A drawing paper sheet is made light sensible by the application of a special mucilage, at low level light. The mucilage is prepared by mixing an arabic gum solution, a coloured pigment and a solution of dichromate. The sheet is dried and exposed to UV light, under a large negative. The exposition makes insoluble the sheet surfaces in correspondence with the negative transparencies. The image is developed in clean water by soaking for 15'-30'. The gum dichromate process is apparently simple. The large number of possible process variations makes it anyhow rather tricky.

This laborious process gives a very ample tonal range. A sheet of drawing paper is covered with a warm gelatine solution containing a coloured pigment to be applied when perfectly horizontal. When set, hang up to dry. At this stage the sheet may be stored for many months or used straight away. The sheet will acquire the sensibility to the light by immersion in a solution of dichromate, letting it dry afterwards.
Expose to UV light under a left-to-right reversed negative. The exposed sheet (# 1) is put to float -gelatine down- on cold water to be renewed until the bichromate yellow stain is removed. A final support sheet (# 2) should have been prepared previously, coating a drawing paper with gelatine. The two sheets -both wet- are packed together gelatine against gelatine, adhesion being improved by gentle squeegee. The pack is placed between two blotting boards for 15'-20' and light pressure is applied by positioning a pile of books on top. The pack is immersed in a dish of water at 38°-42°. When the pigment begins to ooze out around the edges, strip the # 1 sheet and discard. The final image on the # 2 sheet will be splashed with hot water until it clears of all soluble pigment.

Beginners should start exploring this process because of its inherent simplicity. A drawing paper sheet is brushed with a low concentration gelatine solution containing ammonium chloride and hung up to dry. At this stage the sized sheet can be stored or sensitised immediately. A silver nitrate sensitising solution is brushed in safelighting; the sheet is then hung up to dry. Expose under a negative to UV light until a slightly too dark print-out image appears. Wash in running water until no more water milkiness is produced. Fix for some minutes in a standard fixing bath. Wash in running water for 20' and hang up to dry.

Due to the high cost of the noble metals, this process is rather expensive and sophisticated. However the final image consists almost entirely of noble metals, the most stable elements known to man. It imparts this feature to print so it does not suffer from the problems of image instability associated with other metallic processes. Three solution are prepared and stored in dark medicine dropper bottles: A - ferric oxalate + oxalic acid; B - ferric oxalate + oxalic acid + potassium chlorate; C - potassium chloroplatinite or palladium chloroplatinate.
The ferric oxalate commercially available is not always pure enough: it should be prepared by the operator. The sensitising solution is prepared by mixing some drops of A, B and C. It has to be brushed on a drawing paper sheet, which will be dried over radiant heat until crackle dry. Do not overheat. Expose immediately to UV light under a negative. Develop in a saturated solution of potassium oxalate. Wash in diluted hydrochloric acid, wash in running water and hang up to dry.

These exploit the redox system Fe⁺⁺-Fe⁺⁺⁺. Many variations are known, the more typical ones being cyanotypic blue, Van Dyke brown prints, kallitypes and argyrotypes. The blue ones are prepared by brushing a drawing paper sheet with a mixing of two solutions: a) green ammonium iron citrate; b) potassium ferricyanide. The paper is dried with warm air at low level light, avoiding overheating. Expose to UV light under a negative and develop in running water until highlights are cleared. Fix for 2'-3' in 1% hydrochloric acid, wash in running water and hang up to dry.
Many variations to brown printing formulations are known. Among them, a solution of green ammonium iron citrate and silver nitrate, to be brushed on the paper. Dry in warm air and expose to UV light under a negative. Develop in a saturated borax solution, wash for 2'-3' in water, fix in 3% thiosulphate, wash and hang up to dry.

Both processes use a drawing paper sheet evenly covered with a concentrated gelatine solution and made sensible with dichromate. Oilprints need exposition to UV light under a negative, resinotypes need a positive. The exposed sheet is washed in running water until the dichromate yellow stain disappears.
The gelatine picks up water and swells mostly on surfaces less exposed. The swelled gelatine rejects the typographic ink with which the sheet is brushed in order to reveal the image. On the contrary, it retains resinotypic powder.

Suggestive images may be obtained through metal etching. Special light sensible chemicals, known as Photoresists, can protect from etching when applied to metal surfaces and exposed to UV light under a negative or a positive, depending on the process in use.
Toners are additional baths for modifying, even locally, the image tone: the silver black tone of conventional pictures may be changed. Many formulations are known, the recipes of which are anyhow beyond the aims of this document.

The different techniques may be combined: an iron salts print may be used as a base for a gum dichromate one.
The iron salts prints may be easily toned, not only to get a longer duration. Particular tones may be obtained on cyanotypes.
Images may also be modified by local colouring.