Porcelain Paperweights. One important step remains... Hammer and Chisel!

A scary, but unavoidable, part of making a pot with a crystalline glaze is separating it from the little ring of porcelain and the glaze catching saucer. I was a bit anxious about the paperweights as their round shape does its best to deliver the maximum amount of run off glaze right to where the porcelain ring is joined onto the paperweight, and there was a chance that glaze would penetrate the join and glue everything together.

Some crystalline glazers favour using a gas burner with a very thin flame to work around the join, but I find that a sharp chisel and a hammer does the job quite well.

I like to tap right round the join quite gently and try to form a little scratch or fault line in the run off glaze with the chisel as I go. I can generally give a series of smart taps after this, and the ring will pop off.

 The run off glaze leaves a sharp edge and this needs to be ground off.

Wet grinding with an abrasive disk fitted to the wheel head is a great time saver.

I finish off around the edge by hand with a high tech sharpening "stone" that is designed for sharpening chisels.

The paperweights really come to life in sunlight. The crystals scintillate in spectacular fashion, and their intricate structures become more apparent.

  1 percent cobalt oxide gives both blue and purple in this glaze.

1.5 percent nickel oxide gives Prussian blue crystals and a orange glaze around them. Amazing details in this glaze, you could spend hours with a magnifying glass exploring the stars and flowers!

The star shapes in the centre of these crystalline structures are rather fine!

Copper carbonate with just a trace of iron oxide gives golden crystals floating on a green glaze.

A Crystalline glaze recipe

I have played with several crystalline glazes over the past few years, but the one I use most often at cone 9 - 10 with the porcelain clay that I have is as follows;

Frit 4110  47

Zinc oxide  27 (you can use regular zinc oxide, but you may find calcined zinc makes the glaze somewhat easier to apply).

Silica  24

China Clay 0.5

Bentonite  2

+ Titanium dioxide  3 (The titanium is left out altogether if nickel oxide is used for colour, but is part of most other glazes in 2 - 5 percent)

To this you can add copper, cobalt, iron, manganese, or nickel, by themselves or in mixtures with interesting results. Rutile and ilimenite also extend the range of possibilities. You should experiment. The glaze should be applied thickly.

The way that the kiln is fired makes a profound difference as to how crystalline glazes turn out. Temperature and time are used like an artist's paint brush to create crystals of different sizes and shapes.

A way to start with the glaze base I have given here would be to fire as follows.

Fire quickly to Cone 9 fully down and Cone 10 starting to bend --using cones-- not just relying on controllers and gadgets! If possible the kiln should climb at at least 125 Celsius (257 F) per hour for the last hour or two (more is better). Let the temperature fall to about 1100 degrees (2012 F), and hold for 3 hours then switch the kiln off. 

If you follow that schedule you should get crystals, maybe as big as an inch and a half in size.

 

*Note that great care must be taken to protect your kiln shelves from run off glaze. Crystalline glazed pots should have glaze catching bowls placed under them. If you don't know what those are, have a look back at my previous 2 posts.

Have Fun!