|Monthly Tech-Tip |
A type of ceramic glaze, typically fired around 2200F, 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). 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 some versions 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 1800-1600F 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 2100 and 1800F).
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.
This is the G3948A 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. G3948A, on these two M370 mugs, is a product of that. Unlike many, the original recipe we found, G3948, 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, G3948A, 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 G2890C, 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 G2896 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.
Courtesy of Steve Irvin.
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").
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.
Same recipe, same clay, same firing schedule. What went wrong?
Fix obvious issues in Glazy recipes before even trying them
Same recipe, same preparation, same clay, same firing schedule
G2826X - Randy's Red Cone 5
A popular Gerstley Borate based iron-red glaze.
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
Article on iron red glazes at Cone6Pots
|By Tony Hansen|
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