Oil Spot Glazes
by John Britt

Most ceramics glazes are simple to formulate and very easy to fire. But there are some glazes and glaze effects that have the reputation of being elusive or at least difficult to obtain. This was certainly the case early in the 20th century with copper reds and celadon blues. These glazes had a history of secrecy and myth surrounding them. One story even claimed that a Chinese emperor had slaves thrown into the kiln during the firing in order to produce the elusive copper red. Thankfully, that was found to be unnecessary, because we learned that reducing the kiln early in the firing produced beautiful copper reds. Oil spot glazes also have the reputation of being elusive but, as I will show in this article, they are very easy to create, once you know the process which makes them work.

Oil spot tea bowl, John Britt, 2004, dirty porcelain, cone 11 gas oxidation, multi-fired, 4” x 3” x 3”

The most important factor in obtaining oil spot glazes is firing in an oxidation atmosphere. This cannot be overemphasized. Oxidation is essential because of the mechanism by which the oil spots are created. Simply stated, the red iron oxide molecule, Fe₂O₃, will let go of an oxygen atom at approximately 2250° F (1232° C). At this temperature, the red iron oxide molecule cannot maintain its complex structure and will release an oxygen atom to become the simpler FeO molecule, or black iron oxide. As the oxygen leaves the red iron oxide molecule it bubbles to the surface of the molten glaze, dragging a bit of iron with it (image on left). When it reaches the surface and leaves the glaze it deposits the “spot” of iron which creates the characteristic oil spot look. However, if the glaze is reduced early in the firing cycle, the red iron oxide will already have changed into black iron oxide, and there is no possibility of an oxygen molecule being released later to create the oil spot effect. So you can see that without an oxidation firing cycle there will be no oil spot effect.

The second most important factor is the glaze recipe. It must contain some red iron oxide and be sufficiently stiff to hold the oil spots that are created, but not too stiff to prevent the bubbling out of the oxygen. Feldspathic bases are perfectly suited to create oil spots. The following are some excellent recipes:

Bailey’s Oil Spot Cone 10/11 (images 1& 2)
25.51 Custer Feldspar 35.71 NC-4 (Soda Feldspar) 15.30 EPK Kaolin 5.10 Talc 5.10 Dolomite 5.10 Ferro Frit 3110 8.16 Silica 6.00 Red Iron Oxide
1. Cup, John Britt, 2003, Bailey’s Oil Spot, Grolleg porcelain, cone 11 oxidation, 5” x 4” x 4”	2. Detail of Cup, John Britt, 2003, Bailey’s Oil Spot, Grolleg porcelain, cone 11 gas oxidation, 5” x 4” x 4”

John’s Oil Spot #3 Cone 10/11 (Works nicely in electric kilns) (images 3 & 4)
54.62 NC-4 (Soda Feldspar) 29.41 Silica 4.20 Whiting 7.56 EPK Kaolin 4.20 Dolomite 6.72 Red Iron Oxide 4.20 Cobalt Carbonate
3. Cup 2, John Britt, 2002, Grolleg porcelain, cone 10 electric oxidation, 4” x 3” x 3”	4. Detail of Cup 2, John Britt, 2002, Grolleg porcelain, cone 10 electric oxidation, 4” x 3” x 3”
Traditional Oil Spot Cone 10/11	Montmollin Oil Spot Cone 10/11
80.18 Custer Feldspar 5.66 Silica 3.77 Whiting 4.71 Talc 5.66 Bone Ash 8.50 Red Iron Oxide	68.40 F-4 (Soda feldspar) 17.80 Silica 4.90 EPK Kaolin 8.90 Talc 7.11 Red Iron Oxide 2.00 Cobalt Carbonate

Each recipe contains a high amount of feldspar, over 50%, some magnesium oxide, either from talc or dolomite, and between 6.0 - 8.5% red iron oxide. Finally, the glaze needs to be fired between cone 10 and 11, which I will discuss later.

Another critical factor is the glaze application. If the application is not thick enough there will be no oil spotting or only very small spots, while thicker application will give larger spots (see image 5). The final glaze thickness should be at least 1/8” (0.3 cm) and can be as thick as 1/4” (0.6 cm). Usually if you apply a glaze this thick it will run and fuse the pot to the shelf. But this does not happen as frequently with oil spot glazes because the feldspathic glaze is so stiff and, in addition, when iron is fired in oxidation it is refractory which makes the glaze even more viscous. This often results in a characteristic hanging glaze drip at the base of the pot (see image 6).

5. Detail of Oil spot glaze application - thick and thin, John Britt, 2004	6. Tea bowl, John Britt, 2004, Dirty Porcelain, cone 11 gas oxidation, 3” x 3” x 3”

So to summarize, oil spot glazes are usually stiff glazes which contain red iron oxide and magnesium oxide. The glaze needs to be applied thick and fired in oxidation to cone 10 or 11. If you follow these simple steps then oil spot glazes are no more mysterious or illusive than any other type of glaze. The hurdle that many people run into is that they want to fire all their work with a standard early reduction firing cycle. They then put some oil spot test tiles into this firing cycle, only to be disappointed. Since early reduction changes the iron they get no oil spots and think it is a difficult glaze to achieve. But simply switching to an oxidation cycle is all that is necessary.

Once you know the mechanism which causes the oil spot effect then producing it is actually quite easy. The difficult but fun part is then creating variations in the effect and creating forms which accentuate those effects. For example, you can alter the size of the spots by varying the length of time it takes to fire from cone 7 to cone 11. If you go slowly, (4 hours), the spots will be larger, while going faster will give you smaller spots. This is because the process of releasing oxygen takes time and lengthening the firing will allow more oxygen to be released, while firing too fast may leave large unhealed blisters on the surface of the glaze. (This can be corrected simply by refring the piece.) You can use a slightly more complicated firing cycle to reduce these unhealed blisters. In this firing cycle the kiln is in full oxidation to cone 9 and is then is put into reduction through cone 11. This will help to stop the bubbling and flux out the surface iron creating a smoother glaze surface. Draw tiles are extremely helpful to determine when the firing is complete.

Another interesting variation is the “hare’s fur” effect. This can easily be created by firing the glazes very hot, above cone 11, which causes the glaze to run down the side of the pot, pulling and lengthening the spots. Another interesting variation is to put a stiff light colored glaze over the oil spot glaze. This will cause an interesting leopard spotted glaze as the iron is drawn to the surface (images 7 & 8). These are only a few of the innumerable variations possible with oil spot glazes.

7. Incense Burner, John Britt, 2003, Grolleg Porcelain, Bailey’s Oil Spot with yellow glaze over, cone 11 gas oxidation, 3” x 4” x 4”	8. Incense Burner detail, John Britt, 2003, Grolleg Porcelain, Bailey’s Oil Spot with yellow glaze over, cone 11 gas oxidation, 3” x 4” x 4”

From this article you can see how simple it is to achieve the oil spot effect. And knowing the process by which it works eliminates the need to perform any brutal sacrificial rituals on your assistants. Hopefully, you can use this knowledge as a spring board to create new and beautiful variations of this ancient glaze.

Article and images courtesy John Britt. © John Britt 2004. See more of John's work at his website http://www.johnbrittpottery.com. John has also got a new book out: The Complete Guide to High-Fire Glazes: Glazing and Firing at Cone 10, Lark Books, 2004. ISBN: 1579904254.

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