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BCD Special Report on
Historic Churches
18th annual edition
35
Repair leads
Honest and reversible repair methods are
preferable for plain glazing. The insertion of
repair leads or strap leads is good practice,
conserving original material. However, there
is no reason why this sort of repair should be
clumsy and detract from the appearance of
original lead lines. There should be a hierarchy
of leads, separating original lead lines from
later repairs. Lead sections are available in
a huge variety of sizes, or can be custom
milled in-house. A sensitive, considered
approach to the insertion of repair leads is
easily achieved, although often neglected.
It has been common over the centuries
for glass to be cut or ‘grozed’ (the edges
nibbled away with a notched tool or pliers)
to accommodate the ‘heart’ of the lead (the
crossbar of the H section). However, this
rather defeats the object of conserving the
original glass. It is possible to order lead from
suppliers with a very slender heart to avoid
removing glass. Alternatively, it is simple to
make a draw-tool with a sharp cutting edge
to remove shavings of lead from the heart.
Only a very small thickness is required; repair
leads usually serve the purpose of retaining
original material and are not structural, so
slim, tailor-made leads are appropriate.
There is a distinction between repair
leads inserted when a panel is dismantled
in the workshop (middle left) and ‘strap’
repairs, where a crack is covered by a
strip of surface lead applied in situ.
Strap leads should be soldered in place,
which is easily achieved with a small electric
or butane gas soldering iron. A tiny sliver
of lead is removed with a scalpel to reveal
bright metal and fluxed. The tip of a small iron
will readily solder the repair lead in place.
Historically strap leads were often applied
on both sides and ‘glued’ into place with
blackened linseed oil putty, however, unless
leads are required on both faces to stabilise
loose fragments, a lead on the exterior face
only with a little blackened putty forced under
the lead, but not into the crack, and trimmed
off neatly, can suffice. This means that the
weather is kept out, but when the panel is next
re-leaded, the trauma to the glass through
mechanical removal of hard putty is minimised.
Copper foil repairs
The copper foil or Tiffany method was
developed in America by Tiffany and
La Farge in about 1880. It has been employed
for over 30 years for the discreet repair
of broken fragments of stained glass.
In this method a very thin sheet of copper
with an adhesive backing is applied to each side
of the crack and neatly trimmed back with a
scalpel. The pieces are then brought together,
fluxed and soldered with a small soldering
iron. The same process is carried out on the
reverse, resulting in a tiny H-section repair
akin to a miniature lead. The repair can be
darkened by applying chemical patination to
make it blend in with the leaded panel, but the
solder oxidises and darkens with time anyway.
There are some justified concerns about
its use for painted and stained glass as great
care needs to be taken when trimming back the
copper foil to avoid scoring the glass surface
with the scalpel tip. The reaction of heat, fluxes
and chemical patination in close proximity to
delicate glass surfaces and painted surfaces is a
further cause for concern. Nevertheless these
issues are being addressed by conservators
and the method remains a valid option as an
honest and readily reversible repair method.
For plain glazing, the copper foil method is
an excellent choice, the scoring of the surface
can be readily avoided by careful application
and the repair is both discreet and reversible.
Because the foil is so thin there is no need to
remove any glass to facilitate the insertion.
It should be remembered that copper foil
repairs can often be carried out in situ when
fragments of a broken piece can be teased out,
allowing copper foil to be applied to the edge
of cracks. Even in exposed positions when
external access may not be possible, the fact
that solder is readily drawn through to the
other side of the foil can make this viable.
This method can also be seen as an
excellent temporary repair for the retention of
original material pending future conservation.
Re-leading
This is a crucial part of the process. If the wrong
decisions are taken about the lead profile, the
whole balance of a window in its architectural
setting can be destroyed. For example, Arts
and Crafts period slab glass is normally set
into substantial round-section leads. To re-
lead in thin, flat-section lead would radically
alter the external texture of the building.
In the same way, 18th-century plain glazing
formed of very thin crown glass should be
re-leaded in flat-section lead with a heart
depth matching the original. This can be as
little as 1.5mm and must be custom-milled
to ensure an authentic appearance. But it is
not merely a question of appearance: panels
from this period were normally of very light
construction and held to a very substantial
iron frame (or ‘ferramenta’) with lead ties.
The panels overlapped one another.
This dynamic can be easily upset if the
panels are made too heavy, leading to premature
failing of ties and water ingress, not to mention
an overly heavy external appearance. In plain
glazing such niceties are important – they are
the difference between success and failure.
Obviously, historic lead such as
medieval cast sections and later sections
with historically significant date markings
should be conserved wherever possible, or
at the very least thoroughly documented.
Stephen Clare
founded Holy Well Glass in
1995 having spent 15 years working with leading
stained glass design and conservation studios. He
is national adviser for stained glass to the National
Trust, a member of the Court of Assistants of the
Worshipful Company of Glaziers and trustee of The
Glaziers’ Trust, the charitable arm of the Company.
A copper foil repair
This example of 18th century crown glass plain
glazing was dismantled on site following collapse.
All glass was retained and was original. Copper foil
repairs were carried out.
The date 1751 and the manufacturer’s initials (WR)
were clearly visible in this original lead from the
18th century panel shown above. As much of the
original lead as possible was conserved and lead was
specially milled for the remainder.