Is C Harris Temoku a stable Iron Red Recipe for Cone 6

[Christine Stangel]

Just came across the C Harris Temoku recipe for cone 6 firing and wondered if anyone knew if this would be a foodsafe glaze? Thank you for your help!

[George Lewter]

Just ran tests on C Harris Temoku with three different Red Iron Oxides.

Generally there is not a lot of difference in the results, with all tiles developing the typical brick red coloring. 

In the photos above the tiles on the left used "Special red iron oxide" from Ceramic Supply in Lodi, NJ. The middle tiles were with Spanish red iron oxide (unknown source). The right tiles used "High purity" red iron oxide from US pigments.

The RIO differences are pretty trivial to my eye with as much being due to slight variations in the coating thickness as to which iron oxide I used. I give a very slight nod to the high purity RIO, but will not be throwing out the others.

As I've noted before, this glaze tends to develop more darks and richer red over an iron bearing clay body (Laguna #80 in the bottom photo) than over B-mix 5 white stoneware at the top. This batch of tiles was over-fired to a hard cone 7 and pots at the bottom of the kiln went to cone 8. So the firing of this glaze is very forgiving over the cone 5 to 8+ range. I used a Steven Hill cooling cycle here - fast drop to 1700 F, 100 deg/hr to 1600 and hold 1 hour, 100 deg/hr to 1500 and kiln off.

[Christine Stangel]

Hi George!  

Thank you so much for sharing your glaze tests (love your results) and I had wondered if the Spanish RIO would make any difference along with the firing temperature.  I have recently discovered that our Guild's kiln does not fire to a true cone 6 when I put in the appropriate cone, so I am now trying a cone 7 to help with this.  Will also look at why it's not firing (we have a venting system and wonder if that changes things).  The Guild has a manual kiln and I have to manually reset the kiln for a soaking when the cone drops.   It's still a hit or miss for me to get the CHT to go red (usually goes brown), so with a little experimentation on the firing temperatures I hope to have more success.  Thanks again for all your help!  I cannot wait to have my own kiln one day....and it will not be manual!!!!

[Norm Stuart]

Below cone 6 temperatures using a pure synthetic red iron oxide is similar to using a mason stain - you get the color you started with.

Above cone 6, red iron oxide is degraded into simpler forms such as black iron oxide. If you fire to cone 8 or 9, I doubt you will find much difference between using black iron oxide and using the same amount of iron in the form of red iron oxide.

I have found significant variation between various red iron oxides if you fire to a cone 5 or cone 6, then cool without any special hold to try to create red iron oxide. Each product produces creates it's own distinctive color depending on the glaze chemistry. I can tell at a glance which product was used. Generally US Pigment from China is red, Bayferrox aka Special Red Iron Oxide is more purple, while impure Red Spanish Oxide produced by Prominfer creates more yellows and browns, in part due to its lower iron content. Laguna Red Iron Oxide, partially natural with recycled irons and barium carbonate, uniformly produces unattractive browns.

Synthetic red iron oxide is created with crystallized iron sulphate. The crystallization process excludes impurities. This chemical is then oxidized at a temperature which creates very pure red iron oxide.

I have found that synthetic red iron oxide with larger micron particle size, will produce a more reliable red at cone 5 and cone 6. Even at these lower temperatures, a portion of the red iron oxide is degraded off the surface of these particles. The remaining red iron oxide particles present a crystallization matrix for the degraded iron to reform onto with free oxygen. Smaller particles may degrade completely, leaving no matrix - thus requiring longer time to recreate the red iron color.

Economically, using large particle size synthetic red iron oxide to cone 6 or below presents a significant cost savings over using non-red iron oxide or firing to higher cones which require costly holds at a relatively high kiln temperature to recreate red iron oxide. This is the very reason we have all migrated from cone 10 to cone 6, and most industrial users have subsequently migrated to cone 2.

I prefer a very slow cool of 50 degrees F per hour between 1800 and 1500 due to the increased level of crystallization and thus complexity this adds to glazes, similar to a naturally slow-cooled kiln made with hard fire brick, but there is a definite additional firing cost associated with this slow-cool.

White clays without iron will definitely act like a sponge, binding iron from the glaze melt, resulting in less iron in the glaze. This can be readily confirmed by snapping a test tile in half. There will be a dark iron line formed at the surface of a white clay with an iron glaze on the surface.

[George Lewter]

 Norm, I hadn't previously picked up on this conversion of all iron oxides above cone 6 to the same structure, or that differences between them stay evident if fired at or below cone six. This is quite an important point. I will be firing with the US Pigments high purity RIO in in a day or two at cone 6, and will show the results.

Using Strontium Crystal Magic as a base glaze kind of precludes a real slow cool at the upper temperature range unless you really like sanding off rough crystals with 400 mesh wet/dry sanding paper. 

[Norm Stuart]

There are a number of ^6 glazes which "over-crystallize" at a very slow-cool, leaving fairly unattractive crystals exposed above the remaining glass melt.

I've found the solution is adding 10% to 20% of a Ferro Frit which adds both glass and flux, like 3134 at a minimum, or fluxier options with Fluorine like 3269, or even more so 5301.  Adding merely flux can sometimes create an interesting look, as with this example - which shows the side-effect of some borosilicate milkiness.

I've posted this one before. I like it because this glaze shows the greatest undesired transition at slow-cool, and an interesting result with 10% added Ferro Frit.

[George Lewter]

I found a great Derek Au article on an extensive testing of the common ingredients used in saturated iron glazes. It is important to remember that he is working cone 10, where iron undergoes more state changes than maxing out at cone 6.

[Norm Stuart]

Derek Au's curious Yellow Iron Oxide result replicates the results of everyone else.

In Digitalfire Tony Hanson writes:

Theoretically, any form of iron could be used to source Fe in the fired ceramic product (of course they lose different amounts of volatiles on firing so they cannot be substituted gram-for-gram).

However in practice this is not the case. Yellow iron, in our tests, for example, does not stain a glaze but it does stain a clay body. The reason is not apparent.

[Yellow iron oxide] is coarser in particle size and does leave some lighter colored residue on a 325 mesh screen (up to 8% in one specimen we tested whereas the others left zero). 

What is this impurity, whiting?

If Yellow Iron Oxide is 85.0% Fe2O3 and 15% LOIs consisting of water we should be able to use 118 grams of Yellow Iron Oxide in place of 100 grams of Red Iron Oxide.

But instead the Yellow Iron Oxide tends to remain yellow or turn brown or orange. I use this to effect in Orange Street glaze adding two parts red iron oxide and one part yellow iron oxide.

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This is Orange Street using only Yellow Iron Oxide in the center tile, fired to Cone 6.

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For comparison this is all Red Iron Oxide Orange Street glaze with 10% Praseodymium Yellow Mason Stain.

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Tony comments about lighter color inclusions left on a sieve suggests commercial Yellow Iron Oxide is far from the pure product perhaps containing a significant amount of whiting.

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This is Orange Street glaze made of Red Iron Oxide applied over a single coat of a high-calcium clear glaze with 10% Zircopax, which is similar to the result using only Yellow Iron Oxide. So Yellow Iron Oxide with a large percentage of calcium oxide as an impurity could resemble this.

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Back to chemistry

Chemistry texts state pure Yellow Iron Oxide is reduced to brown or red iron oxide between the temperature range of 170 - 200 C (347 - 392 F)

A Patent filed by Pfizer created "temperature stable Yellow Iron oxide" with a precipitated coating of 1% iron metaphosphate by weight to the yellow iron oxide, but they only claim this increases the thermal stable range to 230 C (446 F). Iron in the Metaphosphate can be replaced with aluminum, barium, calcium, magnesium, or zinc.

Aluminum Metaphosphate

This suggests the possibility that Yellow Iron Oxide in a glaze firing might theoretically retain its color by complexing with other chemicals in the glaze or becoming something else by exchanging the water with silica.

This slow-cooled tile of Waterfall glaze (from "Mastering Cone 6 Glazes) containing only red iron oxide creates a yellow-brown similar to yellow iron oxide, but is more likely iron silicate.

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So commercial yellow iron oxide is a mystery. It's primary use in the market is to remain yellow over a broad range of uses. So maybe the product is so improved in this regard it's effectively not pure precipitated yellow iron oxide but effectively something more like a stain - yet still inexpensive!

If Derek proceeds with tests of Iron Phosphate, as he suggests he might, he'll be as entranced as I was with the result.  Synthetic bone ash contains a higher ratio of phosphorous to calcium than natural bone ash, resulting in a brighter red in iron oxide glazes.

Adding even more phosphorous by replacing some of the red iron oxide with Iron Phosphate (bought on eBay) or aluminum phosphate (made with phosphoric acid and alumina) resulted in bright red iron oxide set in inclusions of clear phosphate glass (the sort of glass used in warm glass firing). The only drawback is phosphate glass is softer than a typical iron oxide glaze.

Magruder Red made with Aluminum Phosphate resulting in phosphate art glass.