Wednesday, 21 January 2015

Inspirational pottery glazes

The special ceramic materials and process

Reactive coloured glazes
The first article of my series on glaze making identified the basic ingredients used in glazes as silica, felspar, frit, and clay and explained how a little science would help speed up your artistic flair!.This second of the series on glaze making discusses the non core ingredients of a glaze and explains how a little science in the form of good processing can ensure that you achieve the best result from your glaze making efforts.

The 3 Basic Questions

To make truly inspirational glazes we need to establish at least 3 things
1) What type of glaze we want to make?
2) What type of body it will be applied to? Stoneware?, Earthenware? Biscuit? Clay?
3) What firing temperature/cycle do we intend to use?
Once we have the answers to these questions we can begin to formulate the glaze.

The choice of non core glaze materials

Making a silky matt -what to add to your basic glaze

Silky Matt glaze

Let us assume we want to make a white silky matt textured glaze firing at 1150C on stoneware biscuit.
We have already identified silica, felspar, clay and frit as core glaze materials. This means that at least 2 of these materials are used in almost all glazes.
Using the same cake making analogy as before, these ingredients are considered the eggs, butter, flour and sugar equivalents of the glaze recipe!
The materials we might consider adding to these core materials could be:-
Limestone (calcium carbonate)
Needle-like crystals on glaze surface
Dolomite (calcium magnesium carbonate)
Zinc oxide
Alumina (aluminium oxide)
wollastonite (calcium silicate)
clay (alumino silicate)

These materials help to form crystals on the surface of the glaze on firing and thereby help create the matt (dull) texture. However these materials also influence how the glaze melts and bonds to the body on firing. It is an absolute necessity to match the glaze to the clay body or on cooling the glaze may just flake way like old paint! (Technically the glaze and body thermal expansion need to match so that the glaze is in compression after firing.) But lets not go too deeply into how we do that at this stage.

Alumina does not readily melt at this temperature so the likely (and easier) materials to use are zinc oxide, limestone, dolomite and wollastonite. Adding these materials in the right proportions to make a suitable glaze takes a lot of trial and error.

Developing the Glaze Recipe

If you start a with a basic 1250 C recipe as follows:

Base Glaze

Silica 38
Felspar 40
Clay 10
Limestone 12

and start replacing the silica and felspar with more fluxing ingredients that create crystals and allow you to fire at a lower temperature you might eventually reach a formula

Modified Glaze

Limestone 12
Dolomite 25
Felspar 20
Clay 25
Zinc oxide 4
Boron frit 9
Silica 5

Imagine how many test glazes you might need to make before arriving at such a detailed recipe? Note how different this is from the starting base transparent glaze! The material recipe is not the only part to consider when making glaze. The particle size of the glaze materials needs to be reduced to a fine powder by grinding with ceramic pebbles in water to less than 75 microns. Often, for best results, the mean particle size needs to be closer to 15 microns-the diameter of the finest human hair. This allows the glaze particles to react and melt during the firing process.

Firing the Glaze

The firing process is equally important in obtaining satisfactory and repeatable glaze results. Initially the firing cycle should remain constant as you develop your glaze recipe. A typical glaze cycle might be 150 C per hour ramp from room temperature to the peak at 1150 C, followed by a holding period (soak) at peak temperature of 1 hour, followed by kiln switch off and natural cool to room temperature.
Note that the cooling is often as important as the heating process when firing matt or crystal type glazes.
Clearly making glazes is quite a complex process. However, like the best potters, in time you will come to believe that this is what makes pottery glazing so interesting. You can never be sure what will come out of the kiln each day!
In the next of the series the use of ceramic colouring pigments to create even more interesting coloured pieces will be examined..

Pottery books of value

A great way of backing up your knowledge is to read some simple pottery making books. Some of the best books are those you keep going back to as your knowledge grows!

Thought for the Day

In pottery making it rarely goes exactly as you would like first time. Stay positive! Use it as a learning experience.
Henry Ford Quote:
"Failure is only the opportunity to begin again, only this time more wisely."
~ Henry Ford

Happy Potting
The potters Friend

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

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The Potters Friend