Glaze technology

The information in this glaze course has been written with idea of synchronising practical experiments in glaze making with the theoretical knowledge of glaze chemistry. This course was developed when I was teaching a ¬†glaze course in India, so I have kept out a lot of confusing detail and simplified the concepts where possible. During the lesson sessions additional information is given out if students require either more detail, or if a particular student’s problem requires more information. The analysis of materials and results is also done blackboard style and so no notes are available.

If you intend using this information in a course please credit your source – me!

If you are using this information for your own development then good on you, and good luck with your tests.

Practical Course

This practical course covers the basic ideas of experimentation and ties in with the thereoy pages. It does require access to a range of common glaze materials and equipment, including a kiln capable of reaching 1280 degrees.

HyperGlaze

Much of my early understanding about glaze chemistry arose through the use of a piece of software called HyperGlaze. This software analyses glazes and is immeasurably useful when learning about what materials do in a glaze and how the relationship between materials and oxides works out in a glaze. Of course much testing is still needed, but at least you have a tool that can quickly get to the heart of glaze chemistry. The tests listed have all been initially developed using HyperGlaze.

Engobes
The descriptions and application advice given below are only an initial guide, almost all the rules can be broken in ceramics, however it is best to understand your materials first and to experiment.
Slips
Usually consist of one or more clays made into a liquid, sometimes with the addition of feldspars, frits or other fluxes. These slips can be defloculated but because of their high clay content and the amount of water to make them into a liquid they will shrink the most. So apply them to work that is still damp or leatherhard. If applied too thickly or to work that is too dry they can crack and fall off. Once fired they will usually produce a matt surface similar to any unglazed surface, the one exception is terra sigillata, which can produce a dull shine.
Best use is on leather hard clay that you want to cover with a different coloured surface, with a view to adding colour or glazing with a coloured glaze later.
Use: on damp to leatherhard
Clay content: 60-100%
Flux content: 0-40%
Fillers: 0%
Engobes
Consist of a mixture of materials, with clay forming about half the total. Because of their lower clay content engobes can be applied in a wide range of ways. The great advantage is their ability to be applied on work that is dry or even bisqued; it is also possible to apply them to leatherhard work. You will have to experiment to find the best method of application and thickness. Once fired they can produce a range of surfaces, usually matt to dull shine. Engobes form the base to underglazes, so adding colour to them is recommended, these will need a clear glaze over the top to develop their full colour.
Use: on leatherhard, dry and bisque
Clay content: 40-60%
Flux content: 10-50%
Fillers: 10-50%
Glazes
Consist of a mixture of materials, with clay playing only a minor role. Because of this glazes are best applied to bisque, although in some cases can be applied to dry work and then the piece can be fired just once. Glazes have the greatest variation in surface and colour; remember that the clay surface underneath the glaze will have a considerable impact on the colour and surface of the glaze.
Use: on dry and bisque
Clay content: 0-30%
Flux content: 30-80%
Fillers: 0-40%
Engobes
Below is a chart that refines the proportions of ingredients used at various temperatures to make an engobe.
Cone 08 - 1Cone 1 - 6Cone 6 - 11
China Clay303030
Ball Clay202020
Borax Frit20
Soda Frit or Feldspar20
Potash Feldspar20
Talc10105
Silica202020
Zirconium Silicate5
Borax555
If you have clays with a high shrinkage then you will need to modify the amounts of clay used. One option is the calcine (low bisque fire) half the clay content and then grind it up, this works best if the clay is already in powdered form.
If you are low firing and have no access to a borax frit then crushed clear glass will work as well.
The talc is added partly as a filler at low temperatures, partly as a flux at high temperatures and partly as it imparts a smooth creamy quality to the engobe in the wet form.
The zirconium silicate is an opacifier and help to whiten the slip.
To mix the borax in, first dissolve it in some hot water, then add to the engobe. It is used to make the engobe dry hard on the pot prior to firing.
Below is a list of colourants that can be added and the range of percentages used.
ColourColourantPercentage range
BlackBlack Iron Oxide8%
BlueCobalt Oxide2.5% - light < 7.5% - dark
Brown with specksManganese Dioxide3% - light < 8.5% - dark
BrownRed Iron Oxide8%
Buff with specksIlmenite6%
BuffYellow Ochre2%
CreamAntimony5%
Grey with brown hueCopper Oxide3%
GreyIron Chromate6%
Forest GreenChrome Oxide2.5% - light < 7.5% - dark
OliveCopper Oxide2.5%
PinkRed Iron Oxide2%
Rust-tanYellow Ochre5.5%
TanNickel Oxide6%
TanRutile3% - light < 9% - dark
Glaze Defects
Black Coring
This is caused by too much reduction in the early stages of the firing (usually from 600 to 1000 degrees Celsius). Black coring makes the work brittle and easily broken. Evidence of black coring is a blackening in the centre of clay walls. To fix; keep the kiln oxidising until 1000 degrees, then go into reduction.
Blistering
Blisters of glaze, or craters that are the remnants of burst bubbles.
Is often a fault of over-firing or of firing too fast. If the blisters are more uniform and small then it can be a result of firing a high alkaline glaze too low. Glazes move through a boiling phase prior to smoothing out to a glossy glaze, then if heating continues they can start to boil again, depending on the materials used and the kinds of gasses released.
Bloating
Bubbles appear inside the clay walls, making a dimpled appearance on the surface, or in extreme cases, large blister like bumps. This is a result of gasses being trapped inside the clay, which has been taken to the point of vitrification. Reduction during the bisque firing leaves some carbon in the clay, which causes bloating; always have an oxidising bisque firing. Some stoneware clays will bloat if fired too hot in an oxidising kiln, to fix this problem keep some reduction in the kiln throughout the last stages of firing.
Crawling
The shrinkage of the glaze during the early stages of melting pulls the glaze off the pot leaving bare unglazed areas. Most often caused by dust, dirt or oil on the bisqued pots. Other causes are surface texture, very viscous glazes, too thick glaze slop, or pots that are damp. Applying more glaze to the affected area and re-firing usually saves the pot. Some special effect glazes use certain ingredients deliberately to create a crawling glaze.
Crazing
Is a network of fine cracks throughout the glaze. Caused when a glaze contracts more than the clay body during cooling. A large mismatch in shrinkages will result in fine crazing. Alkaline glazes or very fluid glazes tend to craze, even if not immediately obvious, over time craze lines will appear. Adjust the glaze by adding silica or alumina, or replace the alkaline fluxes (soda and potash) with alkaline earths (calcium, magnesia) or with boron frits.
Dull surface
A glaze with a dry surface texture can be the result of underfiring the glaze, too thin an application, or a glaze with too much alumina present.
Dunting
Cracking in the fired pots that can often be attributed to uneven or too fast a cooling of the kiln. Large flat forms that have a lot of contact with the shelf should be cooled very slowly. A clay with too much silica in it will also cause dunting as the pot cools the last 600 degrees. A glaze that shrinks less than the clay body can also break the pot when cooling, particularly if the glaze layer is thick on a thin walled pot.
Exploding
Is almost always a result of the pot being heated too fast to allow the water present to turn into steam and evaporate slowly. The pressure of steam builds up and shatters the pot. It is important to heat the kiln slowly for the first 300 degrees.
Peeling
Is usually the result of a slip or engobe being applied too thick. Slips should be applied to damp ware, engobes can be applied to bisque, but only with a thin coating.
Pinholing
Is tiny holes in the surface of the glaze, sometimes penetrating down to the surface of the clay. This can also be the first sign of blistering, or crawling. Soaking of the kiln at top temperatures for up to an hour can help the glaze to heal over, or reducing the kiln temperature by up to 100 degrees and soaking at that point. Some viscous glazes or those with fluorine in them will pinhole more easily.
Shivering
The reverse of crazing, shivering is a result of a glaze that has a lower shrinkage than the clay body. It can dunt a pot or flake slivers of glaze off rims or edges. Increase the alkaline content of the glaze at the expense of the alumina or silica.