Thursday, 31 May 2012

How to make a Pottery Glaze

The Glaze Recipe

There are literally thousands of different types of glaze depending on the clay body used and the firing cycle and condition. A virtually unlimited number of colours, textures and designs are also possible. Applying more than one glaze to the piece just multiplies the possibilities.
Each glaze needs its own specific recipe, making process and testing for optimum performance.

However for ease of understanding I shall give the recipe and outline process for a transparent glossy glaze firing at 1050C.

A typical glaze recipe includes:

Borax Frit
China Clay
China Clay

Transparent borax frit               90 parts
China clay                                10 parts
Water                                       50 parts

Glaze milling

(particle size reduction)

In order to melt a glaze rapidly and produce a high gloss finish it is necessary to reduce the particle size to a powder and ensure that it is well dispersed in water. For this to be achieved the particles of the frit and clay are ground to an average size of 5 microns-less than 1/10th the size of a human hair. Overall a distribution in the range 60-70% less than 15 microns is targeted.

Glaze Mill
Ball mills are traditionally used to grind the materials to make the glaze. The ball mill is a metal cylinder lined with hard alumina or silica blocks. The mill is filled with graded ceramic balls with a max size of 5cm. to approx 50% apparent volume of the internal space. This allows efficient grinding of the glaze. The glaze materials occupy approx 1/3 the volume. The glaze is sampled after set time periods eg 12 to 20 hours until the required particle size distribution is achieved.

Glaze Processing

Once the average particle size is achieved the glaze slurry is poured out of the mill into a storage vessel via a coarse sieve. This removes large glaze particles and remnants of the grinding balls from the glaze. Typically a 60 mesh sieve is used.

The glaze is then processed further by passing the slurry over thro a fine sieve typically less than 100 mesh and a strong magnet. The magnet can be rare earth magnets or electromagnets. This cleansing process removes more of the coarse residue and contaminants that can give faults after firing.

Glaze Quality Controls

A few simple tests are used to assess the quality and suitability of a glaze for use and include:-

1. Glaze appearance after firing- by glazing a tile of standard body and firing

2. Particle Size measurement and distribution

3. Solids content of slip = slip density

4. Slip fluidity = Slip viscosity

Additional tests may be carried out depending on the end use. For example for hotelware the physical and chemical durability may be assessed.

Rheology Control

The glaze slip rheology is controlled according to the application method required eg dip, spray or brush etc.
This is achieved by control of density, viscosity and thixotropy. A torsion viscometer and density can are often used to measure and control these key properties of the slip.

Addition of binders and other chemical additives are usually necessary to achieve the optimum properties for specific application methods.

Supply and Packaging;

Glaze can be supplied in containers suitable for use from 500ml jars for brush on glaze, 5 to 50 litres for dip glaze and 5 to 1000 Litre tanks for spray glaze.

For industrial use tankers containing 10000 litres may even be used. However for overseas or long distance supply the glaze is often supplied as a dried powder. In this case the user needs to mix up the glaze and control the slip characteristics themselves.

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend. Go now to sign up for my free newsletter.

No comments:

Post a Comment

Please leave me a message and I will reply asap
Happpy Potting
The Potters Friend