Kiln Unloaded. Exciting Crystalline Glaze Test Results!

 

I was delighted with the results of the recent crystalline glaze test firing. I tested quite a variety of glaze recipes, none of which I had used before, and all but two gave very promising results right away. 

Macro-crystalline glazes that are formulated to produce clusters of large zinc-silicate crystals are often *frit based, for example 50% high sodium frit, such as 3110, with 25% silica and 25% zinc oxide will almost certainly make a basic recipe for a cone 9 or 10 crystalline glaze. The frit based glazes are adjusted to the particular needs of the potter, often the frit content comes back a little from the 50% and maybe a tiny amount of china clay or bentonite is added to make the glaze easier to apply, or to help control the flow of the glaze. Sometimes a secondary flux might be added, such as Whiting, but the frit based glazes are often very similar in formulation. 

 

A Feldspar based crystalline glaze.

**Feldspar based macro-crystalline glazes are rather more varied. Like their frit based cousins, these glazes often have about 25% zinc oxide, and at least 20% added silica, but feldspar introduces a considerable quantity of alumina to a glaze, so there are limits as to how much feldspar it is practical to use when making a crystalline glaze as one of the prerequisites of growing large crystals is to do so with a low alumina glaze base. When looking at Feldspar based recipes I have noticed that many have Feldspar at no more than about 35 percent, the glaze then needs secondary fluxes from other sources in order to get them to flow. The secondary fluxes can add a great deal of character to the glaze, and calcium, strontium, magnesium, lithium, barium, or maybe a rare earth or two, are all useful candidates, possibly singly, but more likely as a pair. 

Metal oxides, used for adding colour to the glaze, can also play a role in making the glaze more fluid. Sometimes the fluxes or the metal oxides can cause small crystals to form in the glaze that can add a great deal of interest to the large zinc-silicate ones. 

I tested all the glazes on cylinders, each with their own built in glaze catching saucer for any run off glaze. I also tested a few of the glazes in small bowls. It is very noticeable how different the glaze can be on the inside of a bowl as compared to on a vertical surface.

This Feldspar based glaze was one of the most exciting of this batch of tests.

I love the starry centres of some of the crystals.

The blue and the white are both the same base.

The blue glaze has some cobalt oxide added to it.

This white glaze is very different in colour when inside a bowl.

Glazing inside a bowl with a crystalline glaze can reveal a lot about the glaze. Near the rim the glaze is thin, then it thickens as it flows down towards the bottom of the bowl. In the middle of the bowl the glaze is too thick so the crystals become coarse and dense, then rough and dull, because the glaze is thick enough to allow them to send up little vertical spikes!

 

I did two tests of each glaze base, one of just the base, and another with an additional metal oxide to add colour. It is a very useful way of getting a glimpse of the potential of the glaze, and to see what the fluxing effect of the metal oxide will do to the crystal growth and shape. You may notice that I also record the glaze name, Cone fired to, and sometimes the whole glaze recipe around the base of the tester.

Some Notes.

Some of the glaze recipes that I used came from the excellent glazy.org website, others from books, that included Peter Ilsley's Macro Crystalline Glaze book. I am guessing that this book is now, sadly, out of print as I can only find very expensive second-hand copies listed on Amazon. 

 

Here is my log of the Cone 9 glaze firing. I work with a manual kiln so there are little notes below the hours that say what power settings were being used at the time. My pyrometer always reads low so, according to it, cone 9 was achieved at 1245 C, when it was probably closer to 1270 C.

And here is a glimpse of one of the untidy sheets of glaze recipes that I used whilst glazing. I mark a dot by each material as I add them to the glaze, so I don't forget anything, or add anything twice!

*frit (also fritt). Glaze frits are really a special type of ground glass. Frits often contain glaze materials that are difficult to work with in their natural state, such as sodium and potassium, these are combined with silica and often some boron, to make a glass. 

**Feldspar. Nature has glaze frits of its own, such as feldspar that contain almost enough of the necessary ingredients to make a glaze for stoneware temperatures. Feldspars contain silica (the glass former in a glaze), alumina (a stabiliser that controls the flow of a glaze), and various fluxes - usually potassium and sodium, some may also contain calcium or lithium. Fluxes assist with persuading the silica that is in the glaze to melt at a sensible temperature... as without fluxes silica would need to reach above 1700 Celsius to become molten.