Silicones are organic polymers, molecules that consists of mainly of silicon and oxygen, which can be modified to produce a great number of wonderful products used by us daily. As printers we are only interested in the common caulking silicones that are used in the construction industry as sealers and adhesives. Even these come in a number of varieties that can confuse even the most chemically gifted, so I will refer to only two main types that I have used. The caulking silicones we are interested in are the unpaintable variety, and this should be noted somewhere on the container. I would suggest you purchase yours as cartridges to save money, instead of small tubes, as you will likely go through a fair amount if you print regularly. These products are designated as RTV, for Room Temperature Vulcanizing. They cure by absorbing moisture from the air, which is used as a catalyst. While two part silicones are available, these have not made an impact in printmaking.
Another type of RTV silicone does not produce acidic acid and known as neutral cure and these are becoming more common. While promoted as having better adhesion to painted surfaces and plastics, I have found them to not be quite as good at rejecting ink.
There are many silicone manufacturers worldwide so you will have to test which is best for your purpose. One thing to consider in purchasing the cartridges, is the type of closure being used. In the past the tip of the nozzle was cut off and hopefully used within a short time. This allowed air to interact with the exposed area and eventually cause the remaining material to harden. Today most companies have gone to a screw-on nozzle that is a great help for printmakers. You don’t cut off the tip of the nozzle - just the end of the cartridge and replace the nozzle after forcing some silicone into the base of it. This keeps the silicone free from hardening, especially if the cartridge is stored in your freezer. Silicones have a shelf life and this helps. Of course you will need a cartridge gun to easily dispense the silicone into a container.
To dilute the silicone to a viscosity that is easy to spread and buff, one has to use hydrocarbons and the one I suggest is odorless paint thinner. This solvent has had the toxic aromatic portion of their hydrocarbons removed, making this the least dangerous material to use. While common odorless solvents should be available in hardware and paint stores, for some reason this is not common in other countries. There is a more expensive version sold as odorless turpentine in art stores, but it seems to be the same for for diluting silicone as the paint store variety. In a pinch, common paint thinner will work but can affect some drawing materials with the remaining aromatic hydrocarbons.
You should find a small wide mouth bottle holding about 3-4 ounces and with a good airtight top. I prefer those with a half twist closure rather than screw on tops, because with use there will be cured silicone getting into the threads, making it hard to open and close the container. One type of baby food jars are perfect and can be reused if the silicone can be removed as time goes on. In humid areas, faster curing of silicone on the plate and in the closed bottle will have to be considered. One may place marbles into the partially used container to keep the air space as small as possible. They should be able to be reclaimed when mixing a new batch. I have found that unused diluted silicone will keep fluid for a long time if air is not admitted, but as one makes plates, this is impractical and so you will throw away a considerable amount of your silicone if you only print occasionally.
Squeeze about a heaping teaspoon (5ml) of silicone into your container and add about 1/4 ounce (~7 ml) the solvent and stir. Do not add too much solvent at first as you will not be able to confine the glob of silicone and break it down. To make mixing easier, I now use one of the new motorized froth makers popular to make cappuccino coffee. You should remove the spring like wire first as it will just become clogged and not be effective unless cleaned. To get a bit of action from these fast revolving units, I just bent the circular section so it is no longer sitting at right angles to the shaft. If you make too large an angle and happen to get a very fast motor, this can throw out solvent from the container as it gets fuller. After the glob has been broken down to be smooth and less viscous, add a small amount of odorless and keep stirring. Do this until you have added anywhere from 1 to 1.5 ounces (30 - 45 ml) solvent. Depending on the viscosity of the silicone, the final fluid should have the consistency of a light cream or salad oil. If you are using grained plates,the viscosity should be higher than for smooth backs of recycled plates.
Unless you have an unlimited supply of containers, good ones can be recycled as the silicone builds up on the inside and closure. I use a small quantity of lacquer thinner for the inside and let the container sit overnight. Next day the broken down material can be scrapped out and the closure cleaned off with coarse steel wool or a wire brush.
Coating the Plate
Place the plate on a flat surface with pieces of newsprint or large scrap paper underneath it, with enough paper sticking beyond the plate to keep the table clean. Pour a small amount of silicone unto the centre of the plate and spread it with a piece of scrap foam urethane, making sure it doesn’t remove any drawing materials. This can be noticed by a change of color if toner or dark pencils are used. Unfortunately damage to the image may have already been done - but not in all cases. You will have to remove much of the excess silicone before buffing takes place, so I use previously used facial tissues as there is still much area with no cured silicone around the edges. These used tissues are placed in a container and added to as each coating session ends. A handful passed over all the plate removes much of the fluid but there will be ridge marks when viewed by reflected light. For the final buffing, use a piece of soft foam urethane about 3 x 5 inches (8 x 13 cm) size and cover one side with a double layer of facial tissue.
It is important to leave nearly a mirror smooth surface as any ridges of silicone can produce tinting in that area. I suggest you place a light near the coating area so you can observe the buffing operation more carefully by reflected light. While silicone cures with humidity, there is still the odorless paint thinner that has first to evaporate. If possible, just set the plate aside and let it cure overnight, which will produce an effective ink rejection film. With the very thin film of around 3 microns, curing is fast after the solvent is gone. For a immediate use or research, dry the entire surface with a paint stripping gun by slowly going over it side to side, much like mowing your lawn.
Washout of the Image
Depending on what you used for the image, you will have to use the appropriate solvent. Water is preferred, but only effective with glue binders and some pencils. For toner and lacquer based marker, acetone has to be used. Because it evaporates so fast, it can redeposit the plastic material unto the silicone and harden. This requires more acetone that I like to avoid. To prevent this, put in a retarder to keep the dissolved drawing materials soft until the plate is washed with plain or soapy water. Of all the materials I have tried, the most convenient and completely safe product is polyalkalene glycol, making up nearly 100% of DOT 3 and 4 brake fluid. Only about 5% is added to acetone to be effective as too much reduces its’ ability of washing out the image. I prefer the convenience of facial tissue for removing the image. Washout the image just before you are going to print as some silicones have enough free silicone oil present that can migrate into the finer open areas of your image and blind them.
Copyrighted Nik Semenoff 2008