A type of stoneware glaze normally fired in a high temperature reduction atmosphere kiln. It is transparent and stained green or blue by the presence of iron oxide.
A green or blue-green reduction fired glaze that has been stained using iron oxide. Celadons were first developed by the ancient Chinese. The celadons that potters are accustomed to firing today are glossy transparent whereas the ancient versions were more waxy and opaque. Thus there is dispute among practitioners and purists about what exactly a celadon really should be or what glaze can truly be labelled 'Celadons'. There are many books and webpages on the subject.
Typically celadon glazes are employed on porcelain but can also be used effectively on stonewares. Modern Celadons usually possess their high gloss because of high amounts of sodium and potassium, these oxides also cause the crazing often seen. However this problem can be solved by substituting some of the Na2O with lower expansion MgO or CaO and increasing the SiO2 (using glaze chemistry of course). Celadons have traditionally been fired at cone 10 but lower temperatures are possible with the addition of more flux (e.g. Gerstley Borate).
Blue celadons typically have high sodium/potassium, high silica, not too much iron, and low titanium (Grolleg kaolin is a good option to minimize the TiO2). Some claim that a little tin oxide and/or barium carbonate will help with the blue color. Some people are investigating creating celadons for cone 6 oxidation using stains to impart the color.
The buff stoneware mug is fired at cone 10R and celadon glazed. The recesses were colored with a tenmoku glaze (on bisque by painting it into the recesses and sponging away the high spots). An outer containment line on the plate prevented the outside line from smearing outward and it provided a definite profile for cut-out after stamping.
The magic of this recipe is the 5% extra frit, that makes the melt more fluid and brilliant and gives the glaze more transparency where it is thinner on edges and contours. The extra iron in the Plainsman P380 (right) intensifies the green glaze color (vs. Polar Ice on the left). The specks are cobalt oxide agglomerates that were made by slurrying cobalt oxide and bentonite, then crushing it to sizes large enough to make the specks.
The outside glaze on this cone 10R mug (made of Plainsman H550) is simply an Alberta Slip:Ravenscrag Slip 50:50 mix with 5% added Ferro Frit 3134 (the Alberta Slip is calcined). This produces a stunning celadon with great working and application properties. Inside glaze: Ravenscrag Slip 90%, talc 10% (a matte having an extra ordinary silky texture). Learn more at ravenscrag.com.
Iron oxide is an amazing glaze addition in reduction. Here, I have added it to the G1947U transparent base. It produces green celadons at low percentages. Still transparent where thin, 5% is producing an amber glass (and the iron is showing its fluxing power). 7% brings opacity and tiny crystals are developing. By 9% color is black where thick, at 11% where thin or thick - this is “tenmoku territory”. 13% has moved it to an iron crystal (what some would call Tenmoku Gold), 17% is almost metallic. Past that, iron crystals are growing atop others. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.
At cone 10R this produces an overly melted glaze. It also crazes.
This is happening because this glaze lacks flux and is not fluid enough to enable their migration. In the upper half they are more evident (double thickness).
This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The fix: A transparent base that fits your ware. Add colorants and opacifiers to that. Another fix: substitute some of the KNaO for a lower expansion flux (like MgO, SrO, CaO, Li2O) and add as much SiO2 and Al2O3 as the glaze will take (using glaze chemistry software).
There is only 0.35% iron oxide in this recipe, as much as ten time less than normal. But this is just enough to provide a darker shadow (where the glaze is thicker) at the edges of the design. This is porcelain fired at cone 10 reduction by Janel Jacobson.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Celadon on Wikipedia