I found this at web , article from 1895.
There are three processes in crystoleum painting — fixing, preparing, and painting— by which anybody without artistic knowledge is enabled to produce life-like pictures which are wonderfully effective, and which have all the delicate tints we see in ivory paintings.
The method is as follows: You have two convex glasses, the small oval or square size -will do for a small portrait, and it is best for beginners to try a small vignette first. When buying your glasses, see there is no flaw in them, and before them thoroughly rub with chamois leather to take off finger-marks.
Now for the first process called fixing. Take an unmounted photograph (which ought to be as strongly shaded as possible), and choose a pair of glasses, sold especially for this purpose, of the nearest size, the photograph to be just a trifle smaller than the glass. Put the photograph in water for a minute, then take it out and press it softly between blotting paper, at the same time leaving it damp. Now with the fixing mixture, sold for the purpose, cover all over the inside (concave side) of the glass, and also over the photograph. Then lay the right side of the photograph towards the glass, putting it on without pressure, and over it a good strong smooth paper of the same size, and begin to rub it with a short broad knife, round which you have smoothly folded a cotton handkerchief, working from the middle towards the rim, proceeding all round, and gradually reaching the border. Be careful to rub everywhere thoroughly, so as to leave no particle of the mixture whatever between glass and photograph. Should the photograph get dry before you have finished, or should you see any shiny little spots (being mixture) on the right side when finished, dip it in hot water and continue rubbing.
We will now suppose the photograph is dry and ready for the second process, or preparing, and for this you fill a saucer of sweet oil and lay the photograph in it. See that it is completely covered; leave it for a week or ten days in the oil, until it becomes transparent; then take it out and dry the oil off it with a linen handkerchief, and proceed to paint.
In the third process the eyes, linen, face, lace, and all jewellery are painted on the first glass— that is, on the back of the photograph, which has been, as described, gummed, or rather starched on the glass. The eyes are the most difficult to do. You hold the glass up to the light, the back of the photograph being towards you, gently touch the eyeballs with the point of your brush, which must be a very fine one. When buying your brush wet it between your lips, so as to see if it comes to a fine point.
For the eyes, oil colours are used.
Blue: Use ultramarine blue, and a small quantity of ivory black.
Grey: Ultramarine blue, Vandyke brown, and silver white; mix together in suitable quantities.
Black: Use ivory black.
Brown: Use Vandyke brown.
After painting the iris, paint in the rest of the eye with white, with the very faintest tinge of Naples yellow in it. Everything that is painted on the first glass is to be clone with thin colour, and the brush to be worked with as dry as possible. Everything on the second glass has to be in as thick a colour as possible.
For blue, carmine and black (which are very thin colours), add, according to the required shade, one that is thick (for this purpose white will prove the most useful.) Some teachers put the lips on the first glass, and it is the easiest method in the end; the colours for flesh, that is, first lips, then complexion.
Lips. — Carmine or vermillion are mixed to the proper tint; colour with silver white. Paint the lips carefully, holding the glass on which the photo is so as to be perfectly exact ; it looks very bad if the least infringement is made on any other part. Always paint the enamels in a good light, and be sure the photograph is perfectly transparent before you take it out of the oil.
Flesh. — Use vermillion, silver white, and Naples yellow; mix to a good colour. For children use carmine instead of vermillion: for dark complexions deepen with Vandyke brown. Before jointing the complexion, you must have all that is to be on the first glass painted — eyes, lips, linen; paint linen with silver white, also lace.
Jewellery: paint gold with Naples yellow, silver with silver white mixed with ivory black. Let this be perfectly dry, then take some strips of gummed paper and gum the other glass, which has also been carefully rubbed with chamois leather to the first glass, the front of the second glass going to the back of the photograph.
Now, after having secured both glasses, so that all air is excluded, turn the back of the glass, and paint the flesh on every part where it is seen; be very careful not to go, say, beyond the face to the background.
After having finished the complexion, go on with the hair as follows: —
Blonde. — Chrome yellow and burnt sienna.
Brown. — Vandyke brown and Naples yellow.
Black. — Ivory black, silver white and ultramarine blue.
Paint carefully, always turning the front of the glass to see the effect of each stroke of the brush. If any thing should go wrong in the painting, a little rectified spirits of turpentine will take the paint out, but in using it care must be taken not to let one colour run into another. The dress is painted on the second glass also; but be certain that the two glasses are gummed closely together, as they will shift about if loosely gummed.
Gentleman's clothes are done also on the second glass, also whiskers or moustache. After the second glass, which, of course, is gummed to the first, is quite, dry, cut a piece of cardboard slightly larger; indeed, it may be better to cut it the same size as the glass, but make sure it is no less. Take some more gummed paper and gum the cardboard firmly on the back of the two glasses which are gummed together. Be exceedingly careful to exclude all air, as it will spoil the enamel. Lay it by in some cool place for twelve hours, and then you will find the enamel completed. All colours should be put on very bright on the second glass, as it comes out refined and delicate on the front, having to be seen through the double glasses.
Observer, Volume 7, Issue 345, 18 July 1885, Page 4
I never saw a more basic coloring and final dramatic rendering. I think as mentioned at Autochrome threads , real success depends on black and white layer. I point you to look above pictures , they reveal lots of things.