Electric Kilns for Ceramics E-Book

Jo Davies

• Herausgeber: The Crowood Press
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 264

Veröffentlichungsjahr: 2023

Beschreibung

This title gives a clear, thorough and practical account of firing, but goes further and explains the techniques and ideas behind this magical stage of making. It highlights commonly-overlooked details that can lead to disastrous results and shares tips to help you achieve the best from your kiln. With over 100 photos, it also profiles leading makers and shows how their use of kilns contributes to their unique and beautiful work. Whether read from cover to cover by the novice or used as a reference book by the more experienced, this book will be your handbook to successful and confident firing.

Leseprobe

CONTENTS

Preface

1 What is a Kiln Firing?

2 Inside the Kiln

3 Running the Kiln Firing and Setting the Firing Programme

4 Kiln Ready?

5 Firing Faults

6 Kiln Maintenance

7 Before Setting Up a Kiln in Your Own Studio

8 Buying Kilns

9 Buying Second-Hand Kilns

Suppliers

Contributors

Index

Acknowledgements

PREFACE

This book has been written from my point of view as a ceramicist with a working knowledge of ceramic kiln-firings. The style and format is based on my practice, which includes the production of a porcelain design range including tableware, lighting and vessels, as well as teaching wheel throwing. Importantly for this book I also ran a kiln-firing service in East London for around twelve years until 2019. My ceramics degree at Bath School of Art and Design and my Master’s in Ceramics & Glass at the Royal College of Art have heavily informed the information in this book but the content is also based on countless conversations with fellow professionals over the years who I owe a great deal and are too many to name. The book includes some Maker Profiles from ceramicists whose work I consider interesting and who illustrate the use of kilns to create unique and beautiful work. Their profiles concentrate on ways that they use their kiln to create their work, including unusual approaches that break some of the normal rules, which are written about in this book. This is because, whilst this book aims to give a structure to work from, it is by no means a doctrine and I would encourage rule-breaking to push innovation forward, create new work and make new discoveries.

Throughout my time offering a kiln-firing service I often came into contact with my customers’ misunderstandings, particularly those who were only just starting out, around kilns and their function within ceramics. I became familiar with commonly overlooked details (which, when ignored, can have disastrous results) and even the problematic idea that ceramics is collectively one subject with a simple set of rules that are easily quantifiable. Many of these conversations have been the foundation on which I have based the presentation of information in this book.

Throughout I have attempted to put across the need to test results ahead of time, not because of a slavish bid for discipline, but in an attempt to encourage experimentation and the honing of ideas alongside a structure. If there is one lesson in ceramics, it is to always give yourself more time to do something than you think.

At the start of my own journey, when I was learning to work with clay, many of the ceramics books I was directed towards used scientific language and cadence, as if it wasn’t an artform I was working with but a science. To a person who is of a creative disposition, and interested in arriving at the making as fast as possible, this was frustrating. The presentation of ceramics as a science was not helpful for me at that stage. Having said this, I do know that a deep understanding through these means can be extremely elucidating for many, but at a later stage. At all times during the writing of this book I have attempted to take into account the creative mindset and tried to arrive at each main point as soon as possible.

In order to be as inclusive to all creative perspectives who wish to work with ceramics, I have also chosen to speak about ceramic ‘objects’ and ‘ceramicists’ for the majority of the time rather than ‘ware’, ‘pottery’ or ‘potter’.

This book aims, for the beginner, to take you through each stage, adding to your knowledge with each subsequent chapter in a way that will be helpful for practical application; it is designed to be read by this type of reader from start to finish for an understanding of electric-kiln firings. For more knowledgeable readers the text can also be used as a reference book to be dipped in and out of as needed.

The firing of kilns will have many factors at play, including external ones affecting the decision-making of the people running them, so some of these considerations are also sometimes discussed in the book. These issues can include anything from ecology, economy, professional deadlines and local politics.

Overall the book has been written to be as understandable as possible with little to no knowledge assumed although, as mentioned above, if coming to this book with little knowledge of kiln firings please take care to read it from the beginning in order to build your knowledge in a way that will make sense of later chapters.

Lastly, all advice in this book should be used in conjunction with the technical specification of the materials you are using, individual manufacturers’ advice for the equipment and kilns being used, as well as relevant professional advice from electricians and kiln specialists or ‘kiln doctors’.

www.jo-davies.com

Pouring Bowl by Jo Davies.

CHAPTER 1

WHAT IS A KILN FIRING?

Kiln firings is the area of ceramics that all ceramicists have in common. It is a leveller, an area of expertise, nuance, radical approaches and common ground. A ceramicist cannot exist without their kiln or some way of heating their work to a high enough temperature so that the clay they have shaped into existence turns into a hardened ceramic. However, there are some exceptions to this, and I feel it is important to mention artists like Phoebe Cummings, who makes her work with care and attention but then actively encourages it to disintegrate, either with the use of dripping water or by other means, to beautiful effect. However, clay is, for the vast majority of time, dependent on kiln firings to be made permanent and finished.

Coral Vases by Jo Davies. High-fired, stained porcelain with black manganese glaze and 24ct goldleaf.

For clay to become ceramic it must be heated, or fired, to above 700°C. Although each clay has a slight variation on this and firing this low would result in a very soft, brittle ceramic, this is the temperature at which it changes, at a molecular level, from clay into ceramic. All kiln firings work on the given that ‘everything has a melting point’ and we are working up to, and sometimes beyond, this point for our chosen materials. Having a knowledge of how our chosen materials will behave at different temperatures is key to developing the kiln firings we use. Thankfully much of the heavy lifting of this has been done for us across generations of ceramic cultural development so the designed, constructed clays we most often use now will give some insight into this information before we begin to work with them.

Kiln controller at Katharina Klug’s studio.

A kiln is any form of insulated chamber that will allow heat to rise inside it. This can be a permanent structure made from non-flammable materials like brick or stone, or it can be a thickly made structure made from flammable materials like wood. Its walls could also be formed from a simple earth pit or a structure made from engineered firebricks which are either encased for permanence by metal or built and then un-built after each firing for impermanence and flexibility of scale. The different approaches to kilns around the world are many. Cultural differences in kiln technology are visible in the objects that are made by each culture because their tools will reflect the values, tastes and preferences of each indigenous population. Ceramics is no different and, in some ways, through its relative longevity, the field of ceramics is a prime candidate for both a whimsical and utilitarian display of evolving culture.

A kiln firing forces all objects contained within it to change through a rise in the kiln chamber’s atmospheric temperature to a maximum point. There is then a gradual cooling of the chamber, along with the cooling of the items inside it, after its top temperature is achieved. A firing is often referred to by its top temperature, for example a 1,260°C or 1,080°C firing.

The top temperature is set depending on the requirements of the materials inside the kiln, what they can withstand and the fuel used to fire the kiln. Within this book I will be making reference only to the composite ceramic materials contained within clay, glaze, slips, enamels and lustres that can be fired within kilns fuelled by electrically heated elements. Historically, wood was the original fuel source for kiln firings (later on, gas and electric joined it as cleaner, more easily controlled fuels) but here we will concentrate on the electric kiln as it is now the most commonly used kiln-type for firing clay objects.

Katharina Klug firings.

The rise in temperature changes the molecular structure of the materials inside the kiln chamber to harden them and give them a greater permanence. The temperatures used take ceramic materials to the point, or just before the point, of melting. This is in order to force the fusing together of particles within the object. However, not all materials are compatible (issues most commonly occurring at the juncture between clay and glaze) so this is not always a straightforward process. Time must be taken to test the combination of materials and for trial and error. Sometimes these experiments will lead to the unexpected and beautiful, and sometimes to disaster …

Throughout this book I will also use examples of ceramicists who use well-known techniques in combination with innovative, or simply unusual, kiln practices in order to gain uniquely interesting results. I will do this to illustrate that the use of conventional practice as a springboard to development is necessary in order to push innovation forward and achieve our creative intentions. There should always be some room for the stretching of rules in any practice.

THE GENERAL STORY ARC OF A CERAMIC KILN FIRING

1. Bone-dry clay or ceramic with glaze or overglaze loaded into the kiln

2. Evaporation of any remaining water and extraction of gases from the clay body as the kiln heats

3. Clay fully dry and starting to change its molecular structure by 600°C

4. Molecular fusing of clay particles above 6–800°C and turning from clay into ceramic

5. As the heat rises the ceramic particles continue fusing together causing the shrinkage of its body

6. Top temperature of the kiln firing, as preset by the ceramicist loading the kiln, is reached

7. The ceramic may or may not sit at the top temperature for a preset duration (a ‘soak’ or ‘dwell’)

8. Kiln cools naturally

THE MEANING OF ‘EARTHENWARE’ AND ‘STONEWARE’

When the words ‘earthenware’ or ‘stoneware’ are used in ceramics it is, at a basic level, a reference to the upper kiln-temperature limits of either a clay, glaze or slip – most ceramic materials can be divided into these two categories and are often prefixed by either word, for example an ‘earthenware glaze’, a ‘stoneware clay’, and so on. They refer to one of two kiln temperature ranges:

• Earthenware clays/glazes withstand firing temperatures up to 1,200°C

• Stoneware clay/glazes withstand firing temperatures up to 1,300°C

These terms give an indication of the temperature range in which, for example, a glaze will melt and be at its best or where a slip will be at its most colourful. On the other side of this coin, it is also a caution telling us that, for example, if an earthenware clay is fired to the higher stoneware temperature (that is, above its upper limit of 1,200°C) then it will lose its structural integrity, slump or even melt.

‘Earthenware’ refers to any ceramic material’s ability to withstand kiln temperatures of up to 1,180/1,200°C and ‘stoneware’ refers to materials that can withstand kiln temperatures above 1,200°C, but usually only up to 1,300°C. Most modern clays, glazes, slips, raw materials – particularly in Western ceramic culture – have been designed to fall within these brackets. Above all, it is important to take into account these factors when considering the compatible combinations of those materials. This is not to say that only earthenware clay plus earthenware glaze, or stoneware clay plus stoneware glaze, can go together but taking their compatibility into account can make things easier.

If firing earthenware above 1,200°C there is a risk of the clay slumping and melting; the same is true of stoneware clays above 1,300°C. Of course, the words earthenware and stoneware carry with them all sorts of other meanings around the look and feel of a ceramic object, such as its colouring and its durability; stoneware clay is considered more durable whereas earthenware clay is often mainly used for domestic ware and would not normally be left outside to endure the weather like its stoneware counterparts. The earthenware temperature range also has a reputation for being more colourful than materials in the stoneware temperature range, which traditionally has been known for its more muted colours, partly because the high kiln temperature can burn out the colour contained in glazes and slips firing to stoneware. That said, ceramic technology has moved on in recent years with many excellent, industrially produced stoneware stains that can be added to base glazes/slips to produce vibrant colour even at high stoneware temperatures, so the reputation for muted colour is falling away. However, it should also be considered that stoneware temperatures bring more risk of firing faults, are harder on the life of a kiln and are less economical and ecological. If we can find clays and glazes that achieve similar results with equal durability – or at least durability for our intended purposes – should we actually be considering the earthenware range as a viable option more frequently than we currently do?

When we first begin to work with clay we most often start by just thinking about the clay, and thought for the surface is delayed or even avoided. Finding the surface finish for the pot/sculpture/jewellery/installation we have spent so much time on is daunting but you can mitigate this by choosing to start with the surface of your object. By this I mean before you have spent the time perfecting the clay part of your work, choose your glaze/slip/gilded finish/and so on. Whether this is a bought-in glaze or studio-developed from raw materials, it can be hugely beneficial to work this way because the technical path to finishing your work will be clearer; this is because once you know this then the type of firings to use, the glaze firing temperature and which clay is best for the intended object will all be much clearer. For instance, the clarity of colour will be partially dependent on the clay that lies beneath it – an iron-rich stoneware clay may not be the best option if a glassy, pale green celadon is the goal as the two will interact, with colour from each material leaching from one into the other and creating a murky finish.

Once your combination of materials is chosen you can determine your kiln schedule. In the West we tend to bisq/bisque/biscuit our greenware (unfired clay) before glazing them and returning them to the kiln for a second firing. Other cultures often ‘single-’ or ‘once-’ fire items with glaze already on the greenware for a single firing.

The reason for bisq firing is in order to ensure we have a stable, durable ceramic surface before applying a glaze. It also limits damage to the kiln as explosions and collapses are more likely when firing greenware – if these occurred with glaze present then the damage would be much more extensive because molten glaze on kiln shelves, elements and thermocouples becomes a much more costly mistake than the comparatively ‘dry’ dust and rubble of a standard bisq-kiln blow-out which contains no glazed items. Single-firing also requires both glaze and clay to have the same temperature range so separating the two allows a wider scope of possibilities. Also, historically speaking, the introduction of bisq firing reflects the early days of a surge in development in the European ceramic industry inspired by the arrival of Chinese porcelain in the 1600s (see more on this below). For quite some time the failure rate of kiln firings was so high that, for the reasons outlined above, bisq firing was introduced.

BISQ FIRING

A bisq firing (also known as bisque or biscuit) is the firing used to turn clay into hardened ceramic but without any glaze. This is the first firing, after which glaze is applied to the ceramic and returned to the kiln for its second glaze firing. In the UK bisq firing, as a concept, came with industrial developments in Stoke-on-Trent but other parts of the world have a history of dividing bisq and glaze firings well ahead of this. The development of the clay we now know as bone china was inspired by the import of Chinese porcelain into Europe, starting in the late sixteenth century. Prior to this, British ceramic culture had been a smaller industry with the use of earthenware clays being prevalent and the colour being mainly in the iron-oxide range – browns, yellows, creams – and was very much a cottage industry. The popularity of the jade-like Chinese celadon, bright white and cobalt blue objects being produced during China’s Ming Dynasty spurred a search across Europe, including Britain, to create similar ware. The Staffordshire potteries developed their own methods to create what became known as English porcelain but the recipes and methods were very different to the ones used for Chinese porcelain – which is a predominantly naturally occurring clay, fired in wood-fired kilns at that time and, more commonly, in gas-fired kilns now. The Chinese firing method was all or nothing, with clay and glaze being fired once; the extra bisq firing of the British potteries would have struck the Chinese as wasteful in the extreme and many Chinese factories and potters still once-fire porcelain as a matter of course to this day. However, in British factories at the time a separate bisq firing became a necessity because of inherent unreliability in the new materials being used; bisq firing’s place in ceramic culture remains the dominant way of working today.

In short, a bisq is the first firing of your clay, without any glazed surfaces, which will give you a reliable and porous surface on which you can apply glaze or slips if you choose to. The bisq also gives us an opportunity to see which items are viable before investing any more time and materials into them. If an item cracks or warps we can choose to discard it after the bisq firing.

Low- or High-Bisq Firing?

We usually choose between two types of bisq firing – low bisq to 1,000°C or high bisq to 1,140°C. I have noticed anecdotally that high bisq is starting to be used less and less but it remains useful if we understand the reasons behind bisq firing.

Bisq firing temperature is determined by several factors. Working backwards in the process, the first factor is the temperature range of the glaze we intend to use, the second is the temperature tolerances of your clay and the third is the creative intentions of the maker. Within these parameters there are several broad brushstroke issues to consider. These broad issues can then be paired with the individual, detailed refinements of the process that each individual maker develops according to their creative intentions.

The key superficial difference between high and low bisq is the top temperature – 1,000°C for a low bisq and 1,140°C for a high bisq. Clay hardens, and becomes less porous and more dense, the higher the temperature it reaches in the kiln. A clay that has fired to 1,000°C will be more porous than the same clay fired to 1,140°C. This is key in terms of glaze adhesion onto the ceramic surface at both the application stage (whether you are dipping and pouring the glaze, spraying or brushing) as well as within the kiln firing itself, when it will bind to the surface. If we think of the pores of the ceramic surface as a ‘key’ for glaze then this can help to illustrate how glaze and ceramic bind together. If a ceramic has larger pores for glaze adhesion then it can be easier to apply the glaze and can result in a thicker layer of glaze. Conversely, a ceramic with small pores can mean a layer of glaze that is too thin.

In terms of the clay body, best practice says that we should fire our clay high enough in temperature in order to mature it to a point where it is hardened and durable. This can happen during either its bisq or its glaze firing. The clay body will harden and become less porous the higher it goes in temperature so, for instance, firing a stoneware clay in a low bisq (1,000°C) and then returning it to the kiln with a low-firing glaze to around 1,020°C does not take stoneware ceramic into as durable a state as it could be. This is because a stoneware clay, by its nature, withstands temperatures between 1,200°C and 1,300°C so the closer we can get the stoneware clay body to this the better. In this example, a better solution would be to either fire the ceramic in a high bisq of 1,140°C before the glaze firing at 1,020°C or, better still, to change the glaze to a stoneware glaze allowing us to low bisq at 1,000°C and then return the item for its glaze firing that will be above 1200°C. When using a low-firing earthenware glaze with a stoneware clay, the high bisq is a compromise that gives the clay greater durability but still leaves it porous enough to accept glaze. However, I caveat this by saying this is a general rule and combinations of clay and glaze should always be tested for compatibility.

If wishing to use a glaze with a top temperature of 1,150°C or above on a stoneware body then a low bisq will be fine as the clay body will mature during its glaze firing.

In the high-bisq example above, allowing the clay to mature at a high-bisq temperature meant that it hardened but maintained enough porosity for the glaze to form an even and reasonable thickness across its surface. All clays have slightly different tolerances so checking or testing the firing ranges, including vitrification points, of the clay we are using will be an important piece of evidence in working out the best combination of firings. Testing different combinations is very important to the development process so we should leave time for this as the combination of clay, glaze, kiln and ceramicist is always utterly unique. The ceramic items we test should not be ones that we are afraid to lose so making sure there are plenty of reject pots, maquettes or test tiles available for this is vital to a thriving practice. However, one of the biggest factors in deciding your bisq temperature is in the glaze adhesion and therefore colour and texture response of the glaze as a direct result of the surface porosity of the ceramic.

The final example above is for the most part not suitable because most stoneware glazes will only be best at a temperature that is higher than the earthenware clay’s maximum temperature. If this combination is attempted, the clay will slump or even melt when fired to most stoneware glazes’ ideal temperature. However, the exception to this rule is when you have a very low-firing stoneware glaze that vitrifies around 1,200°C – a top temperature which most earthenware clays will tolerate but some may be so close to vitrification themselves at this point that they may lose their shape; for this reason, test the exact combination of materials ahead of committing to this on anything precious. This combination may be a good candidate for single-firing (see description of single-firing later in this chapter).

Having tested the above combinations of bisq and glaze firings we may feel that we want to tweak temperatures to, for instance, increase or decrease glaze adhesion when we are dipping/spraying/brushing it onto the ceramic, or we might want to increase fluxing of the glaze in the firing, or even mitigate against a firing fault. In the consideration of adjustments like these we need to examine our creative intentions as these will dictate the potential trade-offs we may need to make between technical issues and creativity. All these factors will be discussed further throughout the book.

In my own practice I have found that the Audrey Blackman porcelain I use won’t accept glaze easily once it has been fired to high bisq so I will always fire to 1,000°C low bisq before glazing and returning the ware to the kiln for its glaze firing to 1,240–1,260°C. As a fine-particle, smooth clay, the very dense closing of its pores is inevitable but most stoneware clays contain a variety of sized particles giving them more tolerance and porosity and less overall density. A grogged stoneware will remain much more porous at high bisq than a smooth claylike porcelain or bone china.

As mentioned, firing your clay too high before applying the glaze can create problems in glaze application but may also result in various issues with the glaze in its firing. It is possible that the glaze could ‘slide off’ the ceramic during its firing because of it not being keyed into the surface enough. The effect of a less porous, dense ceramic body can result in a glaze not adhering to its surface easily and the glaze appearing thin and/or lacking in colour. In short, we are looking for the correct porosity of ceramic plus the correct thickness of glaze according to our intentions.

This is because a more porous ceramic sucks the glaze onto its surface more readily, which can result in a thicker application giving the glaze a more ‘full’ look. However, glaze that is thickly applied may also make the glaze begin to flow, resulting in a dripping effect as the weight/volume of glaze on the surface makes it flow (or ‘flux’) with the rise in temperature of the kiln. Overly thickly applied glaze can also result in glaze crawling so conventional approaches would see this as a fault in the surface (see more on glaze crawling in Chapter 5).

Of course, if our intentions are to harness either of these effects within our ceramic objects then the concept of it being a fault is very much in question. In fact, many ceramicists will use what might be considered traditional firing faults to great effect. Sculptural work tends to have more leeway in this because a matured ceramic body coupled with a glaze that has its structural integrity is of greater practical importance in functional items, so intention is very much everything. However, the historical ideas around what constitutes a firing fault is rooted in the development and production of functional ware because of the practical requirements of this type of ceramic object, but as soon as an object has no function then our conception of a ‘fault’ should be re-evaluated. When viewing ceramic sculpture, my own view is that disregarding objects simply on the basis that they have X or Y ‘fault’ feels very wrong and we should all caution against this. In part because it forces us to bypass the conceptual or emotional relevance of the work, which is where its value lies.

This Firemaster kiln controller shows a diagrammatic version of the rise, plateau and cooling of temperature during a kiln firing.

These two sample firing schedules, which are relatively cautious for hand-built or wheel-thrown items, allow for heatwork in the kiln to take place. ‘Heatwork’ is a term used to describe the effect of heat on ceramic materials plus time. In the bisq firing one of the results of heatwork is for the clay body to gain an evenly porous ceramic throughout and not just superficially. If your bisq firing is too fast you risk the heatwork being very superficial, that is, the outer layer of your ceramic will have a different porosity than the core because the heat has not ‘worked’ on it and the core has reached a lower temperature than the outer skin. This is of greater importance for hand-built or substantially thicker pieces, and very thin ware can have a speedier bisq firing.

Taking this into account, it is common practice in the utility-ware (bathroom/kitchen items) factories of Stoke-on-Trent to use a relatively long dwell/soak of an hour or more at the top temperature of the bisq firing to make sure the heatwork is completed on the thickly made items they are producing, as these items need to be robust because of their function. This is just one example of a strong reason for a comparatively long bisq firing with a view to fully stabilizing the ceramic body.

In ceramics we often talk about items ‘twisting’, which means a piece is effectively in a state of tension, slowly moving, because it has differing density levels running through it. This is of particular importance when it comes to the glaze on a piece’s surface as this molecular ‘twisting’ can be reflected in the glaze through very visible crazing (see more on crazing in Chapter 5). This is another reason for a stabilizing bisq firing with opportunity for enough heatwork on the ceramic. The twisting can be so slow sometimes that crazing can appear years after the item has been removed from the kiln. If you are having trouble with unwanted crazing this can sometimes be resolved by increasing the soak time at bisq in order to create a more even body that is not twisting against the glaze and causing uneven surface cracking. I hasten to add that this is just one possible reason of several for a glaze to craze.

As mentioned, it is possible to speed up the bisq firing a little for very thin ware. However, the first few hours of your bisq firing play a big role in evaporating out the very last molecules of water from the clay. This should therefore still be done relatively slowly with the kiln vents left open for evaporation, and the release of gases, in the first part of the firing and then closed at around 600°C.

The explosive blow-outs inside kilns that ceramics is famous for are most often created by excess water inside the clay body evaporating very fast and the pressure of the steam escaping from the clay breaking it apart. This is why it is important for the clay to be completely dry before being fired. Significant air bubbles can also cause explosions because, as the clay shrinks with the heat of the kiln, the size of the cavities themselves become too small for the air contained within them and so the ceramic is blown apart in a similar way. However, the most spectacular blow-outs I have seen over the years have all been from excess water, which is, unfortunately, a mistake that even the most professional potters can continue to make long into their careers through impatience and looming deadlines. The danger of air bubbles in clay is often overstated and can be easily overcome by using clay straight from the bag or well-wedged clay coupled with a slow bisq firing similar to the above examples. However, you must handle your work in order to test if it is dry enough to be put into its first firing. Understanding what this feels like is a very important skill to acquire and only comes with practice.

If your clay is drying in a space with ordinary room temperature but feels cold to the touch then it is still too wet to go into the kiln. If your work is taking a long time to dry then consider the temperature and ventilation of the room in which it is being left to dry: if there is no airflow and the humidity is high then your work may take weeks to dry. Nothing beats a dry, sunny day outdoors to dry out clay for its firing. However, some clay or certain shapes will not tolerate the speed of this type of drying so slow-drying can be one way to prevent cracking, particularly for items made from component parts.