This week I have been testing iron red glazes in my L & L Doll test kiln. I have been wanting to achieve an oxidation iron red glaze that can be consistently fired in my electric kilns. My first reference to achieving an iron red in oxidation was from the John Britt book, The Complete Guide to High Fire Glazes. Tracy Dotson's Bailey's Red glaze (page 73) became the basis for my oxidation iron red testing. The Britt published recipe contained 8% red iron oxide which I used as a starting point to create a line blend from 8% to 14% red iron oxide using commonly available Spanish Red as my raw colorant.
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| Cone 10 Oxidation — Bailey's Red - 12% Red Iron Oxide |
Achieving an iron red in oxidation can be quite challenging and I amassed a lot of research before beginning my tests. Most of the information I gathered was difficult to understand with various points of views on how to undertake the task. Last March I received an email note from potter friend Ed Kaplan who suggested I look at John Post’s website, which turned out to be a very useful source of precisely worded information on oxidation iron reds. John is a talented potter and public school teacher with an obvious ability to make the challenging simple to understand. He describes a simple and methodical approach to achieving oxidation iron reds in an electric kiln. Keeping it simple was my ultimate aim and John's information proved invaluable and precisely correct.
John notes that iron reds must be slow cooled through a particular temperature range to allow time for the iron red micro-crystals to form as the glaze cools. The tomato red color is produced during the cooling by a surface crystals growth of iron oxide.
John suggests cooling at 150F per hour from 1800F >1300F and proposes cooling at 125F or 100F per hour if you are having trouble achieving iron reds. I decided to start with 150F per hour from 1800F > 1300F. I fired to Cone 10 using L & L slow glaze easy fire program and then programed the kiln the kiln to cool at maximum rate before entering a program ramp of 150F per hour from 1800F to 1300F. The kiln then dropped at its maximum rate from 1300F until it was cool enough to unload.
The Doll test kiln with the Bartlett DynaTrol controller (which I program and monitor from a laptop with Bartlett's KISS software) allows me to have precise control over the firing as well as the cooling cycle. Both a controlled firing program and a controlled cooling program can be entered, either from the KISS software or from the kiln computer control box (though the programing process is a bit different for each). For reassurance on programming the kiln to suit my needs, I contacted Robert Battey (L&L Kilns stellar technical guru and owner of Northwest Potters' Supply). Rob has a boggling grasp of electric kilns esoterics and is as far as I am concerned the most valuable part of L & L customer support apparatus.
The line blend produced a good visual contrast between 8% and 14% red iron oxide. There was a gradual and significant growth of iron red crystals on the surface of each test, with the results displayed below.
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| Bailey's Red Line Blend 8%-14% Red Iron Oxide |
There is a considerable change in iron crystal formation from test 11% through 14%. With 11%, there are indications of speckled red but the brown tone is dominate. On test 12%, iron red is the most prominent and on tests 13% and 14% a darker iron red surface resulted.
I was quite happy with the results, especially the 12% but have decided to fire the kiln again, with Baileys Red 12% dominating the glaze tests which will be applied in various thickness. I am fortunate to be using a clever glaze thickness measuring device which has been described and promoted by John Tilton on his blog. Following posts will detail my continuing iron red test adventures as well as more detailed information about my low cost adaptation of John Tilton's dial indicator glaze thickness device.
