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Here is a method of backing etching plates, devised primarily with economy copper plates in mind. It's fairly simple, uses common materials, is removable, and offers good enough protection that if you want to you can turn the plate over and use the back side to etch an image on.
Tools and Materials:
1. Asphaltum (pure, Senefelder's, the really thick stuff)
2. Kraft paper (brown, comes on a large roll - 35 lb. weight is good, other weights will also work)
3. Reynolds "Parchment Paper" (also called "baking parchment"), from the aluminum foil & waxed paper section of the grocery store
5. plastic Bondo spreader (often available at automotive stores) or something similar (e.g. piece of mat board)
6. Brasso (metal polish)
7. razor blade
8. newspapers and/or scrap cardboard, to protect work surface
9. masking tape
10. heat source, for heating the plate (hotplate or grate & gas flame, etc.)
11. electric iron (like you use for ironing clothes)
Get a piece of Kraft paper that is larger than the plate to be backed. Lay the plate on it and trace a line around the edge of the plate to define the shape & size of the plate on the Kraft paper. Tape down the four corners of the Kraft paper to the cardboard or tabletop so it won't try to wrap around the brayer. Lay the plate face down on some newspaper. Dab some asphaltum on the back of the plate, and on the plate outline on the Kraft paper. With the brayer, roll out the asphaltum in an even film until both the back of the plate and the outline on the Kraft paper are completely covered. Coverage should extend a half inch or so beyond the outline on the paper. Thickness of the asphaltum laid down by the brayer is significant. Be generous, but don't wallow in it. There should be a fairly heavy layer of asphaltum covering every bit of both the paper and the back of the plate.
Allow both items to thoroughly air dry. This can be accelerated by gentle heat (placing in the sun), hair dryer, fan, etc. It's dry enough when it's no longer tacky. Success of this process depends on not applying too much heat to the asphaltum until it is dry enough to take it. Thoroughly dry or baked asphaltum can take very high temperatures, but if it isn't dry enough you get blistering, slippage, or indentations.
When the asphaltum has air dried, plug in the electric iron and set it on a hot setting, like "linen" or equivalent and let it heat up. Pick up the plate, turn it over and place it on the coated area of the Kraft paper, so the two asphaltum surfaces are together and there's some asphaltum showing around the edge of the plate as it lies on the paper. Remove the masking tape and holding the plate and paper together so they don't move, turn them over so the paper is on top. Lay them on the cardboard or folded newspapers. Tape the corners of the Kraft paper again so it won't move relative to the plate. Place a piece of the parchment paper over the Kraft paper for a protective/release layer, since some asphaltum might come through the Kraft paper. (Nothing will stick to the parchment paper.) Using the iron, firmly iron the Kraft paper on to the back of the plate. Try to push air pockets out to the edges of the plate with the iron. This step both melts the asphaltum, causing the paper to fuse to the back of the plate and bakes the asphaltum, making it harder. You can do this several times, and in so doing will progressively bake the asphaltum harder making it more ready to take the heat of baking an aquatint.
After ironing the paper on to the back of the plate, seal the paper by spreading a thin layer of fresh asphaltum on it with the spreader or mat board. The idea is to scrape fresh asphaltum across the paper sparingly so as to fill and seal the paper fibers but not leave any buildup or layer of asphaltum on the back of the plate. Go over it with parchment paper and hot iron.
Thicker asphaltum and paper takes significantly longer to dry and bake to the required hardness than thinner paper and asphaltum, though it offers more protection against scratches. It also takes longer to remove.
When the paper is well stuck down to the back of the plate, trim the paper to within 1/4 inch or so of the edge of the plate. (You could trim it to the edge of the plate, but when you get it hot enough to cook an aquatint it will shrink a little bit and pull back slightly from the edge of the plate. This may or may not be significant.) For the final bake of the preparatory process, put a few crumbs of rosin on the plate and gradually heat it until the rosin melts. If you are using a hot plate, you may want to put a piece of parchment paper under the plate to keep asphaltum from getting on the hot plate. Allow it to cool down. You can now finish trimming the edges of the paper, and remove the rosin with some alcohol and proceed to do your etching on the plate. When heating the plate to cook an aquatint, apply heat gradually. If blisters appear while cooking an aquatint, go ahead and do the etching of the aquatint and when you next clean the plate you can flatten the blisters by ironing. (You can iron down blisters with a rosin aquatint on the plate, but you have to be careful not to get the plate hot enough to melt the rosin.) If the backing is too directly exposed to a flame, it will blister. To avoid this, I'd suggest using a pad made from several layers of metal window screen between the flame and the plate backing.
To remove the backing, lay the plate face-down on a level surface on some newspapers. Swamp the back of the plate with some Brasso and let it sit for a half hour or so. You can peel the paper off and finish cleaning the plate with more Brasso or solvents. (If you put it on thick, it will take longer.)
Obviously, while working on the plate you will want to be somewhat sparing in exposing this backing to solvents, especially around the edges. However, once you have removed the backing from a plate and seen what it takes to get this stuff off, you probably won't worry much about spilling some solvent on it.