>The above device is known in the UK and doutbless the USA as a Hegman
>Gauge. As I understand, it does not measure the size of the individual
>pigment particles, but gives an indication of how well the pigment is
>dispersed in the resin or medium.
This is true and a welcome enhancement of what I wrote: being on holyday
one has to do everything from his mind... The particle size will be
indicated though for well dispersed pigments. For absolute particle size
measurements I wouldn't know which device to use, maybe it can be done with
a microscope?
>The particle size is determined by the pigment manufacturer. Because the
>individual pigment particles tend to stick together due to various
>attractive forces, it is necessary to disperse or break up the pigment
>agglomerates, by 'grinding' the pigment in a mixture of resin and
>solvent. In a ball mill, roll mill, sand mill, or even a specially
>designed high speed stirrer.
>This 'grinding' has the effect (in theory) of surrounding each pigment
>particle with a film of resin, which prevents the individual particles