Washes that fume?

[Pyewackette]

I was reading this:

Which talks about fuming-on-purpose using cobalt salts which seems to be a bad idea but isn't exactly what I'm concerned about.

We have been told not to use copper or cobalt washes at the studio because the studio manager says they can fume and damage other pieces, brick and shelves.  So not cobalt salts, specifically he said copper and cobalt washes (he did not specifically say carbonate that I can recall, keeping in mind my recall is often somewhat suspect).

Yet this guy uses OXIDE washes apparently without concern for possible fuming effects:

Digitalfire notes of copper carbonate

Quote

This material is considered volatile during firing and thus can affect the color of other pieces in the firing.

However re cupric oxide:

Quote

Under normal oxidizing conditions the CuO molecule remains unchanged and produces clear green colors in oxidation glazes

I could not find any mention of fuming for cobalt oxide on digitalfire.  However of cobalt carbonate he says:

Quote

The carbonate form of cobalt is very fine-grained and disperses better in the glaze slurry and the glaze melt, it gives a more evenly distributed color than cobalt oxide. However, as with any carbonate, it produces gases as it decomposes and these can cause pinholes or blisters in glazes if they need to escape at the time when the glaze needs to solidify.

Further there are pictures of color contamination due to fuming from rather large chunks of copper carb and cobalt carb.

All of Richard McColl's washes appear to be oxide forms (he says metallic oxides).

So are the oxide forms "safe" from fuming issues and I only need to avoid the carbonate forms?  If so why have the carbonate forms around at all if we can't put them in any of our kilns (excepting, apparently, raku, which possibly answers itself)?

Are the carbonates also high risk for fuming in a glaze recipe?

Edited November 17, 2023 by Pyewackette

[Hulk]

Good questions.
I don't have answers, will share an observation: the tin-chrome red glaze I'm using* picks up blue flecks, I'm guessing from neighboring pots, however, some of the flecking could have been from the kiln**.
Sometimes there's more blue flecks on surfaces facing upward-ish, sometimes more on one side than the others...

My glaze loads typically have a lot of blue in thar, a semi-transparent blue, a variegated blue (with rutile), and a light blue; all use cobalt carbonate.
 

*got the recipe from local JC ceramic lab (matches "Chrome Red" from John Britt's book)
**my guess is that the same will happen in my brand-new kiln, tbd.
The old kiln had seen almost thirty years of service before it came to me! The prior owner definitely used blue glazes...

Edited November 17, 2023 by Hulk
and->an

[Callie Beller Diesel]

9 hours ago, Pyewackette said:

All of Richard McColl's washes appear to be oxide forms (he says metallic oxides).

He’s saying oxides for simplicity’s sake, but his copper wash is the carbonate form. Copper oxide is usually black or red. 

 

9 hours ago, Pyewackette said:

So are the oxide forms "safe" from fuming issues and I only need to avoid the carbonate forms?  If so why have the carbonate forms around at all if we can't put them in any of our kilns

Short answer is price, and because the base glaze plays a part in fuming as well. Some base glazes will release or pick up things better than others. They’re useful in different applications.

In some instances fuming may be desired. Some soda firers will place small cups of copper or cobalt at points in the kiln so it flashes onto the adjacent pots. They use the oxide or carbonate form because it’s less toxic than the salts, and having it contained in the kiln is less hazardous than applying it with a spray bottle post raku kiln. Still wants to wear your respirator though. 

As mentioned in the digitalfire articles, carbonates disperse differently in glazes than oxides do. Cobalt oxide, unless ground very finely or put through a 100mesh sieve, tends to leave specks in a glaze, while carbonate disperses quite neatly. But the light purple colour of carbonate can be hard to see if you’re using it for brushwork, so the oxide is a better choice for that.

For copper carbonate, in high fire glazes it doesn’t usually give too many people grief. But the pinholing at cone 6 is a right nuisance, so some people are starting to use the oxide instead. 

[Kelly in AK]

I’ll throw this out. The worst/best/most dramatic fuming effects I’ve seen were chrome and tin. The tin glazed white pot is now pink on one side because the one next to it had a chrome wash. Next is copper, in reduction at least, pots near the pot with high percentages of copper in the glaze, um, may be affected. Sometimes a red splash is provident, others it’s the ruin of a pot. Cobalt is insidious. Practically invisible until fired good and hot. Then it’s the blue that just doesn’t quit. I haven’t had so much a “fuming” problem as a “How did that get there?” problem with cobalt.

Soluble metallic salts? No thank you. I prefer most metals in a non soluble form. Except sodium.