A chemistry question for the glazing community

[DHettinger]

I'm not currently a ceramics artist (though upon reading,  this about mixing glazes is fascinating) but rather I'm exploring wood ash chemistry for an upcoming  "Waste Not" workshop at our farm in Western NC - these workshops are about diving deep into applications for what is conventionally considered a waste product.  I'm admittedly in over my head a little with this one. 

I've stumbled across a brilliant green (with a touch of blue) color after soaking wood ashes that have been calcined for 2 hours at 1500F.  What is causing this?

Some details of the experiment:

• I have two samples repeated 4 times.  One sample is presoaked wood ashes - soluble compounds have been removed for making potash 'lye' water.   The other was not yet soaked.
• Each 1 cup sample was roasted at 1500F for over two hours. 
• The presoaked, calcined ash was soaked again with water (in an exploration of portland cement alternatives) and the excess water was clear and very alkaline.
• The un-soaked, calcined ash was soaked w/ water revealing almost instantly a brilliant green color in the excess water.
• I've had soil application tests that reveal each soaked and unsoaked sample have similar amounts of Mn and Cu... two elements I suspect would attribute to the green color but are not necessarily soluble in water. 

Is it that I've oxidized the copper and therefore somehow made it soluble? and that only a very small amount (135ppm dry, 165ppm soaked ash) is enough to change color that dramatically? 

My plan is to decant and recrystallize what's in this small amount of vibrant green water.   Exercising the normal precautions - glasses and gloves - should I be otherwise concerned?

I'm asking the ceramics glaze as I suspect what I've stumbled on was perhaps some kind of middle ages technique someone here may be familiar with? I understand that in your craft, impurities in the glazes can have a huge impact, and I know wood ashes have a role in some glazes.  Is this particular case at all relevant to your work?

Thanks for reading

-dan

 

 

 

[Rae Reich]

Do you know what kind of (species) wood was burned to make the ash? Where was it grown? Local soils might affect chemistry.

Sounds like an interesting experiment for fabric/yarn dyes, too.

[liambesaw]

I doubt it is a soluble salt of copper or manganese, soluble salts tend to degrade at fairly low temperatures.  Could be wrong though!  Copper chloride decomposes at a higher temperature (1800ish f) but would be pretty hard to accidentally make.

Could be some kind of colloid or suspension of copper metal, you could try filtering it through a few coffee filters and see if it stays green.

[liambesaw]

If you have some sodium metabisulfate handy you can add some to the green solution, and any copper chloride will immediately crash out

[glazenerd]

I like a good puzzle- so I will weigh in.

11 hours ago, DHettinger said:

I'm not currently a ceramics artist (though upon reading,  this about mixing glazes is fascinating) but rather I'm exploring wood ash chemistry for an upcoming  "Waste Not" workshop at our farm in Western NC - these workshops are about diving deep into applications for what is conventionally considered a waste product.  I'm admittedly in over my head a little with this one. 

I've stumbled across a brilliant green (with a touch of blue) color after soaking wood ashes that have been calcined for 2 hours at 1500F.  What is causing this?

Some details of the experiment:

• I have two samples repeated 4 times.  One sample is presoaked wood ashes - soluble compounds have been removed for making potash 'lye' water.   The other was not yet soaked. Lye is potassium hydroxide.
• Each 1 cup sample was roasted at 1500F for over two hours. 
• The presoaked, calcined ash was soaked again with water (in an exploration of portland cement alternatives) and the excess water was clear and very alkaline. Correct- potassium hydroxide (lye) is classed as "caustic"- above 13PH
• The un-soaked, calcined ash was soaked w/ water revealing almost instantly a brilliant green color in the excess water.
• I've had soil application tests that reveal each soaked and unsoaked sample have similar amounts of Mn and Cu... two elements I suspect would attribute to the green color but are not necessarily soluble in water. 

Is it that I've oxidized the copper and therefore somehow made it soluble? and that only a very small amount (135ppm dry, 165ppm soaked ash) is enough to change color that dramatically? Not oxidation, but rather amphoretic- copper dissolves in the presence of strong alkali (lye= potassium hydroxide) creating copper hydroxide CuOH2: which is green/blue green. 

My plan is to decant and recrystallize what's in this small amount of vibrant green water.   Exercising the normal precautions - glasses and gloves - should I be otherwise concerned? Advisable- caustic alkali eats skin as fast as acid.

I'm asking the ceramics glaze as I suspect what I've stumbled on was perhaps some kind of middle ages technique someone here may be familiar with? I understand that in your craft, impurities in the glazes can have a huge impact, and I know wood ashes have a role in some glazes.  Is this particular case at all relevant to your work? Might work in raku as a copper salt?

Thanks for reading

-dan

 

 

 

 

[liambesaw]

21 minutes ago, glazenerd said:

I like a good puzzle- so I will weigh in.

 

Copper hydroxide is insoluble and degrades at 170 Fahrenheit, this has survived sustained firing to 1500?

I suppose it could reform the hydroxide when rehydrated, but would still be cloudy particulate? Dunno

[glazenerd]

I was suggesting that 1500F was enough heat to cause a chemical reaction: which presented itself later when water was added. 

[DHettinger]

Thanks all 3 for pitching in on this.

Ash source was from the wood stoves of friends and coworkers -  from common mixed hardwoods native to my part of WNC.  Nothing fancy there. 

Filtering has little effect save for slightly improved clarity.  interestingly, after filtering I noticed some yellow staining on the inside rim of the original mason jar I had stored it in , just above the fill line. 

Going to check the homebrew shop in the morning for sodium metabisulfate.  Thanks for the tip!

Amphoretic - new word for me.   My pH strips indicate I'm in the 12 range on both samples.  Seems clear this is CuOH2 then?  It is definitely the characteristic blue/green I've seen from acid flux stains on copper pipes.  Still, my soil application tests show such low concentration of copper, I'm baffled by the aggressive color change.

Tomorrow I'll repeat the same process at a lower temperatures.  Will repeat with an all new known-species of wood ash sample at 1500F for fun.   Will post updates if anything interesting comes up.

thanks again everyone

[liambesaw]

3 hours ago, DHettinger said:

Thanks all 3 for pitching in on this.

Ash source was from the wood stoves of friends and coworkers -  from common mixed hardwoods native to my part of WNC.  Nothing fancy there. 

Filtering has little effect save for slightly improved clarity.  interestingly, after filtering I noticed some yellow staining on the inside rim of the original mason jar I had stored it in , just above the fill line. 

Going to check the homebrew shop in the morning for sodium metabisulfate.  Thanks for the tip!

Amphoretic - new word for me.   My pH strips indicate I'm in the 12 range on both samples.  Seems clear this is CuOH2 then?  It is definitely the characteristic blue/green I've seen from acid flux stains on copper pipes.  Still, my soil application tests show such low concentration of copper, I'm baffled by the aggressive color change.

Tomorrow I'll repeat the same process at a lower temperatures.  Will repeat with an all new known-species of wood ash sample at 1500F for fun.   Will post updates if anything interesting comes up.

thanks again everyone

You can tell if it's copper hydroxide by neutralizing it, it should precipitate and fall out of solution.  It dissolves only slightly in alkaline solutions.  This means it would be cloudy regardless of the pH.  

If carefully dropping some acid (5% vinegar dropwise) into it doesn't bring out a blue cloud of milky copper, we need to keep looking!

 

[Babs]

Some species will take up minerals or specific minerals at higher rates than others. Actually a way of identifying what lies below...your samples would be pretty concentrated

[Tyler Miller]

https://en.m.wikipedia.org/wiki/Green_liquor

It’s prolly this.  All the necessaries are in the ash and iron is more likely a metallic contaminant than copper—occurring in an abundance orders of magnitude greater than Cu.

This paper maybe helpful:  https://www.nrcresearchpress.com/doi/pdfplus/10.1139/v78-462  (edited to make functional link)

 

Kind regards,

 

TM

[DHettinger]

Yes! thanks Tyler!  A chemist friend of a friend of family bounced back that it's more likely from iron.  Fascinating stuff. 

Nevertheless, I haven't been able to recreate it under other lower temperature circumstances.  Seems the 1500-1600F range is important. 

Also, I checked it a day after passing through the coffee filter and it's dropped out on it's own.  Still at 12+pH, it's now a very clear slight yellow w/ what could be rust or Cu on the bottom.  Photos below of the same jar 24 hours apart.