crystals with a limiting reagent amount of clear slaked lime in solution.

5) Filter the resulting precipitate (Ca carbonate and a small amount of excess Na/K carbonates)

6) Evaporate off water again, then use the crystals (K2O and/or NA2O depending on plant) in glaze in large proportions as flux, mixed with as much local clay as I'm able to reach a melting temp for, for its silica and alumina base

Yes?

[Colby Charpentier]

Yes, you may achieve a glaze with that process. "Reasonably safe" would not be likely. Using an excess of akali fluxes as John suggests would likely yield a weaker glass, but have a higher likelihood of melting. I was always told Boron additions could yield a MORE durable glaze, though (specifically in cone 6 applications, not sure about low fire applications). This was determined through measuring gloss degradation through identical wear simulation.

[Tyler Miller]

No need for the slaked lime step.  K2CO3 and Na2CO3 decompose on their own in the presence of heat.  Your glaze is essentially going to be a lime-soda glass.  There are historical precedents for this type of glaze in around Iraq and Iran.  The efflorescence of the soluble Na and K is used to benefit of the strength of the ware.  I'm fairly certain they craze pretty heavily, though, or they tend to make use of quartz-based bodies like Egyptian paste.  Snooping around archaeological sources on the Achaemenid empire, a little earlier, and a little later, might take you a long way.

 

I'm with Colby, I wouldn't call it "reasonably safe", especially not if you want to colour it.

[jrgpots]

To concentrate your K20 and Na20, you can wash the ash, then take the wash water and evaporate a large part of it. The remaining solution will have high concentrations of K+ and Na+.  Wear gloves as mentioned above... Toxic stuff.

 

Jed

[JBaymore]

I was always told Boron additions could yield a MORE durable glaze, though (specifically in cone 6 applications, not sure about low fire applications). This was determined through measuring gloss degradation through identical wear simulation.

 

This is true, but is based upon some pretty specific ceramic chemistry relationships... and is likely WAY outside the "local materials" , traditional folk-potter approach that is wanted here.

 

best,

 

.......................john

[JBaymore]

Good comments there, Tyler .

 

best,

 

....................john