Developing glazes - Ceramic Arts Daily

Developing glazes
Greg Daly

First published in Great Britain 2013
by Bloomsbury Publishing Plc
50 Bedford Square
London WC1B 3DP
www.acblack.com
ISBN 978-1-4081-3495-5
Published simultaneously in the US by
The American Ceramic Society
600 N. Cleveland Ave., Suite 210
Westerville, Ohio, 43082, USA
http://ceramicartsdaily.org
ISBN 978-1-57498-331-9
Copyright © Greg Daly 2013
CIP Catalogue records for this book are available from the British
Library.
All rights reserved. No part of this publication may be reproduced
in any form or by any means – graphic, electronic or mechanical,
including photocopying, recording, taping or information storage
and retrieval systems – without the prior permission in writing of
the publishers.
Greg Daly has asserted his right under the Copyright, Design and
Patents Act 1988 to be identified as the author of this work.
Typeset in 10 on 13pt Rotis Semi Sans
Book design by Susan McIntyre
Cover design by Sutchinda Thompson
Edited by Kate Sherington
Printed and bound in China.
This book is produced using paper that is made from wood grown
in managed, sustainable forests. It is natural, renewable and
recyclable. The logging and manufacturing processes conform to the
environmental regulations of the country of origin.
title page Greg Daly, etched
lustred bowl, see p. 11.
contents page Two glazes
applied in four different
ways, see p. 27.
Contents
1 Introduction to glazes ....................................................................................... 5
2 Getting started ................................................................................................. 28
3 Systems for glaze testing ...............................................................................35
4 Fluxes and colour ............................................................................................ 75
5 Developing colour ........................................................................................... 78
6 Glaze recipes ..................................................................................................... 91
Ingredient conversions 126
Suppliers 127
Index 128

2
Getting started
Before we begin testing, you need to consider how you will record your glaze testing
and mixing. We need to record these tests in order to be able to make sense of them
later, and to be able to identify the glaze test and recipe that goes with a particular
test tile. It is important to keep records of recipes, firings and outcomes, and to have a
procedure for mixing and applying the tests. It is easy to develop a mental block with
glaze testing, due to the time you assume it will take. Record-keeping, weighing the
glaze ingredients, mixing and applying the glaze to test tiles or rings, as well as firing
the tests, seems like a lot of work, but if you are prepared and organised, the process
doesn’t take long at all, and the effort is small compared to the rewards.
Recording
You should record all your glaze tests in one large book or using a centralised electronic
system like a spreadsheet, rather than multiple scraps of paper, which are easily lost.
Use a simple numbering system for ease of identification and reference – there is
nothing wrong with starting at 1, 2, 3, 4, 5… Alternatively, you may choose a prefix for
a particular series of glaze tests, e.g. for copper reds, as in CR1, CR2, CR3, etc. It doesn’t
matter if you use shorthand markings on the glaze tests, as long as you know and can
identify the mark the following week or in five years’ time.
Another, quite different approach to numbering is to code the various elements of
the test, a particularly useful tactic when you are using a number of glaze bases. Give
glazes a letter from the alphabet. For example, the E base glaze referred to in this book
is nepheline syenite 55, whiting 16, talc 13, kaolin 16. The right-hand colour blend in
Figure 5.5 (p. 83) shows the glaze test tile E-1/27 OX-S. In this case, the base glaze
is E and the colour blend number is 1. The tile number from the colour blend is 27,
i.e. the 27th tile when counting left to right across the rows. OX stands for oxidised
firing, S for stoneware. No other test would have the same marking. The code identifies
the base glaze, the colour blend the test is from, the tile number, whether the test is
reduced or oxidised, and the firing range. This is the system I use to identify a number
of glaze tests. It is handy to do when you use the same base a lot to explore colour,
especially using colour blends.
Have a system and stick to it. It is important to make comments in your testing
book next to the glaze recipe (or, these days, on a spreadsheet, with a photo). Note the
clay body, the materials used (feldspar, kaolin, etc.), and the firing – whether it was
overfired, reduced earlier or later than normal (e.g. a delay in starting the reduction
of the kiln can influence the result). Firing an electric kiln may take longer than usual,
Figure 2.1. My glaze
testing area.
and this could also influence the result. Minor changes may seem nothing at the time
but could be the missing piece of information when a glaze comes out one way in a
test and another on a pot.
Weighing and mixing
Measuring spoons, cups, hats, boots, pinches and handfuls are all volumetric measures.
Glazes can be made using the volumetric method, but more commonly a measurement
of weight is used to find the correct proportions of glaze ingredients. Accurate scales
are required. A set of triple-beam balance scales or digital scales using grams or ounces
as a unit of measurement is ideal. Don't make your weight measurements too small.
For example, when trying to weigh out a test that you have reduced to a total amount
of 20g, inaccuracies may occur when weighing very small amounts – say, less than
1g. On the whole, materials are relatively inexpensive, so it is worth doing the test
properly and weighing out no less than 50g, or better still, 100g, a quantity that will
enable you to weigh the glaze correctly and apply it with confidence. Keep the mix
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Developing glazes
until after the firing; if a glaze looks promising, you will be able to try it in the next
firing on a larger surface – and it is already made up!
Scale is another factor to consider. How often does a colour on a wall-paint chart
look right, but application to a larger area changes its character? After a successful
test, one is apt to mix up a large amount and glaze most of the work for the next
firing. Mix 1kg of glaze and apply it to a few pots. Place them in different parts of the
kiln and see what comes out. You may find that the glaze responds differently in some
areas; it may take time to get to know a glaze completely.
Safety
It is good practice to keep your studio clean and wear a good dust mask. Keep this
in a sealed container between uses and wash it out with warm soapy water after
use, taking off the filters first. Replace dust masks regularly and never bring food
and drinks into the studio. Some of the materials are toxic and carcinogenic, while
dust can lead to silicosis. Your supplier can provide you with Material Safety Data
Sheets (MSDS).
Don’t underestimate the dangers of any of the materials. Keep the glaze-testing
area clean. Because materials can accumulate in the body, some potters I know have a
blood test for heavy metals every few years. Be sensible.
I would suggest that you keep a small amount of each of your materials in an
easily accessible container (100–500g) rather than scooping small amounts from
large bins. This allows for easy access and quick, less messy weighing up. Carrying
material from bulk storage can result in more being dropped than is needed for
the test.
Mixing
There are many approaches to mixing and preparing glaze tests. One of the most
common is to use a pestle and mortar. A small amount of water is poured into the
mortar, then ingredients are added and mixed with the pestle, with more water
added if required, before the glaze is sieved. This slow process is what can put potters
off testing. I originally used this method, but now my procedure is quite different,
with no noticeable change to the finished results – I’ve described the process below.
I use materials of a fine mesh size, silica at 350 mesh, for example; your supplier
can tell you what size the materials are ground to. If I use an un-ground material
such as wood, ash or rock dust, I sieve it first. Materials such as natural bone ash
and some barium carbonates are lumpy and hard to break down, and both sieving
and the use of pestle and mortar may be required. (This problem is caused by the
materials having been water-ground and heat-dried by the manufacturer). Synthetic
bone ash (tri-calcium phosphate) will give the same or a very similar result without
the lumps.
30
top left and right
Figure 2.2. Adding material
to water (left) and mixing
the glaze (right).
above left and right
Figure 2.3. Dipping a test
ring (left), and brushing
three glaze thicknesses
onto a tile (right). Note that
the tile has a texture for the
glaze to break on.
Getting started
As the glaze is weighed, the materials are put into a container with some water.
Adding the dry material to water means there is less dust and results in a glaze that
will mix easily with no lumps. If it is left for a few minutes while other tests are
weighed, you will find that the glaze will mix without trouble. For mixing the glaze, I
use a nylon paintbrush, 2–3 cm (¾–1¼ in.) thick, which breaks down and mixes the
ingredients quickly. An electric hand mixer is also very effective. Water is added to
achieve the desired consistency. There is no set ratio of water to material. A glaze with
a high clay content may requires nearly twice as much water as a glaze without clay,
while a glaze with magnesium carbonate light (a very fine material) will require more
water than the same glaze with magnesium carbonate heavy.
For a glaze test, you should add more water. To obtain the most from your glaze
test, varying thicknesses of glaze need to be applied and thus a thinner mix is
required. The first application is the thin application (if it was being applied over an
iron body you would just be able to see the colour of the body). The second coating
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Developing glazes
gives a normal thickness of application, 1–2 mm, while the third application is thick,
2–3 mm. You won’t know at that stage what the best thickness will be, but you do
know that thickness will play an important role in the surface effect and colour
response. By applying only one thickness of glaze you are limiting the amount of
information to be gained, and as time has been invested in the recipe decision, as
well as weighing and mixing the glaze, so the time invested in application (not to
mention the nature of the test tile) is all-important. How to apply the glaze test is
again a choice – it may be by dipping, pouring or brushing. All the test tiles in this
book were brushed. Brushing has advantages, being quick and clean, but the brush
can also give an uneven application, which affects the possible outcomes. If a ring
test is used, then dipping is best.
Using white-firing clay test tiles or rings gives a brighter colour response than using
a coloured clay. If you normally use an iron-bearing clay, do use it as the base for your
tests, but try the tests on other clays as a comparison. As we have seen in Chapter 1,
even different white-firing clays can change the glaze response (p. 17).
Figure 2.4. Application of
10 glazes, with thick and
thin application next to
each other.
Figure 2.5. A variety of
glaze test forms – rings, tiles,
extruded tiles and bowl.
Potters like using different objects to test their glazes on. Keep this in mind when
deciding what kind of test shape to use: it should be large enough to see colour
development, the effects of different thicknesses of glaze (with space for an oxide
stroke to gauge future colour development), and various brush decoration qualities;
and to observe texture from a stamp, raised trailed slip, fabric pattern, finger lines left
from making rings, and so on. It is important to see how and if a glaze breaks or pools,
and how it reacts on a thin edge such as the rim of a bowl or cup. This information is
acquired from the object you decide to test the glaze on. A shallow bowl or large tile is
ideal when testing variations of one glaze, as all the tests – e.g. the additions of silica
in a glaze to overcome crazing – can be contained on one object.
Figure 2.5, above, displays several testing units. Tiles are made very easily with an
extruder; in one morning, a few hundred can be made. Note the free-standing tiles
on the left: these are extruded and cut, the design of the die allowing the base to
be broken off easily for storage, while also allowing the glaze test to be fired on a
75-degree angle for observing possible glaze movement. The test tiles used in this
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