Window putty

Amanda Clydesdale <mandyc [at] aocscot__co__uk> writes

>We have a set of 200 year-old windows that are about to undergo
>refurbishment prior to re-installation. The architect has specified
>that the original hand-made glass should be removed and then
>re-instated after the woodwork has been repaired. Has anyone come
>across a safe and commercially viable way to remove said putty with
>minimal damage to the glass?

I am aware of two effective methods for removing the putty from the
glazing channel. It is assumed that the sash will be removed and
secured flat on a workbench.

Historically, glazier's putty was typically made with linseed oil
and chalk whiting, although often white lead was also added. If
preserving the glass is not critical, then a heat gun works well
with softening up the linseed oil until you can easily pull the
putty away. To preserve the glass from breaking due to the heat
stress, a sopping wet towel is placed on the glass up against the
putty to diffuse the heat. Also use a metal deflector for the heat
gun. I have also heard of using a manufactured heat shield that has
an asbestos or similar core. There used to be available a clever
tool similar to a soldering iron that had a heated wire that would
sit against the putty. I cannot find this tool anymore; it has gone
the way of the heat plate. Perhaps you could cobble something
together using a soldering iron and copper wire?

The putty usually comes out easily using heat. However, removing
glazier's points is often a problem since they were usually set
tight to the glass. I have broken glass trying to work out the
points, but have never cracked a pane from the heat.

However, with 200 year old glass, I would not risk using heat, but
would use a methylene-chloride based stripper to soften the putty.
This method is much slower, but safer (except of course for the
lovely fumes). Once the bulk of the putty is removed, apply more
stripper along the edge of the glass to help release it from any
remaining bedding putty. Traditionally, a caustic soda stripper was
used to soften putty. However, there is the risk of such a stripper
damaging the wood muntins. With any chemical stripping method, the
risk of gouging the muntins and glazing channels becomes greater as
the surface of the wood is softened up.

There is a drill mounted cutting tool available that will probably
rout out putty pretty quickly, but of course there is the risk that
it will plane down the glazing channel. Such a tool, while possibly
useful on 20th century windows may not be acceptable for
conservation of fine early 19th century sash. For your info, this
tool can be seen at <URL:http://www.praziusa.com/puttychaser.html>.

After the putty is removed and the glazing channels primed, the
glass should be set in a bed of putty to cushion it since it is
typically so irregular. A colleague of mine once lectured on the
fluidity of window glass, and has observed that glass with 200 years
of age on it is typically quite thinner at the top. He always
advocated documenting the top and bottom of each pane of glass (in
addition to documenting exactly which light each pane came from). He
argued that when reinstalling old glass, the glass should be flipped
top to bottom, installing the thicker edge up. The theory was that
the sag in the glass would then be reversed over the next 200 years
(should that our restorations survive that long).

John Horton, RA
Restoration Specialist
NC State Historic Preservation Office
828-274-6789
Fax: 828-274-6995


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                  Conservation DistList Instance 15:43
                 Distributed: Monday, December 17, 2001
                       Message Id: cdl-15-43-006
                                  ***