Janet Hamer outlines the new glazes of Avril Farley
describes how these sculptural crystal shapes are formed.
are all keen to make a mixture of minerals, take it to a temperature and
wonder at transformations –or we should be. It is like pushing seeds
into a garden and, with a bit of nurture, flowers open and spread their
colours. Making crystals is like that: a lot of science, patience and
beauty. Large crystals grown in a glaze even look like flowers. They often
resemble lichens and three-dimensional fans and feathers.
Crystalline glazes are those in which the oxides in the melt reform in
new associations as the glaze cools to give a glass with crystals visible
at the surface. A mass of crystals too small to be seen individually can
give opacity and matt surfaces. Larger crystals can be grown upwards of
15 cm, appearing to float in a glassy matrix. These wonderful shapes,
of distinctly different colour from their background are the result of
manipulating the glaze formula and the cooling rate of the kiln. This
is the science and the patience. The beauty manifests itself magically
from these processes.
Typical crystal shapes are soft flat rounds which may impinge on each
other in clusters, modifying the symmetry. A central star or bundle of
needles can be seen in a smooth area before a fibrous ring fans out as
a halo. Further haloes edge the shape where it meets the background glaze
or matrix. The whole of this shape is coloured by the penetration of a
colouring oxide into the crystal and is clearly seen against the background.
There is often a delicate fringe of a slightly different colour where
crystal meets matrix. The haloes can be deliberately placed centrally
or around the border of the crystal. Where the glaze is thicker, as in
the well of a bowl, the three-dimensional forms can be seen as fibrous
fans filling the depth. Time is needed during the cooling of the glaze
for crystals to form.
In the early stages of cooling, if the temperature is held around 1090ºC
for approximately two hours, the crystals begin as simple needle shapes.
These can fan out at each end into ‘double-axehead shapes’.
These attractive and intriguing crystals can be retained, frozen at this
stage by cooling the kiln rapidly after this growing period. The fuller
rounder shapes develop when the temperature is subsequently maintained
for further crystal growing periods. These periods, or pauses, are programmed
into the cooling graph of the kiln controller and may last from three
to eight hours.
often the choice of body for use with crystalline glazes. Bright
colours show up welland there is little contamination from the bodyduring
the slow cooling. The main glazeconstituent is a frit. This provides
most of theglass which melts at the appropriate temperature.
Ferro frit 3110 analysis
Silica (Si02) 69.8Sodium oxide (Na2O) 15.3Calcium oxide (CaO) 6.3Aluminium
oxide (A12O3) 3.7Boron oxide (B2O3) 2.6Potassium oxide (K2O) 2.3
Avril Farley glaze recipes:
1. Ferro frit 3110 47
Calcined zinc oxide 23 Calcined china clay 3
Flint 23 Titanium oxide 4
2. High Alkaline Frit 2275 46
Zinc oxide 24
China clay 40
Titanium oxide 8
Oxides of copper, cobalt andmanganese are added totalling a
maximum of 8%.
Each glaze component has a particular role but these
are not single elements and their contributions overlap. The frit
is designed to make the glaze melt quickly at top temperature. This
presents a fully molten mix which is immediately ready for the new
bondings to be formed. The rapid firing up to and down from the
top temperature avoids the formation of a body-glaze layer which
inhibits the forming of large crystals. Zinc oxide combines with
flint and provides the zinc silicate for large crystals. The china
clay gives stability and hardness to the final glaze. The flint
is almost pure silica. It can be a different type from that provided
by the frit for the main glassy ingredient and supplies nuclei for
crystals. Titanium oxide contributes nuclei as ‘seeds’
for the initiation of crystals. It also brightens colours and assists
the movement of colour in the glaze.
The oxides (or carbonates which lose their carbon and excess oxygen
in the fusion) of copper, cobalt and manganese, colour the glaze
matrix, or the crystals, or sometimes both in specific ways, according
to their ‘field strengths’.
Crystals grow in the cooling glaze by the isolation of particular
oxides from the surrounding glaze. Zinc silicate is most often the
material of large crystals. In the molten glaze the molecules of
the glaze minerals are loosened from their original combinations
giving a fluid mixture of individual molecules. In a normal glaze,
as cooling begins, these molecules link together to form irregular
chains. This creates the amorphous substance, glass. For crystals
to develop, the temperature is held for those periods when molecules
orientate into more specifically organised chains. They establish
bonds which produce lattice structures which are the framework of
crystals. The unsatisfied valencies existing in the melt link to
sites where they form new combinations.
||Left: This complex crystal shows the fan-like growth
and three-dimensional appearance in the depth of glaze. The blue staining
ofthe crystal is incomplete due to the small percentage of cobalt
oxidein the recipe. The pot was glazed first with a Ferro Frit-based
glazeand over this a High Alkaline Frit 2275 base glaze. Each onecontained
0.5% cobalt oxide and 3% manganese carbonate.
Crystal formation is a selective process. As some constituents are precipitated,
the remaining matrix is changed. The isolation of some of the constituents
upsets the previous balance. Some of the remaining oxides can no longer
remain unattached. They combine as larger molecules and stiffen the matrix.
The matrix then sets quickly and crystals can no longer develop.
are orders and preferences for how the molecules which jostle freely in
the melt will re-bond into new lattice structures. Each element is characterised
by a value number or valency. This number is based on the number of electrons
in each atom and establishes its combining power. Valencies are balanced
to match. For example, hydrogen is 1, oxygen is 2, therefore two hydrogen
atoms are needed to match one oxygen, giving the familiar chemical symbol
The elements which are frequently used to colour the zinc silicate crystals
are cobalt, manganese and copper. They have valencies of cobalt 2 and
3, manganese 2, 3 and 4, and copper 1 and 2. They have 2 in common with
zinc and therefore compete for the same sites when new combinations are
being formed. In the Periodic Table the colouring elements are grouped
together as ‘transition’ elements. Other elements in this
grouping have similar properties and are likely to be useful in a similar
There are further rules which govern the selective process by which crystals
are positively coloured, why blue on an ochre ground predominates whereas
green can be subtly combined. The electrons of the atoms, which are negatively
charged, exert forces of attraction or repulsion on others which are in
close proximity. This activity is referred to as an energy field. Colour
separation is explained in the following extract from the section on crystalline
glazes in The Potter’s Dictionary:
In order to colour the precipitating zinc-silicate crystals, the
colouring oxides must be able to fit into the lattice structure. To
enter the crystal, the metal colouring atom must be able to occupy one
of the six sites otherwise held by zinc in the zinc-silicate lattice.
Cobalt, nickel, copper, iron and manganese are transition metals and
are adjacent to zinc in the periodic table. They are polyvalent and
can be divalent to match zinc. Their atom sizes are also similar to
that of zinc. Therefore all these metals can enter and colour the crystals.
|Right: The colours here are from ilmenite (FeTiO3,
iron and titanium oxide) andcerium oxide (CeO2) which has properties
similar to tin oxide.
The reason for the order of precedence is that they have different liquid-to-crystal
partition co-efficients, or field strengths. Cobalt oxide and nickel oxide
have high field strengths. Manganese oxide is intermediate and copper
oxide is low. Zinc oxide has a higher field strength than does copper
oxide and, therefore, copper oxide tends not to partition strongly but
will stain both the matrix and the crystals at the same time.
Avril Farley has been making crystal-glazed ceramics for four years.
Her workshop is a small, neatly organised outbuilding. The workroom and
its surrounding work areas are next to the cottage where Avril lives with
her husband, Ken. The setting is rural Gloucestershire in the Royal Forest
of Dean, England. The cottage and workshop stand above a delightful sunny
garden which slopes steeply down to a stream. The slope and footbridge
are the route up and down which all clay, equipment and finished work
must be portered. In the late 1780s this cottage was a pub humorously
named ‘The Sow with Three Tits’. Here cider was made and served
to the iron workers who toiled immediately opposite, across the stream.
Avril Farley’s production consists of thrown plates, bottles and
bowls. She prefers Limoges porcelain body, from Potterycrafts. She uses
a Mervyn Fitzwilliam Craftsman wheel. She does a small amount of turning
to make neat footrims which accommodate the inevitable glaze run and grinding.
The two 4 1/2 cu ft electric kilns are fired at night for economy with
the use of two Cambridge 401+ controllers. She usually achieves two biscuit
and one glaze per week, alternating with one biscuit and two glaze firings.
|This fringe-edged crystal is approximately 4 cm across. Itappears
in a glaze with 1.5% vanadium oxide (V205) and1.5%
ilmenite in a Ferro Frit base glaze.
||This glaze shows how copper oxide can give a green stain toboth
crystal and matrix. The glaze has 3% copper oxide and 3% barium carbonate
(BaCO3). The barium carbonate shiftsthe colour towards
are brushed on, thicker above than below to allow for considerable glaze
movement. Calcining the zinc oxide removes water and helps to avoid flaking
of the glaze. A binder, ‘CMC’, is used to make the glaze less
friable. Every pot is fired on a ‘catcher’ made to measure.
Surplus run off glaze is contained by the ‘catcher’ which
must be separated after firing. Glaze and foot are then ground smooth.
This is a demanding process requiring specialised grinders for the particular
shapes and a skilled operator who is efficiently clad in protective clothing,
goggles and helmet. Every firing contains tests. New shapes show their
effect on glaze run and positioning of crystals. The permutations seem
infinite. Variations of amount and combinations of colouring oxides are
studied in glazes with The diagram shows how felspar, Alkaline Frit 2275
or Ferro Frit 3110, sometimes layered cobalt oxide can replace zinc together.
Firing is usually to between 1245ºC and 1265ºC with many crystal-oxide,
being the same size, growing pauses in the cooling cycle. The first at
1080ºC for 30 minutes may be and fit into the lattice followed by
a pause at 1060ºC for 30 minutes, then a rise to 1080ºC again.
structure of a growing crystal There may be as many as six pauses but
Farley also likes to vary the timing of of zinc silicate. these periods
to give more interestingly placed haloes.
Farley declares her approach to ceramic chemistry was instinctive. From
a nonscientific early career she is now developing a firm understanding
of chemistry through methodical practice and application. She enjoys the
discipline which the creation of crystalline glazes demands. In a detailed
directive to herself, ‘The Learning Curve’, she stipulates
every practical rule steering a controlled course through trial and error
Record keeping is unquestionable. Glaze making is precise and meticulous
with thorough attention to care of equipment. Firing records include identification
of all tests, positions in the kiln, weather (for its influence on cooling
rates), temperature readings and digital pictures are stored. Suggestions
for variations on every aspect of the making follow with practical guidance
for control of glaze fusion, and ending with warnings against impatient
kiln opening. Avril Farley is currently experimenting with more materials
based on elements from the lanthanoid series sometimes called ‘rare
Janet Hamer, with co-author Frank Hamer, are the compilers
of The Potter’s Dictionary of Materials and Techniques now in its
5th edition (A & C Black).