Developing glazes. Greg Daly

Transcription

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2 Developing glazes Greg Daly

3 Contents 1 Introduction to glazes Getting started First published in Great Britain 2013 by Bloomsbury Publishing Plc 50 Bedford Square London WC1B 3DP ISBN Published simultaneously in the US by The American Ceramic Society 600 N. Cleveland Ave., Suite 210 Westerville, Ohio, 43082, USA ISBN Copyright Greg Daly 2013 CIP Catalogue records for this book are available from the British Library. 3 Systems for glaze testing Fluxes and colour Developing colour Glaze recipes Ingredient conversions 126 Suppliers 127 Index 128 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.

4 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 28 29

5 Developing glazes Getting started 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 ( g) 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. 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. 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 30 31

6 Developing glazes Figure 2.4. Application of 10 glazes, with thick and thin application next to each other. 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 2.5. 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 32 33