Copper Raku Matt Glaze

[tomr]

Does anyone have a good copper matt Raku glaze and some tips for firing to get the rainbow colors.

[Kohaku]

Hi Tomr...

 

The 'Rainbow' hues can be a shifting target- they often are a function of spot reduction in the post-fire process.

 

However, I've had good luck with a range of the copper sand glazes.

 

The thing with many of the 'Matte' glazes is that they're inherently low in silica, and the colors can be quite unstable. There's a really good article on Digital Fire that you may want to check out... and it includes a number of glazes.

 

The reality is that you'll need to experiment to find the conditions that work with your system, however...

[PeterH]

Kohaku,

 

Either we're having a little terminological confusion, or I am very very confused.

[i'll admit from the start that I haven't the faintest idea what a copper sand glaze

looks like. AFAIK it's not a term we use on this side of the pond, so I've never

knowingly seen one in the flesh.]

 

To me copper mattes look like highly coloured velvet: http://tinyurl.com/ngl359b

requiring both fairly strong reduction and judicious re-oxidation.

... and in John Weeldon's case contain something like 10% hi alk frit and 90% CuO.

Not so much an unbalanced glaze as slightly sticky copper oxide.

 

On the other hand the glazes in Tom Buck's excellent paper http://tinyurl.com/omq3p8c

have always looked to me (on paper) like copper glazes that flash, and in the the photos

appear shiny rather than velvet-matte. http://digitalfire.com/4sight/images/articles/xujfudusij.jpg

 

Do we agree that these are two distinct styles of "glaze"? 

 

Regards, Peter

[Kohaku]

Fair enough. I thought you were looking for a matte style raku glaze that could yield an 'oil on water' type of color effect.

 

It sounds like you're after something more specific than that though... so I'll take a step back and see if anyone else can answer.

[PeterH]

TomR,

Does anyone have a good copper matt Raku glaze

Yes (definitely).

and some tips for firing to get the rainbow colors.

Maybe/perhaps. Personally, I am just about to the point were they are starting to

almost look like copper matte.

 

A glaze that can definitely give good results (in the hands of a master like John

Wheeldon) is 10% hi alkaline frit and 90% copper oxide plus a little glaze binder.

 

As to the firing and post-firing, I quote from an earlier email to a friend:

 

In physical terms the copper matte is an optical thin-layer of copper

oxide on a reflective layer of metallic copper. The metallic copper is

itself a thinnish layer on top of a very high copper "glaze".

 

Chemically the high-copper glaze is fired in oxidation to give a CuO surface.

This is then reduced to give a layer of copper. The thin film of copper oxide(s)

is then formed by a light oxidation of the metallic copper. This all requires fine

control of the temperature and atmospheric redox during the process.

 

The masters of the craft do this all by sleight-of-hand: taking the pot out of

a hot kiln; sprinkling a little sawdust on the pot; waiting  "until things are right",

covering the pot with a tin can; waiting "as required"; "burping" the can (to let

just the right amount of oxygen in); and finally letting things cool down. Piece

of cake really.

 

... using small tins:

... at dustbin scale:

 

 

I do it the pedestrian way, requiring zero skill. Remove pot from the kiln and put

into a tin can, and arrange for heavy reduction (some charcoal in the bottom of

the tin, throw meths on the pot, seal with a thick layer of soaked paper, a board,

and a brick). Wait for it to cool. All very much belt-and-braces using up the oxygen

and trying to get some positive reduction as well (alcohol, water-gas, hydrogen, ....).

If the kiln gods are willing you finish up with a pot covered in copper (the salmon-pink

electroplated kind) and a surprisingly strong vacuum. Take the pot home and bake at

400F until the oxide thin film is ready.

 

... at any point if things haven't worked refire the pot and start again.

 

Regards, Peter

[Marcia Selsor]

I am on the road without recipes but google copper sand or dry alligator . They are copper Matt glazes. Tom Buck had three versions of copper sand. I use number two. it flashes well. Tom recommended never using more than 5 % copper in a glaze because it would be unstable and oxidize fading the colors. look up raku glazes in CAD articles you can download. Dry alligator is there.Apply thick and it gives alligator skin texture. I spray it on.and careful not go that thick.

Marcia