Volume 16, Number 3, Sept 1994, p.28

Conference Reviews

Colour Measurement

A Course held in the Department of Optometry and Visual Science at City University, London 21-23 March 1994.

This three-day course was very good even though I expected more time devoted to application and practical exercises. It was well organised by Dr. John Sanders and the supplementary materials included a 91 page course outline and the second edition of "Measuring Colour" by Professor Robert Hunt, the main lecturer. The outline included a schedule, a glossary of colour terms, lecture notes, diagrams and experiment notes. The text is a good reference with colour plates and diagrams, and was useful throughout the course. The fee covered the supplementary materials as well as excellent lunches and coffees and teas. There were 19 delegates including managers and researchers from many scientific backgrounds, though Kodak Limited sent five. Approximately half of the delegates attended two additional days of lectures on colour reproduction.

The course was easy to follow because Professor Hunt is an organised lecturer. Topics included terminology, the visual system and colour physics, as well as visual colour matching and numeric systems used to represent colours. Light sources were also discussed with comparisons made between the colour temperature, colour rendering and spectral distribution of different lamps. Other topics included colour atlases, such as the Munsell colour system, and definitions of lightness, brightness, hue, colourfullness and saturation in relation to this system. Problems with colour classification systems were also discussed, such as the manner in which different browns plot on the same point of chromaticity diagrams. The impact of the geometry of the illuminant and viewer was also discussed. Finally, metamerism, a phenomenon familiar to conservators, was explained in relation to metameric matches, or colours that appear the same using one light source but different in another. The definition that I preferred came from the text, and was made by Fairman in 1986: "Spectrally different colour stimuli that are a visual match for a particular real observer under specified viewing conditions.

Since I attended the course because an important aspect of my research involves measuring colour changes in materials, I was happy when the following topics were presented: calculating tristimulus values, colour matching functions, standards and formulas for calculating colour difference. The lecture covering practical precautions in colorimetry, was also informative, and there is a chapter devoted to this subject in the text. Another interesting point involved the relationship of colour perception to the two CIE systems for measuring colour, CIELAB and CIELUV.

Dr. Hunt also discussed spectrophotometers and made a point to say that chromameters are not accurate, but that the readings are precise; in other words, the reproducibility of the data is good. This point suggests that chromameters may not be accurate enough for research involving the measurement of colour changes in materials. Even though chromameters are much less expensive and more portable than spectrophotometers, an investigation is warranted in view of their considerable use in conservation research.

In addition to the lectures, the course included four practical sessions. Unfortunately, I felt that these sessions were the weakest aspect of the course and could have been improved if they had been used to clarify the information in the final lectures. Also, it would have been useful if practical exercises had been used to demonstrate the advantages and disadvantages of the different systems and equipment for measuring colour. Finally, I would have appreciated some exercises involving calibration, methods of sample preparation and guidelines for producing statistically valid data.

Nevertheless, my overall view of the course was positive, even if it didn"t address my specific desires. The course provided a good theoretical background in colour measurement which will aid in my research. The information will also be useful when preparing lectures concerning colour measurement. Perhaps my only unbiased criticism is that the course was very expensive.

I would like to thank The Conservation Unit for providing half of the financial support needed to attend this course. I would also like to thank Derek Gillman, Keeper of The Sainsbury Centre \for Visual Arts for offering me time and financial support, as well as Ruth Goodall, Head of Staff Development and Training of The University of East Anglia, for offering matching financial support that made my attendance possible.

Testing Materials for the Storage and Display of Artifacts

A Workshop at the British Museum, jointly sponsored by The Conservation Unit, 11 and 25 May 1994.

This one-day workshop was organised by Lorna Green and David Thickett, conservation scientists at the British Museum. The course fee included a 30 page course manual with a bibliography, diagrams, materials suppliers and reference photographs for interpreting the metal coupons used in the Oddy test. The fee also covered the cost of materials for the practical session, as well as coffee, an excellent lunch and afternoon tea. There were 12 delegates including conservators, conservation educators and scientists from museums, regional centres and training programmes.

The day was divided into two sessions: the morning began with short talks covering the theory behind the testing programme and the afternoon session consisted of demonstrations and practical assignments. The talks were augmented with slides illustrating objects damaged by harmful gases emitted from materials used in display and storage cases. It was pointed out that more damage was caused by the gases in cases, than those in the surrounding atmosphere, because of the high concentrations that build up in the enclosed case environments.

The morning continued with an overview of the eight tests used by the British Museum to evaluate the suitability of materials for display or storage of artifacts. The speakers emphasized three important considerations when using these tests. First, it is crucial to take a representative sample. In other words, if case construction materials include a wood with a varnish coating that is adhered to a support, the sample tested must include the wood, the varnish, the adhesive and the support material. Second, it is important to consider both the materials used in case construction and those of the artifacts displayed in a case, because artifacts can also emit gases that are harmful to others. For example, wool emits sulphur which corrodes silver and copper, therefore woolen objects should not, if possible, be placed in a case with artifacts containing these metals. Third, it is important to consider all materials in an object when choosing the tests to evaluate material suitability. For example, materials that are not obvious in some artifacts, such as lead pigments in paintings, silver leaf in Japanese scrolls or metal fibres in textiles must be considered. Finally, there was some discussion of the use of sorbents to remove damaging gases from display and storage cases.

The following tests were demonstrated in the afternoon workshop (More detailed information is found in the course manual.):

• The Oddy test is used to detect volatile emissions from materials used in cases holding copper, silver and lead objects, but can be adapted for zinc, magnesium and aluminum objects.
• The azide test detects reducible sulphide gases, such as hydrogen and carbonyl sulphide, which corrode silver and copper.
• The iodide / iodate test detects volatile organic acids which corrode lead or copper.
• The chromotropic test detects formaldehyde which corrodes lead and copper.
• The Beilstein test detects chlorine which has the potential to emit hydrogen chloride and volatile chlorides which corrode copper and iron.
• The pH test is used to evaluate the acidity of materials in direct contact with organic artifacts.
• The phloroglucinol test detects lignin which emits acids that degrade paper and textiles.
• The aluminon test detects alum which degrades paper.

The practical session was held in the Conservation Research Laboratory and began with COSHH and Health and Safety considerations. Before testing, delegates were given a lab coat, gloves, samples, test materials and allowed to choose a lab place. The first experiment involved setting up an hour long aqueous extract for determining the pH of materials. This was followed by instruction and practice with the Oddy test, following the procedure presently used at the British Museum. This session was extremely useful since there are many modified versions of the Oddy test currently practiced and one aim of the course was to teach a standardised version.

After demonstrations of the remaining tests, the delegates were given time to practise their technique on the azide, phloroglucinol and aluminon tests. Delegates were also asked to interpret the suitability of materials after examination of metal coupons following the Oddy testing procedure. Since the test takes 28 days, these coupons were obviously "prepared earlier". Before the workshop ended, Lorna went over the results sheet giving her interpretation of the test coupons. Finally, delegates had a chance to see the British Museum database of materials tested for their suitability for use in the display and storage of certain materials.

The course was a great success. It was well organised, the information was clear and interesting, and the supervised practical was extremely useful. In fact, the only change that these reviewers would suggest involves extending the lab time to allow the delegates to indulge their desire to practise all the tests. Well done!