I sent this e-mail to several close friends to see what I could learn about how much of what colorants to add to push things over to a more red red color after reduction.

From: Phil Hamling
Sent: July-19-12 1:07 PM
To:
Subject: FW: Your Advice

I have been working (playing) to try and get a handle on “molybdenum crystalline glazes”. They certainly are a unique beast and there is not a lot of useful “how to” info out there.

This sample contained 4% rutile and 2% black copper oxide.

I reduced it lightly and got this.

It seemed to me that:

· The color is a characteristic mud red associated with reducing copper and titania.

· A possible explanation for why it looks like the edges of the crystals were reduced is that the background was reduced but re-oxidized. The crystals, floating on the top of the glaze, initially blocked the inner most part from being reduced and finally blocked the outer most part from being re-oxidized.

I started another series where I added 1.5% black copper oxide, .75% cobalt carbonate and .75% manganese dioxide and varied the rutile content from 0 to 4%.

Here’s a look at some results from the first firing.

I found it interesting (and totally unexpected) that higher rutile content resulted in fewer and larger crystals.

Never the less I intend to reduce some of these and expect that much like in zinc silicate crystalline glazes the copper will produce a metallic copper look. But The Fat Lady has not sung yet and we’ll just have to wait and see what actually happens.

I was wondering if I could get some advice from you in terms of how much of what colorants to add to push it over to a more red red color after reduction. The base glaze contains frit 3134, calcium carbonate, kaolin and silica.

What do you think?

Phil

And got some very interesting responses.

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I believe that your muddiness really comes from using rutile, which has equal titanium and iron, sort of.  Iron always seems to me to muddy a copper glaze in reduction.  I believe using no iron is a good idea, or very little. Ii also think a DROP and I do mean VERY little cobalt can enhance the vibrancy of the copper reds.

I think tin helps for some reason I am not sure of.

Now, those are comments I am making just off the cuff and from memory of those old copper red days.  My glazing is always trial and error and these are the results…let me know how you are doing and if I think of/find anything else that might help I will send on the info!  I'll talk to a few friends who do consistently good copper reds at high fire reduction in production.

Oh…and I do like the 4% rutile test best as you can more clearly see the crystals!

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One initial thought is this.  The red is in the lowest temp growth ring.  Something happens there – don’t know what or why as you will want to understand... but when I have reduced things with titania and gotten the typical purple ground, the lowest temp ring takes the reduction differently than the rest of the crystals... ie the 2000 degree + growth areas turn dark green, but the 1800 deg outside ring stays lite beige...  Same happens to a lesser extent with the copper....

All very interesting! I’ve not spent a lot of time chasing the moly... yet... just a few tests, but nothing yet.

Sounds like you are having some fun!

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I have used some moly but not to the point of its crystals. this looks good tho. For your red...secret a slight amount of cobalt......maybe only .10 persent or even less.
let me know if this helps ya.

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What I know about copper reds is that they only develop in an alkaline glaze, and they develop in the cooling after reduction with a clear at the end. The amount of copper for the best ones is surprisingly small, like 1/3 to 1/2%. It's also important to have about twice the amount of SnO2 as copper -- so 2/3 to 1%. The color can be so intense with just 1/3% copper carbonate that it is really surprising, and if, for some reason you don't have reduction, the glaze is the faintest green, as if it were not colored at all. 

But the bottom line is that copper reds have very small amounts of copper. My favorites have 1/3%. 

I like those tiles you are getting. The rutile stumps me. 

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Those molybdenum glaze recipes sure are strange with all those traditional crystal killers, boron, calcium carbonate (color killer) and kaolin, but I suppose they are tiny additions.   Also, those tests are all small so the crystals are really small.  And why is the first test at the top shiny with sharp crystals when all the others look matt with fuzzy blurry edges?

And perhaps you don’t need to add anything to further the red.  It looks like you didn’t hold the reduction long enough so it re-oxidized.  It looks pretty red to me, not so brown as you seem to infer.  I would try 1% and ½% copper oxide and perhaps try a little tin, and reduce the amount of kaolin.  But tests take time and I know you have little of that.  Maybe you could use an apprentice?

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I'd suggest 2% red copper, 1/2 – 1% iron, 1/2% tin – In a copper reduction glaze I use these three together, though the copper would be less than 1%.

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Try to reduction with different  materials.

The best reds I ever made with crystalline were reduced with H gas.

Use N gas before and after fulfilling the gastight kiln with H to remove Oxygen !!!!

You must be extreme careful working with H gas !!

I like best the piece on the second photo.

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I don’t have a shade of a clue for you.

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Wow, youve totally got me on this one.

Awesome results though.

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that is probably about as red as you will get it, likely from the titania.

I usually don't work in reduction often because the results tend to be inconsistent. 

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Second, when I had my Fallonator working - I got my best reds by checking out some of the cone 10 copper red glazes to see what mixture worked for those glazes. I found between .5% and 1% copper along with a small amount of Barium Carbonate but more importantly tin oxide. I will get back to you on exact amounts used. The important thing to realize here is the glaze in oxidation is quite unattractive because it is designed specifically for reduction. While I have my notes with several recipes - My pieces with the results are in Florida. I believe the best results were .5 to 1% copper carb and 1% Tin Oxide - the only reference I have as to the Barium has been 6% but I think that is way high - I think I only added about 2%. I do not believe there was any Whiting in any of the glazes but I can check that once I get back to FL.

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Is it necessary to use TIO2 at all? It seems to me that the metals are taking over. Copper in reduction likes to be low, and it also likes a little tin along for the ride, or it is muddy

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The Moly Crystals are looking killer! We have been known to get some pretty muddy reds and bright reds in the same kiln load, so I would say you are close to something great!

I have scribbled in a notebook that Copper Red glazes are typically...

.3 - .5% Copper

.8 - 2% Tin Oxide

These two are usually in a .3 - 1 ratio.

Some other associated notes that may not be as useful...

Low Alumina

High Alkaline

Low Silica

High Feldspar

I know tin isn't something that is typically used in crystalline glazes and honestly I am not sure exactly how it reacts with copper. I think it helps keep the glaze stable so that the copper doesn't escape but I could be wrong. I know that putting our copper red on top of my tin bearing Winter White glaze has given me the best reds. In general the copper red does better over specific glazes though I don't know what the common theme is.

The Moly Crystals are looking killer! We have been known to get some pretty muddy reds and bright reds in the same kiln load, so I would say you are close to something great!

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I think the physicists would say that the amorphous background would be more receptive reduction than the crystalline phase, but that assumes the same composition. Maybe the moly crystals that have precipitated are vastly different in composition from the background? I think your explanation is probably the most consistent with theory.

For more of a red, I would be tempted to try less copper, less cobalt, less iron (from rutile), and maybe some tin? Maybe a small amount of chrome?

Most copper red glazes use a really low amount of copper and the actual layer in the glaze that causes the red is extremely thin. We tend to use a lot of copper in our red zinc crystalline, but that is just because of the "ain't broke so don't fix it" mentality.

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Most of the good results use 1% Cu with .5%Co - but that glaze is layered over another glaze that could be Fe 2, Co .15 Ti 4 - BUT, it also has another glaze over it - Fe 1 Mn 3 Ti 3 .
This one is Cu 2 Co .5 Sn .2 - and post fire reduced TWICE. It just had some red blotches after the first reduction - and it was reduced at 890 for 100 minutes the second time, used 164 gm of propane.
I DO think that the 20 minute reduction at 1350 down to 1300 is the way to go - the pieces are just more interesting - and the 2nd reduction of these same pieces made them even more interesting!!
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 I get a good striking red with 2% copper carb and .5% cobalt.  I get a metalic luster coppers from 4-8%.  I have no idea how the metals effect the moly.  I have been getting some cool lusters with tungsten moly and cobalt.  Looking good phil get that glaze on some pots.

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