Monday, October 2, 2017

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!

Friday, September 29, 2017

Glazing and Firing Paperweights

In my previous post I tried to give a glimpse of how a lump of porcelain clay is transformed into a paperweight on the potter's wheel, but this is only a small part of the journey from soft and sticky clay to the finished object.

The paperweights had to dry for about a week until they were bone dry. I sat them on offcuts of plaster board that I changed daily. When they were nearly finished I put them on a wire rack where they could get air right around them to complete their drying.

The paperweights then had a firing to 1000 degrees Celsius (1832 F) in the electric kiln. This temperature gives porous and somewhat fragile pottery that is reasonably easy to glaze and decorate. This firing is known as a bisque firing.

Here is a view of the newly fired paperweights. I fired them upside down to allow steam to come out and heat to go in easily.

I supported the paperweights on the porcelain rings that I made for them for when they are glaze fired.

The rings were unloaded then carefully ground with an abrasive diamond disk that I stuck to my potter's wheel with a ring of clay.

Each paperweight was carefully ground smooth underneath where it was going to contact its matching porcelain ring.

A good tight fit is important.

The rings were then stuck to the paperweights with a mixture of a water based general purpose wood working glue and Alumina Hydrate. The glue burns out in the firing leaving a thin protective layer of alumina between the paperweight and the ring that it sits on. This layer of alumina will help paperweight and ring part company after the firing.

When the paperweights were having their bisque firing, which is essentially a two day process (one day with the kiln heating up, and a night and day with the kiln cooling down!), I was busy making up some glazes. Glaze making is a bit like cooking, there is a lot of weighing of ingredients, mixing with water, and sieving with 20, 40, 60, and 80 mesh sieves to get rid of lumps and bring all the tiny particles of the glaze ingredients together in close company.  It is rather time consuming, and I can easily spend a whole afternoon making 4 or 5 glazes.

Here are some freshly glazed paperweights sitting comfortably on their ring of porcelain in their individual glaze catching bowls.

I like to pour my glazes, and for this I use an old milk saucepan that has a pouring lip. With a degree of confidence, and some sleight of hand, it is possible to get quite a nice coating of glaze all the way round a piece of pottery with one continuous pour. Crystalline glazes are generally put on much thicker than a regular glaze, so, holding the paperweights upside down by the glued on ring, I poured two layers without stopping, and also gave the top of each one a quick dip in the glaze bucket. You really need the glaze to be very thick at the top tapering to fairly thin at the lowest point. The glaze will run like mad when it is at high temperature, so you need to allow for that! After 2 or 3 minutes I then applied a little more glaze with a soft brush, and smoothed off any really high spots with a very slightly damp sponge.

The paperweights each have a stoneware glaze catching bowl put under them. Any drips and runs have to be contained. A lot of glaze can run off a crystalline glazed pot when at high temperature, and it can destroy kiln furniture and stick a pot to a kiln shelf with destructive enthusiasm!

Crystalline glaze firings really need a kiln that will get to temperature quickly, and also lose temperature fast when required, so it is usual to fire with little or no kiln furniture. I have just one half shelf in the kiln in this firing.

This kiln is manually controlled, there is no controller or over temperature safety cut out, or any frills at all! Just me! The pink, white, and yellow pointy things are pyrometric cones that will bend over when they reach their designed temperature. I also have a pyrometer to give some idea of the air temperature in the kiln, this reads between 45 to 55 degrees (113 - 131 F) low at high temperature so the temperatures in my kiln log book look a little odd. Cones are definitely the best guide of all.

The firing was 18 and a half hours from switching on the kiln to switching off. The first 700 degrees (1292 F) was fired slowly overnight whilst the weary potter tried to sleep, leaving the rush to peak temperature for the next morning.
A peak of Cone 10 was reached just before midday, then the temperature was allowed to drop to about 1100 (2012 F), and then held for just over 4 hours to allow time for crystals to grow. I made several adjustments to the growing temperature to try to affect the character of the crystals. Crystals grown at high temperature tend to be spiky, and crystals grown at a lower temperature are much rounder and more like pansy flowers. By varying the growing temperature, it is possible to exploit these different characteristics.

I am happy to report that the glaze firing was a successful one, here is a quick look into the kiln that I opened just 3 hours ago.

Tomorrow I will have the "exciting" and nervous time of separating the paperweights from their porcelain rings. I hope that the alumina did its job!

All going well... more photos will follow of the completed paperweights and I'll share a glaze recipe or two.