A Potter's Geology

By Eric Rowe

To help a potter find and prepare his own clay and glaze materials.

• Language: English
• Publisher: Lulu.com
• Release date: Apr 12, 2011
• ISBN: 9781447644729

Book preview

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A Potter's Geology

Clays and glazes from natural materials

Eric Rowe

Copyright Eric Rowe 1999 All Rights Reserved

9781447644729

Table of Contents

Title Page

Copyright Page

A Potter’s Geology - INTRODUCTION

Part I

Starting to poke around.

Testing

Miniature clay taming procedure

Melting rocks into glazes

Next part of the recipe--Wood-ash

Clay preparation--Old Style

Part II

GRANITE ANALYSES

GEOLOGY

Part III

The disintegration of feldspars and the formation of clay

Some notes on the atomic structure of Clay

A few remedies

Fire-clays or residual clays

Chalk in Clay

More Notes on Clay Preparation

Clay, Silica, Fluxes

Preparation of Kaolin from the raw material

Rock Recognition

General Recognition Hints

Recognition of: - Quartz

Feldspars...

Calcium

Gypsum

Clays

Gneiss

Quartzite

Schists

Basic rocks

Generally

Crystal formation and Rosenbach’s Law

Experimental Archaeology

About...

Part IV

Appendix I - Places to find materials

Appendix II - --More Glazes

Appendix III - -Atoms

Appendix IV

Bibliography

GLOSSARY

A Potter’s Geology

INTRODUCTION

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● Gouache: Peggy Spence-Sales

As the title is intended to indicate, this book is about how a potter can use a little geological knowledge to help him find his materials around him and also perhaps deepen what he may already know about his clay and glazes. It might also point him towards an understanding of, and possibly cures to, problems he may have. This is based on my personal experience--all those heartbreaks and joys I’ve had over the years. Perhaps we all learn more from the failures than the triumphs--I intend to tell you some of mine in the hope that you can learn It is a busy life, being a potter, without someone like me coming along to say, Look! Don’t go on buying all that stuff, you can go out and find it for yourself!

You would be quite right if you answered that you have better things to do. But (a big but) even if you only try the experience once or twice you will certainly find it rewarding. There are even some (like myself) who once they have felt a strong, well-prepared, ‘wild’ clay under their hands, will never again want that floppy dead mixture that some pottery suppliers try and fob us off with.

On the other hand ceramic suppliers are naturally geared towards supplying what they are asked for and the best of them will go out of their way to help. For those without much access to the countryside, I hope this book will help you to have a better understanding of your materials and that glazes and clays are not the complicated things in labelled packets that you buy and apply as instructed. One of the things I have tried to do is to show how basically simple it all is. At the end I’ve put some pages of more complicated stuff as some understanding of what exactly is happening in a kiln at high temperature could and should help when things go wrong.

Part I is about going out and looking for your materials and how to treat and test the samples you manage to get. Part II is about what these earthy looking materials are made up of and just how simple a high temperature glaze can be when made from them.

Part III is the more complicated bit dealing with firing problems, the why and the possible solutions, plus some help on rock recognition. For those who get interested seriously, a simple book on Geology, another on Petrology (rock study) and a geological map of your area would be very helpful and save many hours searching around when someone has already done the work for you. It will also help to bridge that unnecessarily yawning gap between geological terms and familiar potters’ materials.

I also take it for granted that you have a good working knowledge of the making and firing of clay objects. If not, there are plenty of good books around to help you. Although I concentrate on stoneware, much will also apply to earthenware.

There is an extensive glossary that includes information not dealt with in the main book.

I would like to thank:

Russ Collins for his patience with me while he was taking the digital photos. Tony Hansen for making the whole CD project possible and for transforming my piteous diagrams.

My wife for her support, meticulous proofing and helpful suggestions.

Part I

Starting to poke around.

Out of his struggles with the raw materials of Nature a potter builds a fabric of technique and technical knowledge which not only provides a solid foundation for his aesthetic expressions but actively nourishes it. Says Michael Cardew in his book ‘Pioneer Pottery’. From my many years as a potter I fully endorse that.

Well, if you’ve got a geological map you can start ‘poking around’ on that. You might learn a lot, but be a bit puzzled by some of the terminology. They do use the word ‘clay’, however, and that’s a start. Next look to see if there are any clay pits marked, working or abandoned, sandpits or anywhere you think someone has dug a hole for you. I’ve had some of my best luck with sandpits where they hate that sticky clay stuff that makes work so hard for them. If you do find a clay seam in a sandpit, don’t hire a lorry just yet. Just carve out a few pounds and label it, showing the exact spot where you got it. I’ll come back to that later. Do the same with anything else you find. If you are a stoneware potter, refractory clays are much rarer than earthenware ones. Look for light coloured clays; they may have less iron in them and be therefore purer. This is not always the case, of course, as iron can give clay a greenish tinge rather than red. In all cases it is the kiln which will give the final judgement.

Other ‘holes’ to look for:

Brickyards

Coal pits or their dumps

Any pit extracting anything

Road cuttings

River banks

(see also Appendix I)

One great advantage of finding a working pit is that machinery is just waiting there to dig it out for you. Doing it yourself is hard work for any quantity as wet clay is stubborn stuff at the end of a spade! After a while I found my eye getting trained and I started to see clay deposits in places I had driven past before. Smaller deposits often do not get mentioned on the geological maps, but an understanding of the area can lead to finding a more workable site when farmers are rather unwilling to have holes dug in their fields.

Testing

Be sure you get this right give a code to each sample and note it down somewhere. I once took a second sample from a place less than fifty yards from the original, refractory, clay. Being so sure of myself I gave it no protection in the kiln, it melted, gluing down all the nearby pots!

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Pummel your sample of clay about until to some degree workable (it’ll still be pretty lumpy).

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Roll out a coil two fingers thick, cut into 2 or 3 15cm lengths and flatten them. It doesn’t have to be a work of art, but at least flat enough to be scratched on accurately.

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● flatten...

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Make a clean scratch exactly ten centimetres long on each with an upright at each end, as below:

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● AB for Albert’s Brickworks

Date and put it aside to dry along with any other coded samples you have found. Re-measure when dry as this will give you the first important information. Shrinkage plastic/dry is one percent for every millimetre lost. A low plastic to dry figure (less that 8%) is a good sign,