Lasse Östman, Swedish potter. More about Crystal Glazes


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Crystal glazes are very special glazes. Delicate, strong colors, spectacular crystals of different shapes, and other interesting effects.

They are a bit difficult to master, runny, difficult to repeat exactly. They try your patience. They ruin your kiln shelves. They demand great care and cleanliness. They are expensive and often poisonous to handle. They drive you crazy.

I love Crystal Glazes!

I think I have tested about 2000 crystal glazes today. I use about 50 of them often.

When I have invented a good base crystal glaze, I try to vary the coloring oxides and carbonates. In that way I can get many quite different glazes in an easy way. Sometimes the coloring oxides affects the glaze in a bad manner and I have to correct the base glaze. Be patient - donīt give up!

For further tests I change the Feldspars, try different amounts of Zinc, Dolomite, Rutile, Whiting and Barium (maybe Strontium), change the frits and substitute Lithium carbonate and common Feldspars for Petalite or Spodumene.

For coloring I use: Ferric oxide, Copper oxide and carbonate, Cobalt oxide and carbonate, Nickel oxide and carbonate, Manganese carbonate, Uranium dioxide,
and sometimes: Vanadium pentoxide, Molybdenum oxide, Antimony oxide, Rutile and Ilmenite.

All, except a few, crystal glazes fire at 1250-1280oC. You can try Orton cone 7-9.

For my normal firing schedule (even crystal glazes!), look at my "Firing Page".

Best is porcelain and white or light gray stoneware clays. Oxidation in an electric kiln. Later I will add recipes for a silky mat crystal glaze that fires in reduction.

I seldom try to get very big crystals. I think the smaller (2-25mm) ones, often very complex, together with running streaks suite my taste better.
I do not use a longer cooling and I do not use other, not so runny, glazes near the bottom.
I spray all glazes in a normal way, and
I have not to grind away glaze from the bottom.
But I make a lot of glaze testing and control thickness and temperature very careful. And if you are observant you can see that there is not so little Alumina in the crystal glazes!
Of course you can get very big crystals with my lithium glazes, but then you must apply it and fire it in a little different way (controlled cooling).

Here I will concentrate on 4 of my base crystal glazes.
They represent about 80% of my commonly used ones.
Base Crystal Glaze No 6, silky mat
Base Crystal Glaze No 7, glossy
Base Crystal Glaze No 6713, glossy
Base Crystal Glaze No C4, semi glossy

Base Crystal Glaze No 6

with ferric and uranium oxide and
a small amount of cobalt oxide.

with ferric, copper and cobalt oxide.

Potash feldspar LR
Zinc oxide
This is a very simple glaze, only three ingredients. Easy to use and not at all difficult. It was one of my very first crystal glazes, but I use it still very often.

You canīt expect too large crystals. They are round shaped and the difference between the bottom color and the crystals can be very striking. It will give very good streaks on vertical surfaces. Up to 1" crystals.

It is a very good glaze for use with many coloring oxides. The surface is silky mat to semi shiny. Test it substituting the LR feldspar for your ordinary and change the relations between Zinc oxide and Dolomite. Try to put in Rutile, Barium carbonate and Whiting.............

Base Crystal Glaze No 7

with rutile and ferric oxide.

I have no really good picture.

Potash feldspar LR
Barium Carbonate
Zinc oxide
Lithium Carbonate
China Clay
Sometimes it gives good crystals, sometimes not. It gives very sharp colors, flashing orange and blue. It is one of my earlier glazes. I use it very seldom now.
Because of its ingredients it is quite unstable and will flow a lot unless not exactly the right thickness and temperature. It was a starting point for further tests that led to one of my best base crystal glazes, the 6713.

Base Crystal Glaze No 6713

with copper, ferric and manganese

with cobalt and ferric oxide

with rutile and cobalt, copper and
ferric oxide
Potash feldspar LR
Barium Carbonate
Zinc oxide
Lithium Carbonate
China Clay
This is one of two favorite bases. I have it in at least 100 variations. Always good crystals and not too difficult to master. Crystals up to 1", maybe more.

It was developed from a mix of bases No 6 and 7, about 1/4 of No 6 and 3/4 of No 7. It is much more stable than No 7, and gives good crystals easier than No 6.

I use it with a number of coloring oxides, often 3 to 5 together. In that way I can have both Primary and Secondary crystals of different shape and color on the same piece. Very distinct colors.

It can be sprayed on right to the foot ring, but a little thinner of course.

I have made variations with other Feldspars, have substituted Barium for Strontium or Calcium. A small amount of Rutile is often good. For other temperatures correct the glaze with the Quarts and Lithium.

It is a good glaze to combine with the next, No C4.

Base Crystal Glaze No C4

with cobalt oxide and rutile
Borax frit MOK1
Zinc Barium Frit 90420
Zinc oxide
Rutile or Titanium oxide
China Clay
The previous 3 glaze bases are not containing Frits. This one is based on Borax and Zinc Barium Frits. I have tested a lot of other Frits too, Alkaline and Lithium ones. Many are very good. Try it with your Frits and correct it with the China Clay and the Quartz.

This type is as easy to use as No 6713. It often gives 2 or 3 sorts of crystals in the same glaze. Smaller and bigger ones. It is not so shiny as No 6713.

Good with many oxides and carbonates. In some color combinations you must decrease the Rutile.

If you refire it, the crystals may be smaller but more numerous. Test it together with No 6713. Very fascinating results may occur.

At last: Remember one thing - without a good body, the form,
an interesting glaze canīt make a good pot. Crystal glazes needs simple
forms with sparse or no decoration. Else it will be "too much".

Good luck with your testing of one of the most exciting glaze types!
Donīt use crystal glazes for food ware and be aware of the toxic risks
with some of the ingredients. Have fun!


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