Exploring Iron Reds and Fast Cooling Cycles
Randy’s Red – A classic glaze recipe
“Randy’s Red” is a recipe that has been around for a long time. I’m not even sure who the original “Randy” is. This glaze was one of the staple studio glazes at Cedar Hill Rec Centre where I worked as a studio technician for 6 years.
It was a very stable glaze for us and came out of the kiln red every time, unless it was applied very thinly. This image shows a very thick application on the left to a very thin application on the right.
But not everyone gets this lovely iron red colour when they use Randy’s Red.
When red doesn’t turn red
This image was posted on my Randy’s Red recipe on Glazy.org from someone who didn’t get a red result. They say “Not much red sadly. 🙁 I did fire at cone 5 not cone 6. Perhaps that’s what went wrong.”
Perhaps it’s the firing temp and they didn’t fire hot enough. We always fired our kilns to a hot cone 6 and this person says they fired to cone 5. But another factor to consider is the rate of the cooling cycle.
At the Rec Centre, we let the kiln cool naturally with no programmed slow cooling cycle, but a large ConeArt 2827 kiln (2.5″ insulating firebrick plus 1″ of block insulation) takes over 24 hours to cool naturally. If I started a glaze firing on Monday morning, we couldn’t unload it until Wednesday morning. It would be firing most of Monday and cooling all of Tuesday. And even by Wed, the kiln would still be too hot to unload without cracking the lid and waiting an hour or so.
The size of the kiln and the thickness of the bricks/insulation mean the kiln retains heat very well. Compare this to my own smaller kilns at home that are cool enough to unload the next day. Load a glaze firing on Monday, unload Tuesday.
Iron reds turn red because of the crystallization of iron that happens during the cooling cycle. I don’t know what the exact temperature is that the crystallization occurs, or the exact cooling rate requirement is for the iron to turn red. I am still learning. There are some links to resources with more info at the end of this article.
Different cooling rates = different results
Here’s what I know. The same Randy’s Red glaze fired in a large kiln with slow natural cool looks like this glaze on the left and fired in a smaller kiln with faster natural cool looks like this glaze on the right.
What about bone ash?
Another thing I have noticed about the many Iron Red glazes posted on Glazy.org is that they all contain bone ash. Bone ash is tricalcium phosphate and phosphorus (P2O5) is a glass former that can help iron to turn red. Most iron red glazes seem to have 8-15% Bone ash in them.
But then here’s our Randy’s Red recipe, that’s super red with only 1.26% Bone Ash.
This got me thinking. I wondered if adding more Bone Ash would help our Randy’s Red to become more red without needing to slow down the cooling cycle. Many potters still use manual kiln-sitter kilns, which makes it really tricky to slow the cooling down in any consistent way. I was using kiln-sitter kilns in my home studio for 12 years before I finally bought myself a new programmable kiln.
I decided to do a test on Randy’s Red, to better understand how both the iron and the bone ash change the colour of this glaze. All these tests were fired in my ConeArt 1822 with no slow cool, so it naturally cools to unload temps in about 12 hours.
And more importantly, it cools from cone 6 down to 1400F/760C in about 2 hours. This is the part that matters, because most glazes have cooled and solidified by this temp. Once a glaze has solidified, it is set and no more reactions are taking place to change the colour or the surface so the rate of cooling isn’t relevant below this temp.
I wish I had tracked the cooling rate of the large kilns from cone 6 down to 1400F/760C at the Rec Centre while I still worked there to compare. I just know that it cooled much slower.
My fast-cooled, iron red bone ash tests
I started with the base recipe, removed the iron and bone ash and then increased the EPK slightly to make it all add to 100%. I often like to test a glaze before adding colourants to see the base glaze on its own. I tested it on both a light and dark clay body to see the opacity.
Then I added the red iron oxide. Instead of adding the 15% iron all at once, I added it in 5% increments. Here, you can see the effect of 5%, 10% and 15% red iron oxide on this glaze. Not even a hint of red, just shades of brown.
Then I added the 1.2% bone ash that’s included in the original Randy’s Red recipe, giving me the complete Randy’ Red glaze recipe. There may be a very slight red tint to the glaze with this small amount of bone ash, but nowhere near the reds we were getting out of the large, slow cooling kiln.
So I decided to see what happens if I increase the bone ash to levels more in line with other Iron Red glazes I had seen on Glazy.org so I added 2 increments of 5% bone ash to the original recipe, giving me 6.2% and 11.2% bone ash in these 2 recipes.
As you can see, with 6.2% bone ash, the glaze turned RED! And it’s still red with 11.2% bone ash. So somewhere between 1.2% and 6.2% bone ash, there is a sweet spot that brings out the reds with a fast cooling cycle.
The next test I would like to do is a line blend between the 1.2% and 6.2% bone ash versions of this glaze to see the transition from brown to red and where exactly it changes colour. To be continued…
Here’s my Glazy Bookmark for these recipes.
Here are all the test tiles next to each other, starting with the base glaze on the left, then with 5%, 10% and 15% red iron oxide, and then with 1.2%, 6.2% and 11.2% bone ash.
Important:
This is just one series of tests with one particular iron red glaze. It’s important that when we’re glaze testing and we discover something interesting, that we don’t get too excited and draw over-simplified conclusions. We need to say “Hey, that’s interesting! Let’s change some variables and test it again to see what happens.”
So I’m not saying that adding bone ash to any iron red glaze will make it more red with a fast cooling cycle.
What if we raise the Al2O3 level or change the SiO:Al2O3 ratio?
What if we change the fluxes or the flux ratio?
What if we add or remove B2O3?
What if we add or remove iron, or try different sources of iron?
Any of these changes could potentially reduce or improve the glaze’s ability to turn red. More testing and exploration is needed.
Any comments and questions can be posted in the “Comments & Questions” section of these glaze recipes on Glazy.org. I’d love to see any similar testing you’ve done with this glaze or other iron reds.
Resources:
Here are some of the articles I’m reading to learn more about iron red glazes:
The Many Faces of Iron: An Exploration in Cooling – Dr. Carol Marians
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