179 / 208
T W E N T Y S E C O N D E D I T I O N
T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 5
1 7 7
INTER IORS
5
blue and manganese for purple. After
dipping the biscuit tile into the tin glaze
and allowing it to dry, the decorator could
then apply the design by hand painting in
blue or purple glaze directly onto the dipped,
unfired surface.
The tile was then ready for a second
firing of up to 800°c. Repeated patterns
were achieved using ‘pouncing’, a method
of tracing designs onto tiles by drawing the
outline of the design onto paper and then
making pinpricks at regular intervals along
the lines of the pattern. The perforated
design is then placed over a tile and dusted
with burnt bone or charcoal, transferring a
dotted outline of the pattern onto the tile.
Because the pattern is only transferred in
outline no two tiles are exactly alike. The
charm of Delft tiles lies in this elegant
variation, in the ease and swiftness of the
hand-drawn design and in the way that the
blues and purples migrate into the white
background during firing (see page 176).
In Liverpool, in 1756, John Sadler and
Guy Green introduced the technique of
transfer printing onto tiles which produced
an entirely different style of decoration
formed by a printed outline transferred
from a copper plate engraving fired onto the
surface of the tin glaze. This allowed repeated
intricate designs to be printed onto tiles at
great speed. While transfer printing later
became a mainstay of decoration for the
pottery industry as a whole, no other tile-
makers followed suit at the time.
INHERENT DEFECTS
Delft tiles in dairies and kitchens were for the
most part swept away in favour of the new,
more robust Victorian tile. Those that remain
are usually conserved with great care or taken
away to be displayed as ‘art’ pieces. In the UK,
Delft tiles are rarely found undisturbed in
their original location in significant numbers.
The chief failure of Delft tiles is the
inherent problem of the glaze sufficiently
melding with the clay body to form a single
robust entity. The glaze and body too often
shrank at different rates when cooling from
the kiln resulting in the glaze ‘crazing’
(forming a network of fine cracks) and flaking
away from the body. The resulting clay
body, in ceramic terms, would be described
as ‘soft’, meaning that it tends towards high
porosity, friability – particularly around the
edges – and easy fragmentation. Much of
the conservation work on 17th- and 18th-
century tiles is associated with consolidation
of damage directly resulting from the
manufacturing technique.
The soft body of the tile is particularly
prone to moisture absorption, which in turn
can result in efflorescence caused by salt
crystallisation. Salt crystal growth can force
the glaze to become detached from the tile
body. This is a well-known and understood
problem with soft-body ceramic objects.
Moisture absorption also leads to the ready
ingress of stains into the tile body, particularly
if tiles have been used in fireplace surrounds.
The glaze itself is also soft and will
readily become worn on the surface, losing
the case of 18th-century tile work because the
lime plaster used as a fixative often contained
gypsum, which is particularly prone to
salt action where there are prevalent damp
conditions. The only remedy in this situation
is to create an ambient environment with a
relative humidity no greater than 60 per cent,
which will prevent crystallisation.
the typical soft shine which tin glaze
produces and again leaving the glaze open to
absorption of dirt.
GUIDANCE ON CONSOLIDATION,
CLEANING AND REPAIR
The most damaging and frequently
encountered inappropriate repair is the use
of Portland cement either to reinstate loose
tiles or to install them in a new location
following the failure of the original lime
plaster fixative.
The relative hardness, lack of plasticity
and ability to hold soluble salts all combine
to make the use of Portland cement as an
adhesive for Delft tiles highly damaging.
Little can be done to reverse the use of
cement without the loss of original tile
material.
Cleaning
Delft tiles can be cleaned using a solution of
de-ionised water and a neutral pH detergent
such as Synperonic A. Usually, a moistened
soft cloth is sufficient to clean the glaze but
particular attention can be paid to dirty
grout lines using a tooth brush. Buffing to a
shine with a dry soft cotton cloth brings out
the soft lustre of the glaze. If tiles are over-
wetted there is a danger that crystallisation
of soluble salts may take place, and if loose
tiles are soaked over a period of time in
distilled water to remove salts or staining,
care must be taken to dry the tiles quickly
and thoroughly.
Organic staining, such as soot, if left
alone, will continue to migrate into the tile
body. Removing organic and iron oxide
staining is difficult and requires the expertise
of a ceramic conservator.
Site-related problems
Lime plaster was used to fix tiles during
the 18th century and it is usually found to
be fractured behind the tiles (below right).
Shrinkage at the time of drying causes a
complex polygonal matrix of fractures to
form which can cause the tiles to sound
hollow or to rattle when tapped, but it may
not be problematic if the tiles are adhered
and the grout between the tiles is sound.
Fireplaces
It is quite common to find 18th-century
tiles in fireplace surrounds. The severe loss
of glaze surface on hearth tiles due to wear
cannot be repaired. Impact damage such
as fractures can be repaired using normal
ceramic restoration techniques especially if
the tile is also loose from its setting.
The most common problem is soot
blackening at the edges of tiles. The worst
effects of this can usually be removed as
described above but often the best option is to
remove and replace the old grout altogether.
Salt crystallisation
Salt crystallisation is a well understood
problem. In damp conditions, chemical
changes resulting from salt crystallisation
degrade lime plaster causing it to soften and
crumble. The problem can be exacerbated in
Tile panel, Triana, Seville, Spain
Typical use of Delft tiles surrounding a fireplace in
the UK
Hand painted images in cobalt and manganese
Lime plaster adhesive which has shrunk and
fractured while drying