Preservation Analysis and Treatment of Dutch Navigation Charts
Tension drying of a map
Attaching threads
The end result framed and ready for display
The preservation department at the National Library of Australia recently undertook a major analysis and treatment project of four Dutch East India (VOC) navigational charts on semi-permanent loan from the Kerry Stokes collection, as discussed in a previous article ‘Loan of Dutch Navigation Charts’ in the October 2009 issue.
The four charts are written on parchment with various inscriptions. Before commencing treatment, the charts were examined under the microscopy, infrared (IR) and ultraviolet (UV) fluorescence light sources and X-ray fluorescence (XRF). Examination enabled conclusions to be drawn about the date and composition of the parchment, pigments and ink.
Parchment is made from animal skin that has been treated to create a smooth surface for writing. It is durable, flexible and easily rolled for transport, making it perfect for use as voyage charts. These charts are probably calfskin, due to their size and colour pigmentation.
XRF spectroscopy was used to discover the elementary composition and identification of the pigments. XRF identifies the inorganic elements contained in pigments. Analysis showed a mercury peak in the red pigment, suggesting it to be vermillion (mercuric sulphide). The green pigment showed a strong copper peak, from this result the pigment is malachite (copper carbonate) or verdigris (copper acetate). The blue pigment also produced a strong copper peak, confirming an azurite pigment (copper carbonate). The yellow pigment had no clear spectra on XRF, indicating that it was organic and required further testing. The possibilities for organic yellows based on the time of manufacture were gamboge, Indian yellow and saffron. Observations under UV light were unable to differentiate between the different yellows. From conservation literature we understand that gamboge is pH sensitive and will turn orange-red in an alkali solution. A spot test was performed on the yellow pigment, resulting in a positive test. Afterwards, acetic acid was dropped onto the orange spot to return it to its original colour. The purple pigment showed no clear XRF spectra and is still unidentified.
The inks were believed to be iron gall, due to its age and brown colour. Iron gall ink is ferrous and made from the galls of oak trees. XRF testing produced a strong iron peak, confirming our initial observations.
Identifying the pigments is interesting from a historical point because it can help date the charts as well as showing the pigments available and that were readily traded by the Dutch East India Company. It is also important for the ongoing preservation of the charts as it identifies pigments that may be more fugitive than others and steps can be taken to ensure their longevity.
After analysis, the charts were prepared for display. The treatment included tape removal, humidification, flattening and tension-thread mounting.
Three of the four charts had previously been mounted using adhesive tapes on the verso. A toluene poultice was used—a poultice is a gel like substance that holds the solvent and gently releases the solvent onto the area over a period of time. The poultice was placed over the adhesive and left for a few minutes; this softened the adhesive so it could be scraped off the surface of the chart. This work was completed in the fume hood to reduce staff exposure to the solvent.
The charts required flattening before they could be prepared for framing. This was completed using a GORE-TEX® humidification sandwich—GORE-TEX® is a material similar to rain jacket fabric. The GORE-TEX® allows water vapour through the fabric but not large droplets, thus preventing damage to the object. The charts were placed between two layers of GORE-TEX® which had damp blotting paper on each side. Due to the thickness of the parchment charts they were left in the sandwich for two to four hours. They were then pulled out to dry using a tension method. Bull clips were placed around the edges of the chart with a layer of Reemay® and blotting paper in between. Rubber bands were tied to the clips, pulled tight and secured onto a board. The charts were suspended flat by the clips as they dried. This process was completed up to six times before the flatness of the charts was suitable. There are still some minor distortions within the charts due to the nature of the parchment and as they were working charts they will never sit completely flat.
The charts were mounted using a tension—thread mounting technique. Twisted linen threads were cut and washed into one-metre lengths. One end was then adhered to the edge of the back of the chart at 10 mm intervals using a dilute wheat starch paste. The threads are designed to allow some lateral movement as the skin moves in response to its environment. In the event of the skin moving too much the threads will pop off, releasing the tension and preventing any damage to the chart.
The backing board was prepared by punching holes through it corresponding with the attached chart threads. The chart was placed in the centre and the threads were pulled through the holes to the back of the backing board. The chart was flipped over so the loose threads were visible on the backing board. A thread circle was created by folding, twisting and tying a length of linen thread. The thread circle was placed in the centre and the loose threads were slip knotted around it. The slip knots were tightened and tensioned. The thread circle allows all the knotted threads to tension off each other, holding the chart in position.
The chart was then flipped back to its front. Two layers of Coreflute® board were placed around the edge of the chart to create buildup for the window mat to sit on. This allowed the window mat to cover the holes on the backing board without putting any pressure on the strings. It was important that no pressure was placed on the strings as this would prevent the lateral movement the strings allow as the chart expands and contracts. The buildup was also placed on the back before it was framed in a deep custom frame.
This was an exciting and time consuming project undertaken by the preservation staff of the Library. The treatment was a success and the charts look terrific in their new housing. They will be on display in the Library’s Treasures Gallery next year—so keep an eye out.
Alexa McNaught-Reynolds
Paper Conservator, Preservation Services



