Ceramic Technology 2 - Glazes and Glazing

What's in a Glaze?

A glaze is simply a thin coating of glass on the top of a ceramic clay body. The term ‘glaze’ is also used in painting to describe a thin coating of transparent lacquer applied over underlying paint to improve it’s lustre or for it’s protection. In ceramics, a glaze is an integral part of the ceramic work. Heat is required to melt the glaze and during this process begins fuse with the surface of the clay body. This fusion becomes more pronounced with higher firing as the body becomes vitreous, eventually resulting in the translucent glaze and clay body characteristic of porcelain. Further increases in temperature will likely result in the body melting to become a glaze itself.

Imagine crushing a glass bottle, grinding the particles to a fine powder and then adding the powder to a bucket of water. Stir the mixture to suspend the powdered particles, and dip a porous, bisc fired ceramic work into the milky liquid for two to three seconds. Remove the work and watch it quickly dry. A thin layer of the glass powder is now left coated to the surface of the bisc. This is essentially a how a glaze is made, prepared and applied to ceramic.

Bottle Glass left, and Felspar

Although bottle glass and ceramic glass are related, there is a significant difference between them. Ceramic glazes are made from minerals similar to those used in clay bodies, are much higher in Alumina (Al2O3) so they adhere to the clay body better and form a stiffer, more viscous melt than ordinary glasses.

Glaze Chemistry Overview

A deeper understanding of glazes can be found by considering both the physical and chemical reactions that occur during their formation. These conditions, where heat and chemical reactions combine, is called ‘thermochemistry’.

In it’s pure theoretical form, a glaze is usually though of as a colourless, transparent Alumino Silicate Glass. From this basis it is easier to understand how the huge variety of glaze finishes emerge. By altering the ingredients of a glaze, tit’s thickness of application , the temperature to which it is fired, the kiln atmosphere, and the the rate of cooling and heating, a myriad of surfaces, colours and special effects can be achieved. 

A glaze can be thought of as comprising three categories of materials, fluxes, glassformers and stabilizers.

The major stabiliser used in glazes is Alumina, which is also a primary oxide found in Clays. This combines with Silica to form an ‘Alumino Silicate’ glass. Ceramic Glazes differ from bottle or ‘soda’ glasses which have little or no silica present. During kiln firing, Alumina provides a melt that chemically matches the clay and creates a  viscous glass melt which helps prevent the melted glaze from running and adhering objects to the kiln shelves. It follows that by adjusting the amount of alumina in a glaze, it’s maturation or ‘melt’ temperature can be modified. Satin, or Matt glazes are often created this by increasing the amount of Alumina in the glaze. (usually through the addition of  China Clay). The other main contributor to Mattness in glazes comes via crystallization, which is induced by paticular combinations of minerals.

Double dipped glaze with controlled melt

Tang Dynasty Chinese 'Tri-colour' glazed terracotta horse
showing the decorative use of fluid glaze.

Fluxes , in addition to modifying the glaze melt, effect colour reactions and surface quality. Raw materials may behave differently according to their firing. Iron, for example, has more than one form, depending upon it’s degree of oxidation. As Ferric Oxide it is a stabiliser when fired in an oxidation kiln environment. However in a Reduction kiln , Ferric Oxide (Fe2O3) loses oxygen and becomes FeO, which is a flux, and in so doing, alters its colour effect.

See the class notes for more detail on colour an surface in glazes, and check out these videos for a discussion and visual tips on applying glaze.

Form and Function in ceramics - an overview

Via archaeology, much is learnt about historic cultures through the analysis of shard fragments or complete pottery forms. The function of such ceramic objects can reveal much about the way people went about their everyday lives.

When deciding upon the shape to be made, the potter or designer will generally have in mind the purpose of the form, but will also allow aesthetic considerations to influence the form of the vessels, as well as an physical constraints, such as balance, the limits of the material and perhaps their own level of skill.

There are three main approaches to determining ceramic function in archaeology, which are also useful the design context. 

•  vessel form or morphology

how the characteristic shape itself affords a particular function

•  vessel performance

how the execution of the work affects it’s function

•  ceramic use-alteration.

how the experience of use alters function, both physically, aesthetically, contemplatively.

The morphological characteristics of an archaeological assemblage of ceramics, then, can be studied to understand how the vessels functioned, within the limits that such function can only be inferred at a general level. Ceramic function can also be looked at from a performance perspective. A ceramic vessel can have certain performance characteristics that make it useful for some tasks but not others. For example, a potter might form a vessel with very thick walls to make that vessel strong or durable. Thick walls, then, would increase a vessel's strength performance. At the same time, a thick-walled vessel would be heavy, and therefore decrease its transportability. Finally, the function of a ceramic vessel can alter through it’s use. For example, the pot may not be used for it’s intended purpose and be adapted for another, or an emotional bond may form between the user and the pot which transcends it’s intended function, so that it operates on multiple levels including that of aesthetic or ritual significance.

It is difficult to be exact about the function of every vessel, however it is likely that the making of each pot is approached with some fundamental ideas about it’s use. These will include: It’s weight, when full of water or food, or when it is empty. The way in which it must be stood so that it is stable, or tilted to pour, and the way it is held. Many similar pottery forms can be used for a variety of purposes, especially for storing food.

Arising from close visible relationships between human and pot form morphology, most ceramic forms are made to function in relation to particular parts of our body.  Chalice like forms , for example, are obviously made to fit between the hands. Form and function will, therefore, reflect different cultural uses. Some bowls are obviously meant to be held in one hand , others in between two palms, almost as a substitution for clasped hands. Wide rim plates may also be similarly categorised. 

Fine lipped bowls are more difficult to be lifted by one hand, and uncomfortable, especially when full. The shape of some oriental rice bowls, intended to be held below the chin, are more conical, affording a centralised balance in between the splayed fingers of one hand  that allows eating with the other. Other wider bowls, with footrings and more vertical sides will be used on tables. Many larger bowls have thickened or even flattened rims, some deliberately so to aid comfortable pressure of the thumb when lifting with one hand.

 form and function combinations reflect cultural uses. 

One may question whether it is indeed practical to lift some ancient forms by handles when full of liquid. One explanation may be that indeed these small, snug but robust handles were used to tie vessels to the sides of pack animals for transport. The rounded bases on any of these pots would make the vessel easier to pour when tilted, sit stably on uneven gound, and perhaps more easily guided into supporting rings on saddles. In these types of vesel, the neck opening is usually an indicator of the type of storage. narrow for liquid, to stop loss through splashing, and wider for solid grain storage. In other forms, round bases were used differently.The flange topped narrow neck on the Greek Alabastron, was used as a place to tie cord for it to be carried, full of bath oil or hung when not in use.

Archaic Greek Alabastron

 It is easy to imagine how this small sensual object has been considered as something to be handled, a welcome visual and tactile connection to the body in a setting where public bathing was a social activity. Different tactile connections appear in the teacups of the Victorian era and a century earlier, where delicate arabesque handles were fashioned in ways that could not accommodate the thinnest ladies finger. Instead, the device enforce a pinch grip holding of the teacup resulting in the characteristically ‘crooked’ finger of genteel bourgeois society.

Contorted body language scripted by the Victorian Tea cup handle

Indian water pots visually reflect a scripted use in an  almost topographic form. Wide swelling thin walled bellies, light when empty,  keep the weight of water centered above a rounded base , that in addition to aiding pouring, fits snuggly into a ring placed on top of the head for carrying. The height of the rolled concave neck fits a hand comfortably, and although thin, it’s doubling back offers enough strength to allow the heavy pot to be lifted in one hand.

A Indian terracotta clay water pot and a more 'modern' metal
one reflecting its traditional similar design features

Some pottery forms, such as the ‘bourdalou’,the predecessor of today’s bedpan, are designed for concealment, their squeezed, inward curving bowl lip allowing the form to fit snuggly beneath the dress of Parisian’s who used them for relief during long sermons. Other forms where the ‘interior’ takes on significant importance include jelly molds and the beautiful interior forms of japanese tea bowls, designed not only to house a tea whisk comfortably, but to enhance the sound arising from it’s use. 

These examples give some idea of both the rich variety of both functional object types and the way functionality can appear in relation to human interaction with ceramic forms. At the base level, a form may be rudimentarily instrumental.

At the most sophisticated level, all the senses are engaged in the use of an object. The challenge in designing utilitarian forms is not simply to satisfy a mechanical function, but to draw out the experience of it’s use into the aesthetic or transcendent domain , including the weight and balance of a form, whether empty of filled, tactility, the textural feel of the surface against the hands or lips, and of course the visual appearance of the shape, not simply from a single static view, but as the three dimensional form is moved through space,used and viewed at different angles.

Ceramic Technology 1 - Clays & Clay bodies

This week we will begin  to look at the Technology of Ceramics: the relationships between clays and glazes, the different types of ceramics and the way technologies are used for utilitarian and aesthetic outcomes. 

Ceramic Technology - Clays and Clay Bodies

Descriptively, the word clay is likely to raise memories of sticky, earthy material oozing between toes or stuck to the soles of shoes, or perhaps more purposefully, a soft material yielding to our touch in the formation of shapes. It is this characteristic plasticity of clay, arising from extremely fine particles of abundant earth minerals combined with water that give it such excellent malleability. The abundance of plastic clay in many geographical locations has resulted in its widespread adoption as a medium for creating cultural objects, often of a distinctive regional character determined by a  clays particular geological quality. Whilst clays may be found that exhibit ideal working qualities in their prospected state, the desire to develop specific working properties and fired characteristics has led to the deliberate sourcing and mixing of particular clays and other earth minerals for particular purposes, raising a distinction between the terms ‘clay’ and clay body’.In addition, different formulation and processing is used to prepare clay for different typoes of processing, whether in it’s plastic state for hand or wheel forming, pressing or extruding, or as liquid ‘slip’ for casting

Clays are essentially weathered igneous rocks that have their physical mineralogy altered by actions of wind and water. 

The majority of clays are characteristically red, orange of brown burning, indicating they have been weathered from iron bearing rocks or have been contaminated by iron during the process of transportation and weathering. These clays are commonly found in shales, swamps, or other locations where sedimentation has taken place. Other geological forces such as folding may relocate these sedimentary deposits to areas where the clay becomes exposed and commonly visible, such as roadside cuttings. Sometimes, laid clay beds underlying vegetation may have iron leached from the substrate, yielding more refined, pale coloured and even white clays. White burning clays, on the other hand are less commonly found, the most pure being formed at igneous origins, their lower plasticity arising from the ‘in situ’ geothermal weathering of felspar. In circumstances where Kaolinitic clays are subject to transportation weathering with low levels of contamination from iron or other minerals, plastic ball clays form; and in more unusual circumstances, plastic kaolins can form.

The following table characterises four major clay types :

Plasticity imparts a significant working property of a clay body, but it is not the only consideration. Strength, shrinkage and colour are equally important factors at both green and fired stages of the ceramic process. For this reason clay bodies are developed to balance green state working properties with those of the desired fired ceramic state. A clay body may be formulated for a particular forming process, such as hand or wheel forming, pressing, or liquid slip casting. Firing considerations concern colour, surface and application ( eg vitreous china vs. porous gardenware ) hardness and durability. Sometimes a fired property dictates the development of an unorthodox forming process, for example in advanced ceramics where polymer platsicisers are used to enable casting of non plastic alumina.The mineralogy and particle size of ingredients also contribute to the design of the clay body, where particular fired nuances can be achieved through their selective inclusion, for example the use of calcined bone in translucent ‘bone china’, coarse felspar particles in traditional ‘shigaraki’ ware, and powdered soda glass silicate frits in ‘faeince’ paste.

Generally, clay bodies formulated for standard ceramic processes balance the use of plastic and no plastic materials.

Plastic clays exhibit higher green strength and shrinkage rates, the former desirable for workability but the latter contributing to distortion due to closely packed particles that inhibiting drying. Consequently, plastic clays are mixed in proportion with lower plasticity clays and non plastic powdered filler minerals such as felspar and silica to extend particle size distribution and reduce shrinkage. Non Plastics also contribute to the appropriate degree of melt to achieve the required fired characteristics such as durability and porosity/vitrification.The following generalised recipes show two different examples of clay body formulation for different forming processes at different temperatures.

 The following video series provides a good grounding in the identification, selection and preparation of clays for various purposes.

Learn How to make pottery clay. For more Ceramics How-To Videos & Articles, visit WonderHowTo.

Botanica - An Introduction

Although it may not appear common amidst the early chronological development of ceramics evident in archaeological records, the prominence of the vase today argues for it’s status as the archetypal ceramic form. The vase is both utilitarian and an expression of the vessel, however it’s impact goes beyond both of these to carry symbolic properties. In its shape, the vase reflects many of the characteristics of other functional ceramic forms. It is a freestanding shell, an amphora, a bottle, or the foundation of a jug. It is decorative, useful and sculptural. The vase is very often the ‘form of choice’ for ceramic artists, an icon of the medium akin to the framed two dimensional space in painting. It provides a three dimensional field for interrogation of ideas.

                                         

In this work of Peter Voulkos, the orthodox representation of the classic vase shape is upturned. Rather than the smooth, sinuous lines of a Chinese Sung dynasty porcelain vase, the rugged stoneware form is both physically and metaphorically ‘thrown’ on the pottery wheel, fractured and then reassembled. It’s vigorously handled surface is scarred and seemingly accidental in contrast to a more restrained, refined glazed surface, yet it exudes confidence. Whilst using similar techniques to the Sung potters, Volkous imbues the vase with completely different aesthetic properties, an energy and vision characteristic of the 20th Abstract Expressionist movement that launched his work to International prominence in 1960’s. 

Under the above conditions, the role of the flower, and the arrangement of botanical material is suppressed, but this has not always the case. In the work of Andrea Branzi, the celebrated Italian, ex Memphis designer, the flower becomes freed as abstract element in the composition of the designed object that represents the vase as an ‘idea’. In his ‘superego’ series, Branzi uses the ‘picture frame’ literally, as if to ‘script’ this notion of representation, often using the minimal placement of botanical material in a spatially asymmetry that recalls Japanese ‘Ikebana’ floral arrangements, or creating the a ceramic vessel component that mimics botanical material.

These examples reflect on how the expression and meaning of the vase can be conveyed,  not only through its form, but by the role of the material it contains,  it’s shape and configuration; by its relationship to the three dimensional space in which it is sited, the context of it’s location, and by it’s own surface.

Left: Tzu Chou Vase, Sung Dynasty. Right:  Chun glazed vase Sung Dynasty                                          

An example of the vase’s rise to prominence can be found in Yuan’s “A History of the Vase”, an important 17th century early example of the burgeoning late Ming literature of connoisseurship. This work discusses the vase, associated floral arrangements and their traditions, the relationships of vase to it’s surroundings and it’s social context. This rich arrangement develops develop strong symbolic representations of the vase that have endured, influentially, in both China and Japan, and appears one of the prime inspirations of Ikebana. At the time Yuan’s work was published, the Ming Dynasty is described as a world within which everything, including knowledge, had become commodified, where taste was marshalled as   

 “an essential legitimator of consumption” 1

Although this may sound distant from today’s China, it reflects much of the condition of current western capitalist culture in which the role of designed objects, particularly the vase, can be similarly contextualised.

1. ¹ Craig Clunas Superfluous Things: Material Culture and Social Status in Early Modern China (Cambridge: Polity Press, 1991).