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.
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
from re-agglomerating.
Refer to any good book on surface coating technology for a fuller ( and
doubtless more accurate) description of the process.
It would seem to me that as the gloss of a paint or printing ink is
determined largely by the degree of dispersion of the pigment in the
medium, so the sharpness and brilliance of a gum print etc would be
influenced by how well the pigment is dispersed in the gum arabic.
I would be surprised if water colour pigments are particularly well
dispersed. Has any one carried out any tests? I haven't done so myself
as of yet. My Hegman Gauge having vanished into my random access storage
system!
Alex.
Alex Nanson
Gloucester UK