Iron Reds

[Michael Wetzel]

I mixed up two glazes, Pete's tomato red and Ketchup red.  When they are fired, they come out brown with a few red spots.  I fired them in accordance with the firing schedule in "Mastering Cone 6 Glazes", but all I get is brown.  What could be the problem?

[Brent Farler]

I did a semester of trying to get these RIO reds to come out red.  Nothing worked.  I have read a lot of articles on why.  Here is what I have surmised and am going to check in my next shot at the lab:

Black Iron oxide crystals Fe3O4 form above 1000 C 1832 F degrees so once you hit cone six you need to cool quickly down to 1800 degrees.  

Red Iron Oxide crystals Fe2O3 form at 950 C or 1742 F so you need to spend up to an hour there to let them form. However, beyond an hour they start to  turn mottled greenish brown which has been a major theme in my test results.  I was following recommendations to ramp slowly to 1300 F and was getting browns and greens might have been the reason.  Next try I am going to go drop as fast as possible from cone 6 to 1750 F hold for 60 minutes and then turn off the kiln.

The RIO crystals won't form if there isn't a lot of iron around to grow them.  So people attack this problem in different ways.  Some swear by highly purified RIO. Regular RIO is about 88% pure.  Getting 99% pure requires ordering out.  Some swear by Crocus martis or Spanish reds or Black Iron Oxide.  Most of those reported are no purer than standard 88% RIO when it comes to iron content.  Which brings us to thickness.  Putting the glaze on really thick means more RIO on the pot and since the crystals are columnar in nature gives them sufficient depth to mature.  if the glaze isn't thick enough, no crystals and you get either brown or green.  The minimum thickness is about 50 microns or the thickness of a human hair but I think most find that is no problem getting our glaze on that thick.

The last piece is calcium a little of which is required but too much will dissolve the crystals. Magnesia is required if you want to form red magnesioferrite (MgO.Fe2O3) which is why you'll see dolomite in some recipes or the magnesium carbonate in Pete's Tomato Red #13.  

See Ian Curry for information on the calcia effects in stoneware glaes:  http://www.glazes.org/understanding-glazes/some-glaze-principles/pr...  

Here is a link George posted on this site:  http://cone6pots.ning.com/forum/topics/iron-glazes-and-achieving-re...  or for the original post:  http://web.ncf.ca/bf250/glazeiron.html

 

This also led me to having to deal with the accuracy of my pyrometer.  I was just adjusting my controller based on the cone bending but trying to hit multiple temps is driving me to have a second pyrometer to validate results.  

[Jeff Poulter]

Brent, you hit the nail on the head!  You have to be sure(via a second pyrometer) that you are coming down to & soaking at the right temperature for the iron crystals to develop.  if you are not at the right temperature, even if all the other factors are there, you won't get the iron crystals.  Also like I said in another post, one you are getting the iron crystals, you can make them even more red by running it back up to a ^04 bisque to make them "pop" even more(assuming your offset is right on your TC/Controller).  Jeff

[Anne Cunningham]

Hi, all, this is just in time for my new adventure. I have been resistant to a digital control (don't know if it is old fashioned or stubborness) but finally decided to give it a try. I have been eager to try a ketchup red (I LOVE tomatoes) For various reasons, I have to use a smallish kiln (18 x 23) and my glaze palate has been deliberately limited to a base that is a buttery matt with rutile and zinc, with whiting. I had found that the few times I did not pack it tightly the matt crystals didn't form. Now I know I just have to try and see, but as I am getting ready to do a bunch of tests in my never yet fired new kiln, I am wondering if anyone thinks the matt glaze would be harmed by the soak at cone 04 - AND what you mean by rapid cooling. I used to be a full time potter many years ago and have retunred to clay but in a more limited circumstance, so am a little more inclined to ask for advice bfore I just jump in. GREAT to have you all here as a resource.

[Brent Farler]

After a dozen attempts at a RIO Red I did it!!!!!   I took your advice and refired one bowl out of a set from Pete's Tomato Red #13 recipe using plain old, relatively cheap Red Iron Oxide.  The cone in the middle is an 04 self supporting witness cone. I fired to 04 with a 30 minute soak and turned off the kiln.  It is blazing hot outside so the kiln took 14 hours just to fall to 350F - a rate of 110 degrees per hour.  The camera flash doesn't show the red like natural sunlight does but the left bowl turned from the color of it's sister on the right (both originally fired with same glaze, same cone 6 with slow 90 degree cool down to 1300F, both same dull brown with green splotches) into a deep red with the red coming out better in natural light. Not sure what to call the red but something like a sienna red maybe?  Bowl was semi-matte before and after the extra firing. So I am going to refire all of my #13 Tomato red tests with my next bisque firing (cone 04 -how convenient!).  

[George Lewter]

I found this comment on a glaze page of Western Pennsylvania Potters Association. It directly addresses our discussion, so I'm quoting it.

Anne M. BrackerOctober 22, 2013 at 2:59 PM

I recently spoke to Pete Pinnell about this very glaze (the glaze is usually referred to as Pete's Tomato Red #13, after him). Pete is the professor at University of Nebraska at Lincoln. I thought I'd share his comments on this particular glaze: 

This glaze performs best at a true cone 6 or a hot cone 6. The key to developing good color is to hold at one temperature on the cool down (in the range of 1800° to 1600°F) for 1 to 3 hours to allow the crystals to develop. The hotter the hold temp, the more deep burgundy red. The cooler the temp, the closer to orange red color. Do not confuse this with a slow cool down. Pick a temp, cool normally until you hit that temp, and then stay there for 1 to 3 hours, after which you'd resume cooling down at a normal rate.

[Norm Stuart]

Pete's Tomato Red #13 certainly needs something when made with Laguna Red Iron Oxide. 

With each additional coat of glaze it seemed to turn more yellow.  No hold, and no slow-cool for this tube.  Would it look different with a 3 hour hold?  When I get time I'll try it.

Quite a different color for Kate Magruder's Red made with synthetic Red Iron Oxide from US Pigment, fired with a six hour slow-cool between 1,800 F and 1,500 F.

Of course the chemistry between these two glazes is a little different. 

1.)  Magruder Red has far less Calcium which has been reported to interfere with red iron oxide - but has roughly the same amount of Phosphorous which allegedly helps produce a red color.

2.)  Ironically Magruder Red also has a lower percentage of Red Iron Oxide to other remaining ingredients after firing.  But of course these tiles compare impure red iron oxide to high purity red iron oxide

3.)  Magruder Red relies on Lithium as a flux rather than Boron. Perhaps this matters. Tthe much larger percentage of magnesium in Magruder's Red might also be benefiting the red color..

4.)  And of course Magruder has more gloss with a lower percentage of Alumina, resulting in a higher ratio of silica to alumina.

[Brent Farler]

I switched to a type 'S' thermocouple which made all f this much easier to achieve consistently. Thanks Norm for reminding us on the calcium phosphorous impact - this is a very small percentage but important so I am not sure that the variation of source materials doesn't show up here as part of driving different results. Some recipes still give me green mud even with the appropriate temperature control. I do like the idea of a longer hold as I have seen variation of the reds based on thickness of the pot, distance rom the shelf ( rims versus base) and location inside the kiln.

I am still not convinced that 88% pure versus 99% pure is material as long as the total iron is the same but I am accumulating the various forms of iron to run a side by side.

That said my wife is still not convinced and uses stain based reds for her work. That means different firing schedules and makes things less efficient. While she has embraced my base glazes (licorice, majolica clear and white, waterfall brown) the iron based reds don't get any respect.

As a side note we are getting pretty nice results with coyote's shino glazes over Ron Roy's Licorice. I will post some pics.

[Norm Stuart]

Your peak kiln temperature, and the duration of that peak temperature, determines whether the purity and particle size of your red iron oxide matters in the finished result.  John Britt says red iron oxide decomposes into black iron oxide at 2,250 F (1,232 C), which is cooler than Cone 7 but hotter than a typical Cone 6 firing.

If you keep your kiln temperature below that range, the red iron oxide you start with is exactly the same red iron oxide pigment you end up with - just like using a red Mason Stain, with the advantages and disadvantages of a Mason Stain.

As an example, I'm not going to be able to create large yellow crystals by adding a yellow mason stain to a macro-crystalline glaze recipe. The crystal or particle size of the Mason stain will not change during a macro-crystalline firing - in fact Mason stains will be excluded from any crystal formation.  But this will not be the case with pure red iron oxide which can create the nucleus of quickly forming larger iron crystals if additional free iron ions are available in the melt.

I'm fairly certain that the popular Amaco "PC-59 Firebrick Red" is colored with both red iron oxide and a red Mason stain.  Amaco "PC-53 Ancient Jasper" is a red iron oxide glaze without any artificial stain, so it can change to different colors depending on the kiln temperature.  But I can tell by the color of the wet glaze that they use a synthetic red iron oxide, or a some other very pure red iron oxide, in their Ancient Jasper glaze.

Once you fire hot enough to break red iron oxide into black iron oxide, or if you start with impure red iron oxide, you need long temperature holds at the right temperature to recrystallize red iron oxide.  And what those temperature and time parameters need to be will be affected by the glaze chemistry.

Companies like Bayer Chemical, which produce pure red iron oxide with large particle size, have already processed iron sulfate in pure oxygen at exactly the right temperature and time duration to maximize red iron oxide production.

The primary markets for pure red iron oxide are the huge companies using red pigment for inks, paints and cement colorant. So it makes my life a lot easier for an economical price, just so long as the kiln doesn't get too hot.  And I do love our platinum thermocouple.

John Hesselberth, co-author of "Mastering Cone 6 Glazes" spent much of his career overseeing the quality of titanium dioxide production.  Given our wildly differing results with each batch of Rutile, which in North America is a mixture of titanium dioxide ores from Australia and Florida, I wish he had been able to focus his career instead on creating a synthetic Rutile.

[Brent Farler]

Amazing