Tuesday, April 26, 2011

Cannon Ball Conservation

Following on from last week’s call for locals to let us know about the historic objects they may have in their personal care that may require conservation treatment – here is the story of the conservation of some HMS Sirius cannon balls. Janelle Blucher carries out our conservation work – and as you’ll read, it’s been a careful, scientific and long term endeavour.

In December 2002, approximately 20 cannon balls from the HMS Sirius collection were in very poor condition: they were broken, fragmentary, friable and soft.  Some were held together with large areas of wax fill and contained in nylon mesh bags.  They were immersed in a tub of 2% caustic soda solution to desalinate and regular chloride readings were necessary to determine when this was complete.  Well over 8 years later with a few difficulties along the way, we are now happy to report that this treatment is now complete.

Chloride readings are well known to be problematic unless you are fortunate to have access to a chloridometer valued at approximately $6,000, which of course we do not. The original method of obtaining chloride readings was achieved by adding chloride to a solution sample, then adding Merouric Nitrate via a digital titrator until there was a noticeable change in colour of the solution, called the end point.  This method was used for some years however the ‘end point’ was continuously becoming unclear.

The Western Australian Museum suggested using the Quantab© Chloride Strip Method.  A strip is placed into the sample, the fluid rises up the strip forming silver chloride visually seen by a white column.  Once the strip is completely saturated, a moisture sensitive string across the top will turn blue, the reading can then be taken from the peak of the white column, which represents the Quantab© unit value, the value is referred to a table which converts units into salt concentration.  This method offered some success for a couple of years, however these strips are expensive and have a limited range. Unexplained large increases in chloride concentrations began in June 2009 - a battery of tests were carried out to trouble shoot the cause, however none could definitely be established. Contamination of the sodium hydroxide or the water were probable scenarios.

 An alternative chloride measurement technique was recommended by Don Brian, our Science Teacher at N.I.C.S., introduced in 2010 this titration method uses Silver nitrate that reacts with Chromate indicator ions, turning them from yellow to red. This provided an obvious ‘end point’ allowing accurate readings which have now led to deeming the desalination of our cannon balls complete. They are now receiving a number of washes in water to remove any chemical residue. Once this is complete the next step is to look at each one and determine whether wax consolidation or re gluing is required. This is all part of the long term, patient and well thought through work that occurs ‘behind the scenes’ in the museum every day as we care for these highly important collections.

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