These lines plot the firing shrinkages for three versions of L3685Z2 engobe. Notice the terra cotta body I want to match (red line) and the black engobe (green line) do not cross anywhere. That means there is no temperature at which they fit each other (the engobe always has 2% or more firing shrinkage). Notice the L4170B terra cotta fits the white version, Z2, at 2150F (red line crosses blue line, but the body is over-fired by that point). For a fit at my preferred 2000F (cone 02) I need the Z4 engobe to shrink 2% more (a 3% addition of frit 3110 will do that). What about the black Z4? That is the opposite situation, it already contains 5% frit, removing that will drop that green line about 3%, hitting the red line at 2000F (and following it all the way down past 1950 into the cone 04 range). I ignored all of this and used the Z2 white on L4170B, L210 and L215. It looked good on most pieces, but sure enough, it did crack around abrupt contours on some. Of course, this does not assume a thermal-expansion-match of body and engobe.

Before jumping to conclusions consider all the factors that relate. This is M340S, it is fired at cone 6. That temperature is a "sweet spot" for this effect, high enough for the particles to bleed and low enough they do not bloat the body. Such bodies contain only about 0.2% of 60-80 mesh granular manganese (compare this to many glazes that employ 5% powdered manganese as a colorant). Further, the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function the glaze thickness and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal. Consider also that the total area of manganese-stained glass on a functional surface is extremely small for this effect.

I was consistently getting the cone on the left when using a custom-programmed firing schedule to 2204F (for cone 6 with ten minute hold). However Orton recommends that the tip of the self supporting cone should be even with the top of the base (they consider the indicating part of the cone to be the part above the base). So I adjusted the program to finish at 2200F and got the cone on the right. But note: This applies to that kiln at that point in time (with that pyrometer and that firing schedule). Our other test kiln bends the cone to 5 o'clock at 2195F. Since kiln controllers fire cone 6 at 2230 (for the built-in one-button firings) your kiln is almost certainly over firing!

G2926B is a popular recipe and there has been alarm recently because of the difficulty in getting the Ferro frit. This motivated us to get a supply of the Fusion equivalent, F-12. We have done much testing in glazes and with melt fluidity tests like this and so far it is looking good.

G2934B is a popular recipe and there has been alarm recently because of the difficulty in getting the Ferro frit. This motivated us to get a supply of the Fusion equivalent, F-19. We have done much testing in glazes and with melt fluidity tests like this and so far it is looking good.

Both of the bodies are made from materials processed to minus 200 mesh. They were fired side-by-side to cone 6. Both bodies are vitreous, in fact, the buff burning one, Plainsman 3D 325, is more vitreous (the white one is Plainsman Polar Ice). The glaze is G2926B with 10% Mason 6304 stain and 2% Zircopax added. Clearly, iron content in the body heavily affects the development of the color in this glaze. But it is more. Because the 3D is highly vitreous the glaze melt is better able to leach iron from it.

As you can see from the search, this is becoming "a thing". The ash is being incorporated into both clay bodies and glazes. The ash of pets and humans. If you are a potter wondering about doing this here are a few tips. Do testing, better to use up some of the ash for that than have to throw away the ware you make! If the ash has not been ground (likely the case for pets) there will be bone fragments, these won't melt so need to be removed for glazes (by sieving or ball milling). For wedging into the body, testing will be needed (consider the possibility of lime-popping). Be careful to write down your procedure during testing so that production does not bring surprises. While you can add ash to commercial bottled glazes, the percentage will be low. If you make your own dipping glaze, 50% ash should be possible. Do tests without colorants to get a base glaze that is melting well and does not crawl. Add stain powders to test colors, zircon and titanium dioxide to opacify (the latter will variegate more). Color and opacifier additions can introduce crawling, test these well also.

Often the answer is yes. This glaze looks too matte, too metallic, too crystalline. This picture was sent to me by a worried person who had bought it and noticed it discolouring on the inside. The potter may very well have considered this safe just because it was fired to cone 10. It is common among potters to overload glazes with raw metal oxide blends, often 15% or more (e.g. manganese, copper, iron, nickel, cobalt). These percentages cannot be held in solution in the melt as it cools and solidifies, so they precipitate out and crystallize, especially if the glaze is not melting well or has insufficient SiO2. The crystalline forms of these metals might look nice to some people, but the glaze is likely to leach them. It is better to use a ceramic stain to create a black like this, adding it to a stable matte base glaze (one that melts well and has sufficient SiO2 and Al2O3 to create a durable glass). The concept of a limit recipe is helpful in eyeballing recipes for their likelihood of leaching.

Eastend, Saskatchewan hosts the incredible T. Rex Discovery Centre. The centre hosts Scotty, the world's largest T. Rex. The building is an architectural marvel and the exhibits are on par or better than anything else where. The building is actually embedded into the hillside in the same geological layers in which the fossilized skeleton was found, the Battle formation (just above the Whitemuds that are mined by Plainsman Clays). If you ever get a chance to drive from Ravenscrag to Eastend take the south road for the most dramatic views. Do it in the spring when the grass is green. The sheer scale of the valley, the land formations and the countless outcrops of the whitemuds will amaze. At Eastend, have a meal of Jack's Cafe, you won't be disappointed! Then go and see Scotty.

As this cone 6 melt flow test demonstrates, the Fusion F-12 is giving very similar performance to the Ferro. The GR6-A recipe is just 80% Ravenscrag Slip and 20% frit (10% zircopax has also been added to opacify). The degree-of-melt on the glazed tiles in also very similar, however notice that some of the whiteness has been lost on the second F-12 tile (the first is frit 3134). We are attributing this to the better melting of F-12, that is amplifying the color of iron present in the Ravenscrag Slip. For the third tile we reduced the frit to 15%, that has whitened it somewhat. Likely an increase in the zircon and would whiten it more.