Conservation of toilets from shipwreck

Nicola Smith <nicola.smith [at] doi__vic__gov__au> writes

>... Among the artifacts were 3
>stoneware flush commodes. We have 2 very similar toilets from a
>shipwreck of similar age, 1865.
>
>These toilets are composite objects of lead and glazed ceramic with a
>flush made from copper, iron, lead, leather and wood. The toilets are
>currently being desalinated in low chloride tap water (approximate pH
>5.5 - 6) using cathodic protection with impressed current to further
>protect the lead.  However we still have some lead corrosion
>occurring.

The corrosion of the lead in these artifacts is not unexpected. It
is known that lead is very corrodible in soft water, and that
cathodic protection is ineffective in low conductivity media such as
tap water.  I do not have relevant experience but suggest that a
better solution (no pun intended) might be the use of a dilute
solution of calcium bicarbonate (15 mg/liter) which can form a
protective coating on the lead. At this concentration the solution
should not produce an insoluble scale on the ceramic, but the
materials should be observed to confirm this.  It should be
recognized that water that in which lead has corroded may have a
toxic concentration of the heavy metal, and thus may be a hazardous
waste.

Patrick Gallagher
Materials Preservation

                                  ***
                  Conservation DistList Instance 12:5
                   Distributed: Friday, June 19, 1998
                        Message Id: cdl-12-5-003
                                  ***