Lead solder

This is in response to Scott Erbes query regarding lead solder on a
pre-1700's silver object. There are several issues in regards to
silver solder on historic silver.

First, there may be aesthetic issues. When repairs have been
effected during the life of an historic object these repairs can
sometimes be quite inartfully applied, leaving gobs of grey lead
solder evident. If this repair is effected in an unobtrusive place,
such as the underside of a foot or base, then it's visual appearance
is not as great a concern.

Second, there may be corrosion issues with old lead solder repairs.
This corrosion occurs because of the difference between dissimilar
metals in the galvanic series. Silver is a very noble metal while
lead is a base metal and falls significantly below it. When
intimately combined in the presence of an electrolyte (either liquid
water or elevated RH) the noble metal, in this case the silver,
becomes cathodic, while the base metal, (lead), becomes anodic. The
other player in the mechanism of corrosion is the copper in the
silver alloy. Copper ions are quite easily dissolved from their
alloys and will corrode in the presence of dissimilar metals. I have
often seen this form of copper corrosion in both lead and silver
based-solders used in silver objects. Copper depletion from the
alloy will, over a long time, lead to physical instability of the
silver--but I have rarely seen this in historic silver objects.

The thousands of silver objects that I have seen that are
non-archaeological have fairly insignificant corrosion of lead
solder joins and it is quite easily removed by chemical means
(dilute formic acid). Lead solder that is visually disturbing can
only be removed mechanically--and best removed under low-power
magnification. Never try to use a torch to attempt to remove these
lead solders. You can soften and melt other low-temperature solder
joins nearby, and you will impart a risk of making the object
mechanically unstable--the reason why the solder was applied in the
first place.

If you have no evidence aesthetic issues or active corrosion I would
simply leave the piece alone--the repair is part of the history of
the object and if it is stable there is no reason to intervene at
the current time.

David Harvey
Metals & Arms Conservator
Williamsburg, Virginia  USA

                                  ***
                  Conservation DistList Instance 12:87
                  Distributed: Thursday, May 13, 1999
                       Message Id: cdl-12-87-008
                                  ***