Plaster printing has been around for quite a while, but unfortunately I don't have knowledge of the artist who actually invented the process. My friend and fellow printmaker, Sarah Sanford, who attended Edinburgh University in Scotland where this technique is popular, deserves credit for explaining the basic process to me.
I have made quite a few plaster prints but still feel like I am in the experimental stage with the process. Below are some of my findings and a description of the basic technique for intaglio plates (you can also use this process with pronto plates and monotypes).
Plaster Cast Etching A nice impression of etching plates, pronto plates, and even monotype plates can be made by making a plaster cast of the inked up matrix. The sensitivity of the plaster material can capture incredibly subtle nuances and details in an image, and is particularly well suited to images that have a wide range of values. This is a process that bridges the gap between sculpture and print.
Materials needed for etching plaster print:
plaster of paris
2 mixing containers (bowls or pails)
oil base printing ink
Tarlatan and ink card for wiping etching plate
plastic sheeting or saran wrap
How to construct a mold:
In order to carry out this process, you need a strong mold that will fit your intended plate and contain your plaster. You can make a tight frame out of wood that your plate fits snugly into. This type of mold is necessary if you are making borderless pieces that resemble tiles. The depth of this frame depends on how thick you want your finished piece to be. Usually, about 2 to 3 inches deep is sufficient. The Instant Printmaker manual by Melvyn Petterson and Colin Gale provides really good instructions on how to fashion a wooden mold.
You can also use a cardboard box or a cake pan (above) as an "informal" mold. If you use one of these types of mold, the plate may not fit snugly against the sides but, as long as you don't mind a plaster border around your image, this is a fine way to go.
Whichever mold type you decide upon, it is imperative that the bottom of the mold is flat. You may also wish to line your mold with cellophane or thin plastic sheeting to help contain your plaster just in case there are any weak areas. The plastic also makes it much easier to remove the hardened plaster. I like to leave a bit of plastic hanging out of the top of the mold, so that I can simply lift the entire piece out by the plastic when it is finished (instead of having to chip it).
Inking up the plate: No special procedure is necessary for the inking up of the intaglio plate for plaster. Simply wipe it the same way you would if pulling a print onto paper. As always, be careful not to over wipe your plate or the richness of your tones will be reduced.
Place the inked plate face-up in your mold.
Safety Note: mixing plaster can be dangerous and heat producing! Wear protective gloves and follow professional safety instructions given with your product. Study such instructions (also search online for 'plaster safety') before using this method, as the safety description given here is brief and incomplete.
Mixing the plaster: Most plasters work just fine for this project. I have used cheap plaster of paris from Lowes as well as "fancier" hobby and craft casting plasters. They all seem to work fairly well.
Mix the plaster you select according to the ratio of water to plaster directions on the bag.
If you are only doing one small etching plate, you will not need much plaster. There are a lot of different ways to mix plaster, but you should always use clean water at room temperature. One method is to pour your plaster into the water and let it set for a few minutes before stirring in order to let the water fully permeate the powder. I like to gradually mix my plaster into the water a little at a time, stirring continuously with my hand or a wooden spoon, to prevent lumps. I keep stirring until the plaster is the consistency of thick cream.
When it is the correct thickness it should look as if you plunged your arm into a thick milk shake. Once it reaches this consistency, gently pour the mixture into your mold, which contains your ready-to-go, inked up plate. If it is a larger plate, you may want to dribble the plaster from side-to-side over the entire surface as evenly as possible.
While the plaster is still wet, some artists press cheesecloth into the back of the piece to provide extra strength and to aid in evenness of shrinkage. This is also an excellent time to press hangers into the piece.
To get a good impression, you need to let the piece sit for a minimum of 4 hours. A larger piece may take longer to set and dry. I usually let mine sit overnight if possible, just to ensure the richest possible image. If you rush it, parts of the print may not transfer onto the plaster.
Once the piece is dry and set, you need to remove it from the mold. If you used the plastic sheeting and left a little hanging out of the mold, it is easy just to grab it and lift the piece out this way. In some instances, you may have to chip your piece out of your mold.
Once it is released, gently scrape off the plaster that may have leaked onto the back of the plate and pry off the plate. Hopefully the result will be an excellent impression of your image. Remove the ink from your etching plate using your preferred method.
The plaster usually takes a day or two to completely harden, so it may feel a little moist. This is a good time to carve into the block or embed objects into it if you so desire.
Although I haven't included instructions here, a plaster impression can also be made of a pronto plate, a photo litho plate, and a monotype. You just need to make sure that each is charged up fully with ink before pouring your plaster. Pronto plates in particular are wonderful for this process, as they are so versatile and economical, and really well suited to an artist who prefers to work more photographically.