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Oil Spot Glazes by John Britt

John has taught all types of ceramic classes including, basic hand-building, throwing, sculpture, raw materials and glaze chemistry, glazing techniques and kiln building to all levels of students.

Most pottery and ceramics glazes are simple to make and very easy to fire, but there are some glazes and glaze effects that have the reputation of being hard to get or at least difficult to obtain.

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This was 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.
Now, we don't know if this was true or just a old wives tale.
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 actually 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 process by which the oil spots are created. What really happens is, the red iron oxide molecule will let go of an oxygen atom at approximately 2250° F (1232° C).
At this temperature, the red iron oxide molecule cannot keep its complicated structure and will release an oxygen atom to become the simpler 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.

Oil spot Glaze 1

When it reaches the surface and leaves the glaze, it deposits the spot of iron which creates the characteristic oil spot look, which you can see above.
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.
Feldspar bases are perfectly suited to create oil spots.

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 picture below.
The final glaze thickness should be at least 1/8” and can be as thick as 1/4”.
Usually if you apply a glaze thicker than this, it will run and fuse the pot to the shelf.
This does not happen as much with oil spot glazes because the feldspar glaze is so stiff and when iron is fired in oxidation it will retain its strength during a high fire, which gives the glaze a high resistance to flow.
This often results in a characteristic hanging glaze drip at the base of the pot.
The tea bowl picture below shows this very well.

Oil Spot Glaze 3

Close up of Oil spot glaze application showing thick and thin application.

Oil Spot Glaze 4

Tea bowl.
Dirty Porcelain.
cone 11 – gas oxidation

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 hard to get than any other type of glaze.
The trouble that a lot of people run into, is that they want to fire all their work with a standard early reduction firing cycle.
They put some oil spot test tiles into this firing cycle, only to be disappointed.
Early reduction changes the iron and they get no oil spots, so they think it is a difficult glaze to get.
There is a simple solution, all you have to do is switch to an oxidation cycle.

Once you know what causes the oil spot effect, then producing it is actually quite easy.
The difficult but fun part is creating variations in the effect and creating forms which bring out those effects.
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 fire 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 though, by simply refring the piece.
A slightly more complicated firing cycle has to be used to reduce these unhealed blisters.
With this firing cycle, the kiln is in full oxidation to cone 9 and then is put into reduction through cone 11.
This will help to stop the bubbling and flowing out of the surface iron, creating a smoother glaze surface.
Test tiles are extremely helpful to determine when the firing is complete.

Another interesting variation is the hare’s fur effect.
This can 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 their on is drawn to the surface.
These are only a few of the many variations possible with oil spot glazes.
Below is a picture of this leopard spotted glaze.

Oil Spot Glaze 5

Incense Burner
Grolleg Porcelain
Bailey’s Oil Spot with yellow glaze over it.
Cone 11 – gas oxidation

Oil Spot Glaze 6

Close up of the detail on the incense burner.

If you want a different look to your piece of pottery or ceramic, try this oil spot glaze.
It is fairly simple, but when finished it will look like a hard to create finish and everybody will think you had to work hard to get the special look.

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