knowledge TeCHnICAl

Pigments and Engobes for Crystalline Glazes

CATERINA ROMA eXPlAInS A TeCHnIQUe deVeloPed BY JoSeP MARISCAl. ENGOBES The basic technique consists of using a porcelain engobe to
cover a stoneware piece, a clay that is far better to throw but one on which
with a family tree with deep roots in the pottery trade, Josep Mariscal was crystals do not generally develop so well. For the porcelain to correctly adhere
taught by his father from the age of seven to master the rolling of clay. when to the stoneware body, Mariscal covers the piece with a deflocculated slip
he grew up, and after working as a potter in many factories in the traditional when the clay is leatherhard, and he does it slowly and steadily in order to
ceramic village of la Bisbal d’empordà (Catalonia, Spain), Mariscal set up achieve a homogeneous thickness of about 2-3mm. If the piece is thin or
his own studio and kept researching to perfect the techniques that would too wet, it needs to be immediately dried with a torch to avoid collapse.
best express his artistic interests, mainly terra sigillata, raku, and crystalline once dry, it is ready to bisque fire.
glazes. A self-made artist from head to toe, today he’s a great master without If you want your piece to have a white background to show the colour
ever having attended a ceramics class. of the crystals, nothing else needs to be done; do some tests with the clays
with such a long practice in throwing, and strong arms, Mariscal builds you have available to check if they work well together. But if you want to
large pieces with smooth surfaces – the perfect background to grow large experiment with new colours, there are some factors that have to be taken
crystals, floating in deep and shining glazes. His research has allowed him into consideration.
to achieve the brilliant finish of porcelain in large-scale pieces born on the
throwing wheel, and to discover new colours that are different from those LOSS OF COLOUR If you have ever tried to mix pigments into a crystalline
you would get if using classic combinations of oxides. glaze, you know that most of the time the colour mysteriously disappears
The technique is not new – Mariscal just uses engobes and pigments – during the firing. This can happen because of the breaking down of the
but there are hurdles at every step. He has standardised the process in a particles through excessive milling, or due to the chemical attack by alkali
way that, if you stick to it, will allow you to take a step forward with your fluxes or glazes. To overcome this first setback, simply mill the glaze for
crystalline glazes colour range. under thirty minutes.

68 CeRAMIC ReVIew 265 January/February 2014

 1 Crystals with engobes, stoneware with stoneware with coloured porcelain Email ceramics@caterinaroma.com
white and coloured porcelain engobe, engobe, 2013, H65cm 4 Bottle with red Web www.caterinaroma.com
2013, H65cm max 2 Josep Mariscal engobe, stoneware with coloured Caterina Roma is a ceramist who
throwing at the International Crystalline porcelain engobe and crystalline glaze, graduated in literary Theory and
glaze Convention, la Bisbal d’empordà, 2013, H45cm Comparative literature, and studied literary
April 2013 3 Crystals on coloured engobe, Photography Caterina Roma Translation and Publishing at postgraduate
level. She is a passionate researcher of
ceramic techniques and traditions

2 4

However, the second factor presents more difficulties, because crystalline

glazes have a high alkali nature, and there are only some metal oxides
whose colours will ‘survive’ it. That’s why the colour disappears if you mix it
into the glaze. The oxides that work are lead, calcium, magnesium, boron,
and zircon. The neutrals are zinc, barium, and alumina. And the ones that
work against it are sodium, potassium, lithium, and fluorine.

WORKING WITH SLIP The first ingredient of the glaze that melts when the
temperature rises in the kiln is the frit. The most common frits for crystalline
glazes, 3110 or 90208, create a particularly unfriendly environment for the
pigments, but all crystalline glaze frits do. when zinc oxide dissolves in the
frit, on a later stage of the firing, it is too late to protect the pigment. That’s
why the only possibility of saving the colour is to mix the pigment in the
porcelain slip, which works as a protection from the alkalis. You can always
try to make your own frit so it will be more suitable for this purpose, but that
would be another (long) story.
The key point is to find the balance between the glaze composition
and temperature, to ensure that the pigment ‘survives’ and shows its
original colour.
Mariscal recommends trying 10% to 30% pigment in the porcelain slip,
depending on the colour. once the piece is bisque fired, you can glaze it
with your usual recipe. If it still doesn’t work, you can try adding some zinc
oxide or zircon silicate to the slip, from 5% to 20%.
with this simple but challenging technique, Mariscal glazes his pieces
using his crystalline glaze recipes with added oxides over tinted engobes.
The results are rare but fascinating combinations of different colours for the
backgrounds and crystals.
You can find more information about crystalline glazes, recipes, and
techniques at www.ceramicasjosemariscal.blogspot.com (Spanish only).
And don’t miss the macro-crystalline zinc-silicate glaze discussion group
www.s3.excoboard.com/crystal, where you can learn and share with
experienced potters.

CeRAMIC ReVIew 265 January/February 2014 69