Each dial is carefully cleaned and examined in different light sources (including ultra-violet) to get the maximum amount of detail before work begins.
The dial is then painstakingly worked back up to a crisp finish, using original and modern materials, adding nothing more than necessary.
The earlier painted dials (from the beginning to around 1840-50) generally use Indian inks. The colour base for Indian ink is Lamp black / soot (carbon particles). Lamp black is not soluble in water and therefore was traditionally bound with gum arabic or animal tannins/gelatins (often horse). Once bound and dried into cakes however, it does become soluble in water.
This gives the “painterley” quality of early dials, in that you can attain any degree of blackness by simply painting using a thicker constituency. Particle analysis has shown that some earlier inks used for clock dials also contained some bichromate of ammonia. After exposure to sunlight this water bound ink becomes in part an insoluble compound (does not smudge or wash, but must be polished off) which explains why sometimes when the ink is removed from a dial there is a definite “shadow” beneath.
Post 1840 there is a increasing tendency to use ammonia based aniline dye inks, which are much like the ink in your fountain pen (smell fountain pen ink and you will recognise the ammonia). A slight reddish hue, especially on worn dials, is imparted by over-concentration of the dye which results in it crystallising on the very smooth surface, rather than lying in a smooth block of black colour.
In the light of this, we usually use Windsor and Newton acrylic ink. Mindful of conservation concerns, this is easily removable by a conservator should that ever be necessary, and tends not to leave staining. It can also be applied slightly gloss or slightly matt. Those who undertake their own dial restoration will find that the “painterley” thickness they seek can be obtained by allowing the ink to dry out somewhat before use.
All these fall under the term ink by nature of their being in aqueous binders rather than in oil.
Likewise we do not lacquer or varnish the whole of the dial for no good reason. Some early dials were lacquered across the whole, as were some later ones. Many only used varnish on oil painted corners or arches. Each dial must be assessed according to how it was originally made. Again, we use Windsor and Newton artists re-touching varnish for its excellent quick drying nature, coupled with the ease with which it can be removed at a later date.