Oil spot glaze effects on ceramics have the reputation of being difficult to obtain.
But the fact is that once you know the secret of creating them using high rather than low fire glazes, there is no mystery to the process.
The most important point in obtaining this effect is firing in an atmosphere of oxidation.
This is essential because of the chemical process by which oil spots form.
Simply put, molecules of red iron oxide release an oxygen atom at about 2250 degrees Fahrenheit which converts the red iron oxide molecule into a black iron oxide molecule.
When the freed oxygen molecules leave they bubble to the surface of the molten glaze and drag some of the iron with them.
Upon reaching the surface they deposit a spot of iron on the glaze, which is what makes the characteristic oil spot appearance.
The glaze, therefore, must contain red iron oxide, and be stiff enough to hold onto oil spots but not so stiff as to prevent the oxygen from bubbling up.
Unlike similar effects created with ceramic paint, the oil spot technique requires exactitude in applying the glaze.
If the layer of glaze is not sufficiently thick then oil spotting doesn't occur, or else the spots are very small.
The final thickness of the glaze should be 1/8" to 1/4" thick.
Normally, applying glazes this thick makes the glaze run down and fuse the piece to the shelf of the kiln.
However, the glazes used to create this effect are very stiff to begin with and oxidation firing makes them even more viscous so that this is not usually a problem.
Indeed, a hanging glaze drip near the base of the piece is characteristic of the technique.
In short, to create oil spot effects requires stiff glazes containing red iron and magnesium oxides which are applied thickly and then fired to cone 10 or cone 11 in an oxidation atmosphere.
Keeping these requirements in mind makes this glazing no more difficult than using standard overglazes.
One difficulty you might run into is trying to fire all your work using a standard, early-reduction firing cycle.
If you put some test oil spot tiles into such a firing cycle you will be disappointed since the early reduction changes the iron, and no oil spots will be formed.
All that is necessary is to switch to an oxidation cycle.
The technique itself is quite easy.
The difficult but fun part is to create variations in the effects, and to make forms which accentuate the different effects.
As an example, you can change the sizes of the spots by changing the length of firing time from cone 07 to cone 11.
If you fire slowly then the spots become larger, while if you fire faster the spots are smaller since the process of liberating oxygen requires some time, and a longer firing releases more oxygen.
A further interesting variation is the hare's fur technique, which is achieved by hot-firing the glazes above cone 11, which makes the glaze run down the side of the piece and pulls and lengthens the spots.
Now that you see how easy it is to create oil spot effects, you can achieve new and beautiful pieces using this ancient glazing technique.
But the fact is that once you know the secret of creating them using high rather than low fire glazes, there is no mystery to the process.
The most important point in obtaining this effect is firing in an atmosphere of oxidation.
This is essential because of the chemical process by which oil spots form.
Simply put, molecules of red iron oxide release an oxygen atom at about 2250 degrees Fahrenheit which converts the red iron oxide molecule into a black iron oxide molecule.
When the freed oxygen molecules leave they bubble to the surface of the molten glaze and drag some of the iron with them.
Upon reaching the surface they deposit a spot of iron on the glaze, which is what makes the characteristic oil spot appearance.
The glaze, therefore, must contain red iron oxide, and be stiff enough to hold onto oil spots but not so stiff as to prevent the oxygen from bubbling up.
Unlike similar effects created with ceramic paint, the oil spot technique requires exactitude in applying the glaze.
If the layer of glaze is not sufficiently thick then oil spotting doesn't occur, or else the spots are very small.
The final thickness of the glaze should be 1/8" to 1/4" thick.
Normally, applying glazes this thick makes the glaze run down and fuse the piece to the shelf of the kiln.
However, the glazes used to create this effect are very stiff to begin with and oxidation firing makes them even more viscous so that this is not usually a problem.
Indeed, a hanging glaze drip near the base of the piece is characteristic of the technique.
In short, to create oil spot effects requires stiff glazes containing red iron and magnesium oxides which are applied thickly and then fired to cone 10 or cone 11 in an oxidation atmosphere.
Keeping these requirements in mind makes this glazing no more difficult than using standard overglazes.
One difficulty you might run into is trying to fire all your work using a standard, early-reduction firing cycle.
If you put some test oil spot tiles into such a firing cycle you will be disappointed since the early reduction changes the iron, and no oil spots will be formed.
All that is necessary is to switch to an oxidation cycle.
The technique itself is quite easy.
The difficult but fun part is to create variations in the effects, and to make forms which accentuate the different effects.
As an example, you can change the sizes of the spots by changing the length of firing time from cone 07 to cone 11.
If you fire slowly then the spots become larger, while if you fire faster the spots are smaller since the process of liberating oxygen requires some time, and a longer firing releases more oxygen.
A further interesting variation is the hare's fur technique, which is achieved by hot-firing the glazes above cone 11, which makes the glaze run down the side of the piece and pulls and lengthens the spots.
Now that you see how easy it is to create oil spot effects, you can achieve new and beautiful pieces using this ancient glazing technique.