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.
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. 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. (See photo #9 or 10) 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 then 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. They can be used alone or in combination. The following are some excellent recipes:
Bailey’s Oil Spot Cone 10/ 11 oxidation
25.5 Custer Feldspar (Potash)
35.7 NC-4 Feldspar (Soda)
15.3 EPK Kaolin
5.1 Talc
5.1 Dolomite
5.1 Ferro Frit 3110
8.2 Silica
6.0 Red Iron Oxide
John’s Oil Spot #3 Cone 10/ 11 oxidation
(Works nicely in electric kilns)
54.6 NC-4 Feldspar (Soda)
29.4 Silica
4.2 Whiting
7.6 EPK Kaolin
4.2 Dolomite
6.7 Red Iron Oxide
4.2 Cobalt Carbonate
(See photo # 3, 4, 7, 8, 10, 16, 35,) (See photo #5 and 6)
Traditional Oil Spot Cone 10/ 11 oxidation
80.2 Custer Feldspar (Potash)
5.7 Silica
3.8 Whiting
4.7 Talc
5.7 Bone Ash
8.5 Red Iron Oxide
Candace Black Cone 10/ 11 oxidation
65.0 NC-4 Feldspar (Soda)
20.0 Silica
5.0 EPK Kaolin
5.0 Talc
5.0 Dolomite
8.0 Red Iron Oxide
2.0 Cobalt Carbonate
(See photo #17, 21, 22, 23, 36 or 38) (See photo # 1, 2, 5 or 6)
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.
Besides feldspathic bases, another good way to achieve oil spots is with glaze recipes containing slip clays, like Albany Slip, Alberta Slip, Redart, or other local slip clays. These slips clays are high iron, low temperature clays that when fired to cone 10 can become glazes by themselves. But I have found the most success by using them as a part of a recipe. Because these recipes (see below) contain high amounts of clay they can shrink quite a bit and may subsequently flake off if applied too thickly on bisque ware. But with care, they can be applied properly and work well on both bisque and greenware. Often a cover coat glaze is applied over the slip glaze which assists in the oil spotting.
Dorothy’s SG-12 Cone 10/ 11 oxidation
47.5 Albany Slip Clay
38.0 NC-4 Feldspar (Soda)
12.2 Kentucky Ball Clay
2.4 Borax
4.8 Red Iron Oxide
SG -12 Cone 10/ 11 oxidation
47.4 Albany Slip Clay
38.0 Custer Feldspar
9.5 Kentucky Ball Clay
2.1 Bone Ash
3.1 Talc
4.5 Red Iron Oxide
1.0 Rutile
John’s SG-12 Cone 10/ 11 oxidation
38.0 Custer Feldspar (Potash)
40.1 Redart Clay
9.5 Kentucky Ball Clay
5.5 Dolomite
2.1 Bone Ash
3.1 Talc
1.7 Whiting
4.5 Red Iron Oxide
1.0 Rutile
Davis Oil Spot 1 Cone 10/ 11 oxidation
57.4 Nepheline Syenite
19.1 Albany Slip Clay
14.4 Burnt Umber
9.1 Silica
NAO #236 Cone 10/ 11 oxidation
63.2 Buckingham Feldspar (Potash)
21.0 Albany slip
15.8 Burnt Umber
#215 Cover Glaze Cone 10/ 11 oxidation
45.0 Custer Feldspar (Potash)
13.0 Whiting
28.0 Silica
11.0 EPK Kaolin
8.0 Red Iron Oxide
Another critical factor in achieving oil spot glazes is the thickness of 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 photo #16) The final glaze thickness should be at least ⅛”and can be as thick as ¼”. Usually if you apply a glaze this thick it will run and fuse the pot to the shelf, which is why traditionally oil spot bowls were fired on clay wads. But this does not happen as frequently with feldspathic oil spot glazes because they are 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 photo # 21, 33, 34 or 38)
If you are using slip based oil spot recipes apply one or two coats of the slip glaze, followed by one or two coats of the cover glaze. The application of these glazes should be a bit thinner (⅛”) to avoid it cracking off or excessively running, although the stiff cover coat glaze can reduce some running. Remember that small variations in thickness of either glaze coat can create a huge variety of final appearances.
So to summarize, oil spot are generally of two types; a feldspathic glaze or a slip based glaze with a cover coat. The glaze needs to be applied thick and fired in oxidation to cone 10 or 11. If you follow these simple steps then oil spots are no more mysterious or illusive than any other type of glaze. The main hurdle that many people run into is that they want to fire all their work in 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 spotting then producing it is actually quite easy. The fun part is then creating variations in the effect. 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. Firing too fast may also leave large unhealed blisters on the surface of the glaze. These can be corrected by including a peak temperature soak or by simple by refring the piece. You can also 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.
An interesting oil spot variation is the “hare’s fur” effect. This can be created by firing the feldspathic glazes very hot, above cone 11, which causes them to run down the side of the pot, pulling and lengthening the spots. Some slip glazes alone will produce the hare’s fur effect but for others you will need to use the recipes above with the cover glaze. A thinner coat of the cover glaze will tend toward more hare’s fur effect.
Another fascinating adaptation is to put a stiff light colored glaze over the oil spot glaze. For example, try a stiff white or yellow glaze over the oil spot. (See recipe and photo below.) This will cause an interesting leopard spotted glaze as the iron is drawn to the surface. These are only a few of the innumerable variations possible with oil spot glazes.
CHINESE CRACKLE Cone 10/11
83.0 Custer Feldspar (Potash)
8.0 Silica
9.0 Whiting
10.0 Zircopax
(See photo # 32)
LINDA’S YELLOW Cone 10
32.7 Custer Feldspar (Potash)
16.3 Whiting
31.6 Silica
14.3 EPK Kaolin
5.1 Dolomite
6.0 Mason Stain 6464
(See photo # 12 and 13)
From this article you can see how simple it is to achieve the oil spot effect and some of its many variations. 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.
John Britt