Getting a warm, transparent copper green without using lead

[mr_glazy_man]

I’ve been able to yield a beautiful copper green using a lead bisilicate at earthenware temps. It’s wonderfully transparent, enabling one to see right down to the clay body. Recipe as follows:

- 4064 (lead bisilicate) 90

- kaolin 5

- silica 5

———

- copper 3

- iron oxide .5

Using the same quantity of copper oxide in a non-leaded frit like 3110 or 3124 yields turquoise. I knew this would be the case, but thought I’d test it anyway. 
 

I’m wondering if anyone has a recipe for a clear gloss copper green without using lead? Many will suggest a just using a stain in a non-leaded fritt and  they would be right, except that the stains I’ve used yield a flat finish without the lovely depth of oxides.

[Bill Kielb]

A picture would help for sure. Post one of each and folks likely will have ideas. This piece below is mason stain and a base glaze tinted to taste. The base glaze often will have an effect on the the stain color. See Mason stain color reference guide https://www.masoncolor.com/reference-guide as an example.

 

[mr_glazy_man]

 Here is a pic. 
 

Leadless glaze is column one and two. Leaded glaze is column three and four. Each test tile has varying weights of glaze on them. 

The leaded glaze is  not just an actual green, but also exudes this beautiful warmth. It’s that warm, transparent green that I’m after. 

 

[Bill Kielb]

Hmm, my picture above is more in the family and it is stain. I think if the transparency and depth above is appropriate it likely indicates the right combination of stain with a particular glaze probably gets you the tint you want. Maybe Refer to the Mason stain link above for an idea of color variation and flux dependencies.

[PeterH]

Copper in lead glazes gives a very characteristic deep green that was very popular when the use of lead glazes was common. Lots of it about in my youth in the `50s and early `60s, mainly in older buildings. Sadly there seems to be no lead-free equivalent, for technical reasons.

1) The colour is explained by "crystal field theory". See The Origins Of Color In Minerals http://www.minsocam.org/ammin/AM63/AM63_219.pdf

2) In hand-waving terms the glass structure adopted by the lead ions leaves spaces into which the copper ions fit which distort the energy levels of the copper's  electron orbitals. This distortion results in a green colour. Pedantically orbital-hopping electrons remove quite a lot of reddish light from the incident white light, leaving a green colour.

3) I believe that only lead (and lots of it) creates the environment where this happens.

There remain lots of green copper glazes.

From Copper in various cone 6 base glazes (Free Online Glaze Class Pt. 8) https://tinyurl.com/mrxx2ycu

Perhaps this area might be of interest.

... but hard to tell without seeing actual examples.

 

[Min]

Stains aren't going to give you that wonderful depth of colour that the copper lead glaze gives you but to get away from using lead and come up with a next best type scenario perhaps combining a stain with some copper would be an acceptable compromise. I put your leaded glaze tests next to a recipe from glazy that uses a stain plus black copper ox. Colour isn't quite there and it looks underfired but might be a path to explore.

[oldlady]

min, would the recipe you gave me work on his clay?  it has the quality he is searching for.   i cannot determine the cone number he is firing to.  there is the term earthenware temps but it is not clear to me if the clay is an earthenware or stoneware.

 

 

 

[Min]

31 minutes ago, oldlady said:

min, would the recipe you gave me work on his clay?  it has the quality he is searching for.   i cannot determine the cone number he is firing to.  there is the term earthenware temps but it is not clear to me if the clay is an earthenware or stoneware.

 

Nope. The recipe @mr_glazy_man posted has 90% lead bisilicate so it's firmly in the lowfire range and the one I gave you is a cone 6 glaze so it would be immature at lowfire.

[Magnolia Mud Research]

I would try using a low temperature commercial clear and add some copper to that glaze.  A colleague I worked with a few years back used cone 05 commercial clear as his base glaze and added his own choices of oxides (and some stains) for cone 5, cone 10, and Raku fired glazes.  Worked fine.  [no the glazes did not run off of the pots except when applied a quarter inch thick)

To get the pazazz of lead glaze without the lead I would try applying clear glaze layers over the "green" glaze layer; might take several firings (a technique from Ron Nagle).  

lt

[mr_glazy_man]

Thanks everyone, all very useful and helpful suggestions.

To answer oldladys question, the cone I’m firing to is 05. 

In order of preference, I’m going to try:

1) Mixing yellow iron oxide with copper in varying ratios. Perhaps this will yield me a green as it does in traditional colour theory, but I’m not hopeful

2) Mixing copper with some chromium oxide (even though chromium is refractory and a suspension colourant). Likely won’t give me the transparency, but might be a good compromise

I’m less worried about using lead bisilicate than raw lead but still… the less the better in my opinion. 

[Callie Beller Diesel]

Sodium is what pushes copper into that turquoise range, and magnesium can muddy some colours, or send them into a warmer temperature range. Have you tried a frit with no sodium, like Ferro 3249? Or something similar available in Australia? Digitalfire description here.

[mr_glazy_man]

@Callie Beller Diesel no I haven’t tried the 3249, thanks for the suggestion. Ill pick some up, give it a whirl and post my results here. 

[PeterH]

Looking at an old book that gave nice copper green test tiles at 1%, 2% & 3% copper oxide in a cone 04B base glaze (lead bi-silicate 65%, whiting 10%, potash feldspar 15%, china clay 10%).

It said that if you don't want to use lead substitute borax frit for the lead bi-silicate. No pictures of the result though.

Consistent with the route you seem to be taking,  and the fact that the high-boron cone 6 glazes seemed to give more promising greens.

I'm a little surprised at the high Na.

[mr_glazy_man]

Ah, but this is the lovely thing about cone 6: you can get a lovely emerald green at this temp without the lead bisilicate. The challenge is to get it without the lead.