This is recommended in the booklet "15 Tried and True Cone 6 Glaze Recipes". This melt flow tester compares it with a typical cone 6 glossy, G2926B. This recipe is 90% Frit 3110 and 10% kaolin and their booklet recommends adding stains to it. But anyone knowing a little about this frit knows it would run off this flow tester even before bisque temperatures. It is crazy to recommend this. Even as a crackle. For cone 6 it needs to be diluted much more, not just with kaolin but also silica. I knew this would run but I underestimated its melt fluidity. I put a large tile below the tester to catch overrun, yet the melt ran off that and a big three-cm-wide blob melted through the kiln wash and so far into my zircon shelf I cannot chip it off! I cannot imagine how many people have tried this on vertical surfaces and had the same thing happen. The lesson: Use common sense when looking at recipes. Even if their authors did not!

We get this clay from St. Rose, Manitoba. Four tandem loads arrived this week. Just seeing the pile inspires me to make more pieces! It is a red fireclay and it is highly unusual. St. Rose Red has issues. They at first seem to be problems, but in combination they give it magic powers! It fires with very heavy iron speckling. The iron pigmentation is so high that it burns almost black at cone 10R. It has low plasticity. It shivers glazes: The vase on this picture lasted an hour after kiln exit, it spontaneously fractured because of the outward pressure from the under-compression glaze on the inside. But, by combining St. Rose Red with our more vitreous clays, which are highly plastic, we can make H440 and H443. But guess what happens when feldspar is added? A mix of 45 St. Rose, 40 Ball clay and 15 feldspar produces a rustic metallic surface (like the cup shown). Such a body cannot be made from a low fire red clay (like RedArt), it would just warp and collapse in the kiln. It is the refractory character, heavy pigmentation, iron speckling and low plasticity of St. Rose that make metallic ware possible.

Robert has done really valuable work in this research, what an amazing range of color! I am so grateful he shared this with the rest of us. Surfaces are unpolished and unglazed. All are fired to cone 6. Browns are missing, they can be made using iron oxide. For blacks, Mason 6600 is also effective. The blues require lower percentages than shown here, as low as 2% can be effective. Likewise with others, there is an optimal amount for each stain, beyond that, with increases in percentage the color intensity increase will drop significantly. There is another reason to keep stain percentages to a minimum: To reduce the impact on body maturity (and firing shrinkage). Blues, for example, can significantly heighten the degree of vitrification, even melting the porcelain. If you plan to marble different colors, keeping stain percentage as low as possible is even more important, unless you can do fired shrinkage compatibility testing, for example, the EBCT test. Need to develop your own white porcelain? See the link below.

You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).

Stains can work surprisingly well in matte base glazes (provided they are not too matte). Stains perform differently in a matte host glaze. The glass is less transparent and so varying thickness do not produce as much variation is tint. Notice how low many of the stain percentages are: yet most of the colors are still bright. A good reason to minimize stain concentration is to avoid leaching. We tested 6600, 6350, 6300, 6021 and 6404 overnight in lemon juice, they passed without any visible changes. It is known that MgO mattes, like this one, are less prone to acid attack that CaO mattes. A down-side to this matte mechanism is that chrome-tin stains do not work (e.g. 6006), this is because this does not have the high CaO content needed in the host glaze to develop the color. The inclusion stains 6021 and 6027 work very well in this base. As do the 6450 yellow and 6364 blue. And the 6600 produces an incredible gunmetal black. The 6385 is an error, it should be purple (that being said, do not use it, it is ugly in this base).

Stains are a much better choice for coloring glazes than raw metal oxides. Other than the great colors they produce here, there are a number of things worth noticing. The percentages may be lower than what you think would be needed, stains are potent colorants. Staining a transparent glaze produces a transparent color, that means it is more intense where the glaze layer is thicker. This is often desirable in highlighting contours and designs. If you add an opacifier, like zircopax, the color will be less intense, producing a pastel shade the more you add. The chrome-tin maroon 6006 does not develop well in this base (alternatives are G2916F or G1214M ). The 6020 manganese alumina pink is also not developing here (it is a body stain). Caution is required with inclusion stains (like #6021), the bubbling here is not likely because it is over fired (it is rated to cone 8), adding 1-2% zircopax normally fixes this issue.

These two pieces are fired at cone 6. The base transparent glaze is the same (G2926B Plainsman transparent). The amount of encapsulated red stain is the same (11% Mason 6021 Dark Red). But two things are different. Number 1: 2% zircon has been added to the upper glaze. The stain manufacturers recommend this, saying that it makes for brighter color. However that is not what we see here. What we do see is the particles of unmelting zircon are acting as seed and collection points for the bubbles (the larger ones produced are escaping). Number 2: The firing schedule. The top one has been fired to approach cone 6 and 100F/hr, held for five minutes at 2200F (cone 6 as verified in our kiln by cones), dropped quickly to 2100F and held for 30 minutes.

Fired to cone 6. These are not glazed. Polar Ice is very vitreous and very white, an ideal host for stains. However there is a caution: It has a high firing shrinkage. If a stain is refractory it can reduce that shrinkage considerably. On the other hand, some stains will flux it and drive the shrinkage even higher. That means if that if high and low shrinkage stained versions of Polar Ice are laminated the firing will create a tension-time-bomb that either exits the kiln cracked or cracks down the road. This work is courtesy of Robert Barritz.

The clays are Plainsman H450 and H550. Firing is cone 10 reduction. A 50:50 mix of roasted and raw Ravenscrag slip was used. L3954N black engobe was applied at leather hard stage (on the insides and partway down the outsides). We call this recipe GR10-C Ravenscrag Talc Matte, it is on the insides of both and on the outside of the one on the left. The outside of the other is G2571A Bamboo, it is also an excellent matte base. The silky matte surfaces produced by these two are both functional (they are very durable and do not stain or cutlery mark). And they are very pleasant to the touch.

The brick-halves on the left cracked in two during drying, the crack opened at the center. I dried six of them and all cracked in the same way. The one-inch-slices were cut laterally from an extruded slug of clay and sun-dried. The radial pattern of the laminations are clearly visible on the break. These laminations are "a weakness" formed-into this extruded and unwedged clay, they would, of course, extend to fired integrity, weakening the piece. The halves on the right are from a brick that I made by first wedging (kneading) the clay, then forming and cutting it to size. It was likewise sun-dried. But did not crack. I broke it (with difficulty), notice the break followed the stresses of the breaking process, not internal lines of weakness.