Those who are unfamiliar with this printing process will probably think I’m talking nonsense when I say collotype is a process for printing in continuous tone without the use of a halftone screen (ie. NO dot).  Collotype also permits the use of highly pigmented inks so that an intensity of color can be achieved which is unimaginable in the world of four-color process.  Unfortunately the process is no longer commercially available, though there are one or two places where it is still done on an art basis.  In the early 70s I was fortunate enough to supervise the printing of what must have been one of the last few books to be printed by this process.  It was a black & white corpus of Roman coins, and the result was certainly impressive; but worth the expense?

One thing people today don’t realize I suspect is how much art there used to be in getting a book printed.  Straight text was obviously easier, but even that required considerable skill in stroking a rather crude piece of machinery so that it would perform consistently.  When illustrations were involved, especially color, there was an almost miraculous quality to the ability of pressman and other skilled craftsmen to deliver the results they achieved.  Over the past thirty or forty years most of that craft skill has been moved from the craftsman’s brain to the hardware and software of the printing equipment, and nowadays we expect printing to conform to our standards almost automatically.  The “craft” involved in letterpress, offset lithographic, or gravure printing was however as nothing compared to the demands of collotype.  In crude folk terms the process involves coating a sheet of glass with egg white and gelatin, drying it in the sun, and using this plate to print using the same grease/water antipathy as offset lithography does.

The word collotype derives from the Greek, kolla, glue.  “Glue type” doesn’t really give one much understanding of what’s going on when we print by collotype, though it does sound quite fun.  Think of traditional offset, with a negative, and you’ve got most of it.  The magic resides in the plate making.  A glass (or aluminum) plate is used, coated with albumen, gelatin or stale beer!  This is to allow the printing substrate of warm gelatin sensitized with either potassium or ammonium bichromate to adhere firmly to the plate.  After coating, the plate is dried in an oven (formerly in the sun): the oven temperature is critical: too hot and the grain becomes coarse causing too much contrast; too cool and the fine grain will cause the image to wash out.  The image, carried on a negative is them exposed onto the plate.  The bichromated gelatin reacts to the amount of UV light, hardening in proportion to the tones of the image.  The hardening makes the gelatin least absorbent in the tones receiving the most light and most absorbent in the areas receiving little or no light.  The bichromate is then washed from the plate leaving only the gelatin.  Just before printing, the plate is covered with a solution of glycerin and water which is absorbed in proportion to the tones in the image: the shadows absorb least, and the highlights absorb most.  Just as on an offset press the wet areas repel the greasy ink and as a result a continuous tone image is printed on the sheet.  The life of the plate is limited, so long runs are not possible.  The maximum might be 2000 depending on a whole range of variables.

An interesting website is where a British research project involving digital printing with collotype quality is reported.  They really can’t mean “collotype” they just mean “continuous tone” surely.