The Art and Science of Platemaking
CTP and DI; "computer to plate" and "direct image." These modern advances in digital imaging make middle-aged printers wax nostalgic about hand developing three-step offset printing plates and having to walk to work uphill in the snow. If we went back another fifty years to the 1950's, we could see a really involved plate making process.
First, a little lithographic history. The earliest lithographic plate was a slab of Bavarian limestone. "Lithography" comes from words that mean "stone writing." The inventor of the lithographic principle, Alois Senefielder, discovered the basic principle that greasy ink and water mutually repelled each other. This happened in the late 1790's. He made, on the stone plate, a greasy image area and a water absorbing nonimage area. When he wet and then inked the plate, in that order, the nonimage attracted water and the image attracted the ink.
It didn't take printers long to replace the bulky stone with a thin sheet of zinc. Rapid change, back then, meant about 100 years. Soon, the printers had adapted the metal plate to a rotary-design press. Ira Rubel accidentally discovered that when he printed on a rubber blanket first and then "offset" the image to the paper, he got a better image.
Early images were drawn on the stone with greasy crayons. Later photographic images were applied using materials that sounded like they came out of the kitchen. Albumin or egg white, casein, a milk product, and soybean protein were all used as imaging materials.
Just before World War II, there was some interest in making a better lithographic plate. The Lithographic Technical Foundation, LTF, was the forerunner of GATF, the Graphic Arts Technical Foundation. In 1953, they published "How to Make and Run Better Zinc Surface Plates." In this technical bulletin were listed fourteen steps.
Step 1 Graining:
The printer receives thin smooth sheets of zinc. In order to hold the water on the nonimage area, the plate had to be grained. This consisted of putting the plate in a box and rocking marbles and abrasive grit over the plates until tiny scratches covered the surfaces. Inspection and brushing followed to remove remaining grit.
Step 2 Counter Etching:
This step cleaned the grained plate to make it receptive to the image coating. The counter etch consisted of a weak solution of hydrochloric, acetic, and nitric acids.
Step 3 Pre Cronak Treatment:
The bare metal was now subject to rapid oxidation and had to be quickly treated with an ammonium bichromate and sulfuric acid bath.
Step 4 Pre Etching:
A thin film of gum Arabic was then applied to the plate surface. The gum used was a common food additive used in drinks and candies. It was the same gum Arabic used by Senefielder in the first plates. Gum Arabic comes from the sap of a North African tree. Farmers cut the bark and the sap leaks out . To harvest the gum Arabic, the desert dried sap is simply broken off. Printers had to hydrate the dried gum in water before use. This also involved a Baumé measurement of the solution for strength.
Step 5 Coating:
Now comes the application of the egg white solution. In some shops a person was designated to actually separate the yolks from the whites. Many shops bought dried egg whites or egg flake. In order to make a coating, the egg powder was wrapped in a cloth and soaked in ammonia water over night. The liquid was filtered to remove bubbles and applied to the plate in a whirler. The plate was attached inside of the whirler to an enclosed table that, by rapidly spinning, used centrifugal force to spread the coating on the plate.
Step 6 Exposure:
With the coating completely dried, the plate could be exposed with ultraviolet light produced by carbon arc lights. The carbon arc exposure unit look similar to some of today's plate exposure units, except the UV bulbs were replaced with two carbon rods, similar to welder's rods. When placed in the unit, the points of the rods almost touched and an electrical spark was forced to jump the gap creating the light. The rods would burn down rapidly and have to be replaced, sometimes in the middled of an exposure. There was also a lot of dirt and smoke from the unit that had to be exhausted.
Step 7 Lacquer Application:
The plate was next brought-up by rubbing a lacquer to bond to the image area. The purpose was to create a harder image than the egg white.
Step 8 Developing Ink:
A greasy ink receptive and water repellent coating was applied. It was called developing ink.
Step 9 Development:
The unexposed area now had to be removed or "developed." This was done by washing the nonimage areas with a weakened gum solution.
Step 10 Post treatment:
Sometimes called the post Cronak treatment. The purpose was to protect the plate and remove any remaining unexposed albumin left on the plate.
Step 11 Powdering:
Like a baby's behind, the plate had to be powdered with French chalk to keep the ink from smearing.
Step 12 Etching:
Gum Arabic and phosphoric acid were applied to make the image areas water receptive.
Step 13 Asphaltum:
Asphaltum was applied to replace the developing ink when plates were stored or not run immediately.
Step 14 Roll-up or Rub-up:
Off the press, the plates were "rubbed-up" or "rolled-up" with a leather roller and press ink to make sure corrections and additions held.
Fourteen easy steps! Some things about the "good o'l days" weren't!
Information on the fourteen steps was taken from "How to Make and Run Better Zinc Surface Plates." by The Lithographic Technical Foundation © 1953. Thanks to W. Alfred McCulloch of Charlotte, NC for the book and tip.