Composition
of old cement
Old
cement can have a variety of components. One is red lead, another is
portland cement. Both create different problems, but both require
that all removal of old cement should be done with breathing
protection.
Mechanical removal
Removal
of old cement can be attempted first with mechanical means. Stand
the glass on edge and use a lead knife almost parallel to the surface
of the glass to knock off the old cement. This works most of the
time. But if the bond is too strong and begins to cause chipping of
the glass, you need to switch to another method.
First
consider whether the whole panel needs to be re-leaded. It may be
that only portions require re-leading and so the remainder should be
left in its original leads. This is especially true where there is
painted glass, as you do not want to loose the original painting.
Chemical
removal
If
mechanical removal is not working you need to consider chemicals.
Lye
will dissolve linseed oil but perhaps also the paint and so should be
used locally only and with extreme care and caution. Testing on
unobtrusive areas is required.
Another
way to soften the linseed oil is with heat and for this a steam
generator works particularly well turning hard putty into the
consistency of soft cheese which can be scraped off.
If
it actually is Portland cement creating the bond then hydrochloric
acid is what bricklayers use to dissolve and remove mortar stains
from brickwork. Carefully paint it on the old cement and it should
start fizzing and dissolving straight away.
Precautions
What
ever you decide to do, test first with some of the glass that will be
discarded and confine your chemicals to the cement only and don’t
spread them over the entire piece of glass. Wear a respirator at all
times during the removal process.
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