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Refractive Indices - Dispersion Staining

For the chrysotile, amosite and crocidolite asbestos fibre types present in the samples, it is not sufficient to say 'unknown mineral fibres' have been found.  This demonstrates that the laboratory method has an inappropriate set of diagnostic criteria, the analyst does not have sufficient experience, or the analyst is not applying the techniques adequately.

Some fibres have adhering particles or coatings (such as bitumen), that make dispersion staining (DS) colours difficult or impossible to observe if untreated, especially if the fibres are very fine.  Clean-up procedures are then necessary, or Becke lines can be used by some analysts, providing that they have the necessary experience in this technique.  If an analyst uses 'false' Becke lines that are present in every sample, they provide incorrect results.  Interpretation of Becke lines is difficult and requires considerable experience.

Many laboratories have a single set of dispersion staining colours for each of the three common asbestos types used in most asbestos products, such as those seen on http://www.mccrone.org.uk/micro_photos.htm, which apply to 'pure' fibres from a single source.  As described in Appendix B of AS 4964, it is important to note that each specific type of asbestos has a range of RI's and therefore a range of DS colours, mainly due to location of the parent ore body, age and weathering of the asbestos.  To further complicate analysis, when asbestos fibres are incorporated into the matrix of a product such as fibres-cement or block insulation, chemicals from the matrix can leach into the asbestos and cause further changes to the RI's of the fibres.  Changes to RI's also occur due to coatings, heat degradation or when chemically affected or modified.  Because dispersion staining 'measures' the refractive index of the outside surfaces of fibre bundles, it is also possible that moisture can cause significant problems in determining RI's.

The best guide to a complete range of DS colours plotted on a dispersion staining Chart is the one that used to be provided with the McCrone dispersion staining objective, which is out of print, but may be used by some Australian Laboratories.  The chart gives'families' of charted data for a number of different sources of chrysotile, amosite and crocidolite, both parallel and perpendicular to the fibre length.  Observed DS colours can be read directly off the chart, or preferably from a table that each laboratory has created by using the chart.

Various tools and resources are also available from McCrone Scientific at http://www.mccrone.org.uk/Documents/2008%20PRICES.pdf.

Several useful papers on DS include:-

McCrone, 'Detection and Identification of Asbestos by Microscopical Dispersion Staining', Environmental Health Perspectives, Vol 9, pp 57-61, 1974, available from http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1475424.

Wilcox, 'Refractive Index Determination using the Central  Focal Masking Technique with Dispersion Staining Colors', American Mineralogist, Vol 68, pp 1226-1236, 1983, available from http://www.minsocam.org/ammin/AM68/AM68_1226.pdf.

McCrone, 'The Asbestos Particle Atlas', Anne Arbor Science, 1980 or later - to be found in some technical libraries.

The McCrone publications provide some information on DS colour ranges for asbestos fibres, but not as complete as the Hartmann Chart mentioned above. 

The DS colours used by McCrone are not as consistent and are harder to use than those provided in the Wilcox paper. Therefore, it is best to combine the Wilcox colours with the McCrone asbestos data.

1. Standardising on Colours and Colour Names

It is critically important that analysts identify and name colours appropriately, and a colour chart reference can be used to aid this process. 

There is much confusion resulting from numerous colour models, colour perceptions and common usage relating to defining and naming colours, and it is recommended that analysts settle on a consistent line of approach.  The following examples illustrate this:-

Violet - actually a pure spectral colour (very strong blue), but commonly believed to contain red and blue. Therefore, best not to use this colour name.

Purple - is a general term for the range of shades of colour occurring between red and blue, and because there is disagreement over exactly which shades can be described as purple, best not to use this colour name.

Magenta - When using the classical Munsell colour system, magenta is called red-purple. In the Cyan-Magenta-Yellow (CMYK) colour model used in printing, it is one of the primary colours of ink. In the Red-Green-Blue (RGB) colour model, the colour created by mixing the red and blue primaries is called magenta, though this colour is different to the magenta colour used by printing companies.  Magenta is better defined than purple, and should be used by describing magenta as an equal red and blue in the RGB colour model.  Further, 'red-magenta' should be defined as magenta plus more red, and 'blue-magenta' as magenta plus more blue.

Blue - because there is so much difference between -light' and 'dark' blue, it is best to treat them separately.

Light Blue - is also known as sky blue.

Mid Blue - is similar to navy blue and mid-blue, and has maximum content of spectral blue.

Orange - is a mixture of red and yellow.

Gold - is a mixture of orange and yellow.

Red - has the maximum content of spectral red

Green - has the maximum content of spectral green.

Yellow - the maximum content of spectral yellow

The following definitions apply to the different colours generally found when using central stop DS.  The RGB colour model is used for this purpose; is explained on http://en.wikipedia.org/wiki/RGB_color_model; is seen in a practical demonstration on http://johncfish.com/bggallery/otherchart/index.htm; and is downloadable in various forms from http://rgb-color-model.downloadsoftware4free.com.  Note that 'red', 'yellow' and 'blue' are mainly provided because they form the base of the RGB colour mode l:-


Colour Description
R
G
B
R
G
B

Lower Limit
Upper Limit
red
255
0
0
255
70
0
yellow
240
255
0
255
220
0
green
130
255
0
0
255
110
light-blue
170
210
230
70
200
240
mid-blue
0
160
255
0
0
255
orange (red + yellow)
255
100
0
255
160
0
gold (orange + yellow)
255
170
0
255
200
0
blue-green
0
255
170
0
255
200
magenta (blue + red)
255
0
240
255
0
255
red-magenta (magenta + red)
255
0
90
255
0
160
blue-magenta (magenta + blue)
90
0
255
190
0
255



The differences in the appearance of colours amongst most modern computer screens are not significant, especially if they are reasonably well 'calibrated', and result in the colour chart being reproduced on screen sufficiently accurately for our purposes.

If not already done, the monitor should be calibrated prior to viewing the colours. See Microsoft website for a typical PC monitor calibration of Windows XP, and http://www.apple.com/pro/photo/colorsync.html for pointers on calibrating an Apple monitor. See the attached Excel spreadsheet (Colours) for examples of the above colours and colour ranges.  This spreadsheet should not be printed, because of various fundamental and incompatible differences of monitor colours (usually based on additive models) and printed colours (usually based on subtractive models).  In practice, printed colours are significantly different to those observed on a monitor, especially blues and magentas. 

When observing DS colours, the analyst must be careful to remember that the image of the fibre is not 'true-to-life', but rather a set of colour images resulting from refraction of white light into its component colours, superimposed onto dark field conditions which cause diffraction patterns to 'break-up' the refracted image of thin fibres. Hence, the colour images will generally be in bands parallel to the sides of the fibre, and if anything, the outermost colours should be chosen as the DS colours for the fibre in each of the two orientations. The periodic use of 'standard' asbestos fibres in relevant RI oil is important to ensure that analysts stay 'calibrated' to the range of colours present for 'pure' asbestos types.

Great care should be exercised when analysing ores because it is not uncommon for asbestos fibres in some ores exhibiting different optical properties than 'classical' pure asbestos fibres.  Secondly, some non-fibrous bladed or acicular material can give identical DS colours to asbestos, and should be rejected because the morphology does not match the AS 4964 definition.  If in doubt, ask another analyst or laboratory to analyse the sample.  Be mindful of the limitations of alternative methods such and XRD and IR, as noted in Appendix A of AS 4964.

Please note:  The web based links used in this document are examples only, and their accuracy or otherwise is not guaranteed. The links were active at the time of publication, but may become inactive in time.