|Monthly Tech-Tip |
This is the G3948A recipe fired to cone 6 using our standard C6DHSC schedule. The color "breaks" to black where thinner around contours so it seemed like a natural that the inside glaze should be G3914A Alberta Slip black. The contour of the foot ring is important or the glaze will run onto the kiln shelf. My standard fluted ring foot is working well. Perhaps a better option would be to glaze the bottom inch or so with the black as a catch glaze.
This iron red cone 6 glaze, G3948A, is applied thickly and runs during firing. With no countermeasures, it ends up on the kiln shelf (like the one on the left). Since this glaze breaks-to-black where thin on the edges of contours, glazing the base black seems like a natural match. The base of this was first dipped in G3914A black, up to about 1 cm (1/2 in). I then waxed over all of the black up to within 1-2mm of its edge. Then I applied the iron red by dipping in the normal way for liner glazing mugs. For this thickness of the brown the black melt is able to catch and stop it within 5mm or less.
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.
Yes. Ancient Copper, as of Nov 2023, it is no longer available. Right is G3948A, our iron red (a publicly available recipe). Both of these have been fired using the C6DHSC slow cool firing schedule. As you can see the PC-56 crystallizes more, matting the surface in the process. But if cooled normally (e.g. using the PLC6DS schedule) it does fire similar to G3948A. Likewise, G3948A can be made to crystallize more if the iron oxide percentage is increased in the recipe (we use black iron, it is a little less concentrated than red but does not gel the slurry). The recipe offers excellent slurry properties when mixed as a dipping glaze. Our version uses Spodumene (which has 7% Li2O). Of course, lithium materials are very expensive these days, but that is what is needed for this effect. If you make a brushing glaze of it using our instructions only about 70g of spodumene is needed to make a 500ml jar. At current material costs, we could make 3 jars for $10 worth of powdered materials!