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All Glossary
A type of ceramic glaze, typically fired around F, where iron oxide in the cooling glass precipitates out to form a striking red crystalline mesh on the surface.
Key phrases linking here: iron red glaze, iron red, iron-red - Learn more
Iron red glazes are easiest in high-temperature reduction firings, it is just a matter of saturating a transparent base with 12%+ iron oxide to create a "beyond Tenmoku" (Tenmokus have about 10% iron). That being said, iron red glazes are most commonly found in the cone 6 oxidation range, likely because it is a much more accessible process. That is what this page discusses.
A cone 6 oxidation iron red glaze
The red oxidation color is a product of the chemistry and a slow-cooled firing. The iron crystals form during the cooling cycle in the kiln. The growth depends on the melt being very fluid (to provide the mobility needed to orient in the preferred crystal lattice) and there being lots of iron oxide present. Compared to a typical cone 6 functional glaze, fluid melts need high B2O3 (or a combination of B2O3 and Li2O or ZnO) coupled with low Al2O3/SiO2 (a high Si:Al ratio does not seem necessary). KNaO is low and MgO/CaO is high (especially MgO). P2O5 is usually found and thought to be necessary (yet strangely, some iron-red recipes do not contain it). At least one commercial iron red has balanced the chemistry enough that a special firing curve to develop the crystals is not even needed!
A firing schedule thought to grow the crystals slows between -F on the way down, e.g. 100F/hr (the temperature range would depend on melt fluidity of the base). Since the glaze is very fluid (and thus susceptible to developing surface blisters) it is also advisable to do a drop-and-hold rather than holding at cone 6, like the C6DHSC schedule (which could be speeded up with a faster drop between and F).
Thicker applications give more crystals but also dry-crack and run more during firing, thinner sections tend to be gloss black. Iron red glazes can be very messy to work with, the slurry tends to turn into jelly, because of the flocculating action of the red iron oxide they usually contain (in a high percentage). If thinned with water the specific gravity often goes too low. When excessive clay is present the situation gets worse because of excessive shrinkage during drying (with accompanying cracking and crawling). Understandably it is common to use Crocus Martis instead of red iron (of course more is needed since it is not as pure). Black iron is an even better option, it does not gel and only slightly more is needed. Some potters use the purest iron oxide they can find (99%) rather than impure sources, perhaps because this is the most melt-available and therefore crystallizable.
Iron reds can develop more metallic effects when layered over other glazes. Rutile variegates iron reds.
See GA, G and GB.
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GA is a cone 6 iron red. This sample is firing using the C6DHSC schedule. It is a reactive glaze in more ways than one. This closeup reveals just how much is happening on that fired surface. The recipe contains spodumene, an expensive material, but clearly it is worth it.
This spoon jar is glazed with the GA recipe. It was fired using our standard cone 6 C6DHSC slow cool schedule. The body is Plainsman Coffee Clay. The inside glaze is GA6-B. The metallic appearance was achieved because of the thick application. But this glaze has a fluid melt and should have run on to the kiln shelf. Why didn't it do that? Because of the way I glazed it. The inside was done first. Then wax was applied over the rim and down the inside for a couple inches. Then I applied a thin layer of the iron red to the bottom half to act as a catch glaze - by pushing the piece down into the bucket of dipping glaze for only a second. That dried in about 5 seconds enabling flipping it over and pressing it down to do the upper half. I held it down much longer and got a much thicker layer (with a little overlap over the thin section). During firing it all evened out - leaving only a little evidence of this method near the base.
These mugs are Plainsman Coffee Clay. The glaze on all three is GA iron red. They were fired at cone 6, 5 and 4 using the C6DHSC schedule (adjusted for top temperature). As can be seen, the red color depends on the melt fluidity achieved at cone 6.
This is the GA recipe. Iron red glazes are easy to do in high-temperature reduction but not so in medium-temperature oxidation. Most people just try a bunch of recipes they find online hoping that one of them actually fires the way it is shown in the picture! A better approach for us was to study a range of ones claiming to be iron reds looking for things in common with the chemistries and recipes. GA, on these two M370 mugs, is a product of that. Unlike many, the original recipe we found, G, did have a suggested firing schedule. It seemed strange so we just used the standard C6DHSC slow-cool schedule. That one is also ideal for the liner glazes, giving them a better gloss finish. It was not tempting to even try the original recipe (because it measured up poorly against common sense recipe limits), but it did make sense to fix obvious issues and then try it. Unlike every other recipe we have seen, this one suffers no issues with gelling of the slurry because it contains no Gerstley Borate and uses black iron oxide. It has very good application properties and requires only 80 water for each 100 powder to mix as a creamy dipping glaze. And it does not need any lithium carbonate.
These two pieces were fired in the same kiln using the C6DHSC firing schedule. Fluid melts are an essential enabler of crystal growth during cooldown, that is what there are. Both contain significant Li2O to help the B2O3 achieve that fluid melt. Glaze #1, GA, has less iron than is typical yet works! Its high MgO/CaO are very likely key factors as to why. Glaze #2 has much more Na2O and it has both SrO and ZnO that #1 does not have. #2 is much higher in Al2O3 and has more than double the amount of SiO2. So which of all these factors is responsible for #2 having zero crystals? Very likely it is two important ones: The low CaO/MgO levels. And the high SiO2.
This is GC, a cone 6 iron red glaze. It was so gelled that it was unusable! First I measured specific gravity (with difficulty): 1.48. That's too high, so I added water to reduce it to 1.44. Then I dripped in Darvan 811 (as recommended for iron-containing slurries). I added it until adding more did not thin it further (more was needed than for deflocculating the average non-iron-containing slurry). But it was still gelled. The only choice was to add more water, taking the specific gravity down to 1.42. That made the difference, making the slurry thin enough for both better application and preventing it going on in too thick of a layer. But there is an even better solution: Use black iron oxide, no Darvan is even needed for that.
Original development of the G recipe was done to match the chemistry of Randy's Red (a popular recipe). At the time we did not do any special firing schedule to encourage the growth of the red crystals.
This recipe, our code 77E14A, contains 6% red iron oxide and 4% tricalcium phosphate. But the color is a product of the chemistry. The glaze is high Al2O3 (from 45 feldspar and 20 kaolin) and low in SiO2 (the recipe has zero silica). This calculates to a 4:1 Al2O3:SiO2 ratio, very low and normally indicative of a matte surface. The iron oxide content of this is half of what is typical in a beyond-tenmoku iron crystal glaze (those having enough iron to saturate the melt and precipitate as crystals during cooling). The color of this is also a product of some sort of iron crystallization, but it is occuring in a low-silica, high-alumina melt with phosphate and alkalis present. Reducing the iron percentage to 4% produces a yellow mustard color (we thus named this "Red Mustard").
Courtesy of Steve Irvin.
This iron crystal glaze is Ravenscrag slip plus 10% iron oxide fired to cone 10R on Plainsman H550. Since Ravenscrag slip is a glaze-by-itself at cone 10, it is an ideal base from which to make a wide range of glazes.
A GLFL test for melt flow comparing two cone 6 iron red glazes fired to and cooled quickly from cone 6. Iron reds have very fluid melts and depend on this to develop the iron red crystals that impart the color. Needless to say, they also have high LOI that generates bubbles during melting, these disrupt the flow here.
Chazo Chazim Mehmeti made this Stull chart to help explain why my GB does not produce iron crystals. It plots the formula amounts of Al2O3 (vertical axis) vs. SiO2 providing one lens through which to view the chemistry of multiple iron red glaze recipe candidates that work and don't work. His argument is that, among other necessary things like the presence of P2O5, MgO and CaO, the amounts of SiO2:Al2O3 must be within certain bounds (this chart conveys both the ratio and amounts). He points out that the relationship is sensitive enough that one should use the closest possible chemical analysis of the materials (not generic ones) when plotting points on the chart. Over a year of testing with his students, the ones fitting in the small zone on the chart worked every time (#2 being best). That is where my GA, which is working really well, also resides. The GB, which does not work, appears way off to the right because of its high SiO2 content. Of course, there could be other reasons for its failure, but the SiO2 issue is a good place to start.
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