Reusing Aluminium Lithographic Plates
Aluminium is the metal of choice amongst North American printmakers and commercial offset printers. For waterless lithography, this turns out to be a very lucky decision because of the strong bond between aluminium and silicon molecules. While most art printmakers who use metal, are using ball grained plates because of the necessary texture for drawing and for the amount of water they can hold in traditional gum-etch processes, the grain is not as important for waterless techniques. In fact, much better fine detail is possible from smoother plates, as has been discovered by commercial offset printers. For years I have been using quite smooth commercial plates for my waterless process, doing toner washes and flats with Sumi ink, because of the superior quality of the image -- not to mention the saving in plate costs. Because of the many plates I use in the multicolor editions I print, I have been looking for some way to reuse plates, in the hope to save money. I had found that all plates could be stripped of the silicone coating and reused, but the need to use a stronger silicone stripper, made this technique less desirable. I looked for a way also to reuse the many ball grain plates I had on hand, trying sandblasting and graining with levigators or a woodworkers palm sander. None of these seemed the best approach because of various problems that arose. Finally, after much experimentation and many attempts, I have found a simple and effective method. I have included a simple explanation of the technique in the previously updated paper on my waterless process.
Problems to overcome
Because I have found that the smoother the plate, the better the image in most cases, I looked at the back of plates as a source of metal. This would also overcome the deeper grain that had to be eliminated by working down the surface. When one tries to work with the backside, one soon finds that water is rejected because of dirt, oxides and other things that accumulate on used plates. Trying to rid these with counter etches did little good, and regaining with silicone carbide was very slow, as well as more damaging to the thin plate. Using gentler kitchen abrasives alone, similar to the technique of preparing intaglio plates, was not successful, because of the larger size of litho plates and the contaminates. The main problem was to find a method of removing the debris so that water would take to the surface.
Success at last
Since I am in the practice of using sodium silicate or sodium metasilicate on the aluminium in areas stripped of silicone and redrawn, I had these chemicals on hand. They were used to form a thin layer of aluminium silicate, to which the caulking silicone bonded much better to form the ink rejection layer. These chemicals are quite alkaline, therefore the metasilicate can be found in many industrial cleaning compounds since the use of phosphates has been curtailed. I first remove all dried ink with hydrocarbons and/or lacquer thinner to make sure the surface would be as clean as possible to accept the water based solutions. I found that by applying a dilute solution of either silicate, which had a pH over 10, the surface of the metal could be quickly moistened very effectively. If I added some scouring powder like Comet or Old Dutch Cleanser, the preparation of the metal was improved. Using a special constructed pad, I could quickly abrade the entire surface and be ready for imaging. All I had to do was to flush off the silicate solution and scouring powder, then dry the plate for use.
Amongst the many used plates I have accumulated, there were some with surface blemishes that got there from chemicals used in processing traditional litho plates. To improve the surface of these plates and maybe get some grain for use with Omnichrom pencils and my toner chalks; I used 3M scouring pads. They come in different grits that are bonded to some sort of felt-like tough fibers and are very effective for my purpose. Using my special pad as a backup for the 3M material, I could get a satisfactory grain for use with drawing materials. After flooding the back of a plate with the silicate solution, I would simply rub the surface with the pad, trying to keep a random pattern to the grain; or sometimes a grain parallel to one of the edges. Because the grain is not anywhere as pronounced as in ball graining, the pattern doesn't become a problem with any of the drawing materials I have tried. These would be considered very light grained plates by any standards.
Another approach I have tried with success, is the use of pumice, silicone carbide or aluminium oxide in place of kitchen scouring powders. These coarser materials are very effective in putting a bit of a grain on the surface, and also in helping to remove the more difficult blemishes on the metal. They also are useful in dislodging silicone, when you decide to reuse the plates again.
Drawing on reused plates
Because of the smoother surface, some drawing techniques are better -- some are worse, depending on the material. I often use mechanical pens (Rapidograph) filled with Sumi ink to get line images or to touch up areas. These work much better on the smoother surface. Omnichrom and other drawing pencils will take to the slightly grained plates, and not show any of the grain direction. There are a number of different water-soluble drawing pencils that work in my waterless process, other than Staedtler Omnichrom. All the other brands have to be heated to make them bond to the metal surface and prevent them being dissolved by the odorless paint thinner in the diluted silicone.
Toner washes on very smooth surfaces are different than on grained ones. They tend to reticulate more on smoother surfaces as the electrical characteristic of the particles comes into play. On coarser grained surfaces, the toner particles descend into the grain and are held until the water evaporates; on smooth surfaces they are freer to move. Toner chalk drawings can be effective if the grain is coarse enough, but this requires that the coarser 3M pad be used.
The technique of transferring toner images drawn on Mylar (see toner paper) gives mixed results on very smooth surfaces; I would suggest abrading the metal with 3M scouring pads to get some grain to hold the toner. Even toner chalk drawings on frosted Mylar are good on the more textured surface, but the best images are toner washes on smooth Mylar. Just make sure enough pressure is used to temporarily adhere the toner particles to the plate, until heat sets them properly.
Sumi ink brushwork is better on smooth plates and very effective. Since I use a fair amount of flats in my editioning, this technique is an important part of my imaging. I outline the areas with Sumi ink with different sizes of mechanical pen, before filling in with a brush. I use a water soluble glue that is diluted with water and some Sumi ink, as the black color is only needed for visual considerations.
Using water-soluble block printing inks, I have found that it is possible to transfer images much like in traditional lithography. I have used it by picking up textures from wood and other materials and transferring to the smoother plates. After the ink is dried by heat, it will mask out the silicone coating and later be removed with water to reveal the metal for printing. Pick an ink that has a long enough open time to allow the transfer, or add glycerin, glycol or other retarders to give you the needed interval.
I have discovered a way to make my own positive waterless plates that relies on the smoother surfaces of the back of plates. This will be discussed in another paper.
Siliconing the plate
With a smoother surface, the silicone layer does not have to be as thick. This means a more dilute solution can be used, which also allows the plate to cure more quickly. Because there is a thin layer of aluminum silicate to which the silicone rubber bonds, the plates are just as effective as the others. If you want to double coat the plate, special precautions may have to be taken. Since any hydrocarbon will affect the silicone coating, even the gentle odorless thinner can strip off your first layer on the smoother plates, especially if the second coat goes on too soon after. To prevent this, I do not use foam rubber pads because of their tendency to act like an eraser and grab at the silicone. Instead I use a folded facial tissue, which is gentler on the previous layer. I also buff with less pressure. Once the silicone has cured, it seems to hold just as well as on ball grained plates.
Reclaiming the plate again and again
The purpose of all this is to make use of the plate as many times as possible. Because of the very thin silicone layer and the aluminium silicate underneath, it is easy to reclaim the plates, as the layer seems quite susceptible to removal with weak acids. I use a counter etch made up of hydrochloric and phosphoric acids, with a very small component of hydrofluoric acid. About two ounces each of hydrochloric and phosphoric acids are added to a gallon of water. To this is added about a tablespoon of ammonium bifluoride crystals, to help destroy the silicone layer. This should result in a solution with a pH of around 1.5, depending on the concentration of the acids you use. Even weaker solutions have worked for me and I would recommend you could dilute this formulation even more. I have used a number of aluminium cleaning solutions in place of pure phosphoric acid as these contain the acid as their main ingredient. The very small amount of hydrofluoric acid in solution should not be a concern, as long as gloves and other standard precautions are taken. I add some red food coloring to this solution to indicate the danger of the chemicals being used.
I have found on the market some effective products to strip of silicone. Sold in automotive supplies shops, these chemicals are meant to be used as a cleaner for magnesium wheels, and contain a small amount of hydrofluoric acid. On testing the solution, I found it had a pH of about 3.3, which is not that acidic amongst the other chemicals one uses in traditional lithography. One product is called "Mothers Wheel Mist", and the other "Eagle One Etching Mag Cleaner". I have used the Eagle One solution and found it can be diluted by three parts water, and still be effective. I would suggest pouring on the liquid rather than spraying, so that the risk of inhaling the HF molecules is reduced.
Because of the concern in using any solution with fluoride, I have asked our chemistry department to analysis the commercial cleaning products and the resulting diluted one made from these. While the original products contained less than 1% HF, the diluted stripping solution tested at .36%. This would depend somewhat on the particular product you use, as the content of HF varied from .9% to undetectable amount. You could try Eagle One, which has .9% HF, then dilute the bottle with two liters of water. To this add 3 oz. phosphoric and 4 oz. hydrochloric acids, to produce a solution with about 2.9 pH. The hydrochloric was plain 27% commercial muriatic acid. Depending on the silicone coating, you may have to slightly adjust the content of HF by changing all the others.
While the analysis shows there is very little HF in the working solution, it does not mean that one should be careless when using it. Protective gloves and eye shields should be used to prevent any of the solution contacting the skin. If some does spash on your bare skin, quickly remove it with plenty of water and application of calcium chloride to neutralize any HF remaining. The other acids you may have in the solution are just as dangerous, so use the silicone stripping material with care.
After the ink is removed, I generally use a solvent or a bit of lacquer thinner to get rid of any ink that resists removal. This prevents the previous image becoming lightly etched into the metal because of the resist factor of the remaining ink. The silicone stripper etch is poured on the plate and quickly spread to even out the etching action. Using one of the special pads made just for this part of the process, the chemicals are moved around as you see the silicone disappear and the entire surface take water. If you want a more abrasive attack on the surface, I would suggest you use aluminium oxide instead of the other common abrasives. The bifluoride will be depleted if you use abrasive containing silica, like pumice or silicone carbide; and the acids will be neutralized if kitchen scouring powders are used, as these commonly contain calcium carbonate.
Once the surface is taking the stripping solution all over, this indicates that the silicone layer is removed. Flush the surface with plain water and then squeegee off the excess. By applying sodium silicate in very dilute form, a new layer of aluminium silicate is produced for the next image, as well as neutralizing any acid left in the pores of the metal. Flush off with water and dry; you have recycled your plate for reuse.
Using other aluminium
At the University of Saskatchewan, we have for years been using common industrial 16-gauge aluminium for intaglio, which we buy in 24 x 32-inch size. I have experimented with these plates for waterless and have had excellent results. The first time you use one of the sides, use the same process as for the back of old litho plates. After the oxide etc., is removed, the surface is just as easy to work as the litho plates. Because of the thicker metal, there are some adjustments that have to be made for most techniques. First, the greater mass of metal makes it more difficult to set any toner images with heat, using a paint-stripping gun. While it is possible, it takes more time and there is danger of permanently damaging the plate through warpage if one is not careful. I would suggest using the wick flow method on heavier metal sheets as it does just a good a job of bonding as heat. In case your toner is tending towards an A type material, then modify the white gas with a little acetone or lacquer thinner to get the proper adhesion (see my toner paper).
The other problem that arises is in rolling up the plate. It is necessary to roll completely off the surface of a plate with any waterless process, so this means the thickness becomes a minor problem. By beveling the edges to 45 degrees, like in intaglio, the edge is not a likely to damage your roller. You could buy thinner gauges of aluminium, which would reduce this problem.
I would suggest that the scraper bars used for small images on thick plates be as wide as the plate. This would prevent stretching of the metal in the overused of the center portion, which will "bowl" the plate in time. If most of your images are to be about the same size, then cut your metal to appropriate dimensions.
Using thicker plates such as this can save students a great deal of money over the length of their studies, as the plates could be resurfaced enough times to complete all their yearly assignments. The initial cost of the industrial aluminium is less than for ball grained plates, making the investment a good one. Even printmakers could take advantage of using common industrial aluminum sheeting from metal suppliers. I have not tried any other metal as the price and characteristics of aluminium are perfect for this waterless process. If surplus Dow metal for making photo-engravings can be obtained cheaply from scrap metal suppliers, this harder metal could be an interesting choice. While it contains some magnesium in the alloy, this metal should take to silicone as well as the aluminium.
Where to get the materials
Hydrochloric acid can be bought in most hardware stores or swimming pool suppliers as muriatic acid. The phosphoric acid is the main component of aluminium cleaners on the market, but make sure, as some contain enzymes, which are useless in this operation. Ammonium bifluoride is the active ingredient in such compounds as Jack Frost, a glass frosting chemical used by glass workers. In this form it is very expensive, so I would suggest you obtain it from a chemical supply as a pure material. A 500 gram bottle will last you a very long time in the concentrations I recommend. There are at least three commercial magnesium wheel cleaners that I know of, each containing a small amount of HF or ammonium bifluoride, plus phosphoric acid. These are available in the auto section of department stores or Hot Rod supply outlets.
Sodium silicate is available from pottery suppliers as silicate of soda. It is sometimes referred to as waterglass, which is an impure mixture of sodium and potassium silicates. Sodium metasilicate is available from garage suppliers as it is used to remove grease and oil from the garage floor because of its strong emulsifying action. I have also seen it in hardware stores to remove oil stains from driveways.
Making the special pads
The best material for working the surface of plates is the tough carpeting fabrics available these days. I have used both looped and fuzzy types, and think both work well. They can be obtained as scrap from your local carpet store, as only a couple of square feet are needed. I suggest you use pieces of scrap 2 x 4 lumber for a backing to the carpet. Use common contact adhesive to bond the carpet to a piece about 5-6 inches long. Trim the excess fabric and pound the edges with a hammer to make sure there is good contact between the two elements. Make a number of these, as you should not use only one for all the operations. This will neutralize any chemical from the previous chemical application for which the pad was used. With a felt marker, indicate for which chemicals that pad is reserved. Remember that the sodium silicates are alkaline and the silicone stripper is acidic.
The price of plates can be a major component in small editions; any way this can be reduced means more editions can be printed by money-strapped printmakers. While in traditional lithography, the regraining of a metal plate requires special reciprocating tables, in waterless lithography I have not found the need for a coarse tooth on the surface. In fact, the smoother the plate, the better the detail. Commercial offset printers have found this to be true as they discovered better ways to control the fountain solution needed for ink rejection. Fine art printers have been accustomed to using more water to reject the heavier ink deposits they required for textured art papers and the paper to plate contact printing method. Today, printmakers can use pin registration, which allows them to apply more than one pass of ink to the paper, if sufficient ink is not deposited the first time. Offset proofing presses are common enough, which allow many passes over the paper to build up the ink density. I have not found these smoother plates required heavier inking or multiple passes through the press.
The time it takes to resurface a plate is much less than it takes to grain even a very small stone -- and there is no expense for graining abrasives. Plates can be reused a great number of times, until other damage makes printing or imaging difficult. Since we all go through the trouble to recycle aluminium soft drink and beer containers, just consider how many cans could be manufactured from one .012 of an inch thick litho plate. Since it is so easy to do, recycling printing plates should be a matter of course for most printmakers, as quality of editions should not suffer from the practice. With the exception of ammonium bifluoride crystals, all chemicals are easily obtainable and not very dangerous to use. Even the bifluoride is not dangerous in the concentrations needed to strip a properly applied silicone layer. While it would be admirable to have no dangerous materials in any printing technique, it is not always possible; but cautious and knowledgeable use of such materials is more important than their complete elimination.
Printmaking is an expensive pursuit and bringing down plate costs should bring it more in line with ones ability to produce more complicated editions. I hope this method will help printmakers produce these better editions as they worry less about putting on another color or texture.
This information is copyrighted material, but I urge printmakers to copy the paper to hand to others interested in the method. Publication in any commercial or educational journal is prohibited without permission,because I may have up to date changes and the only reason I reserve the copyrights.
Published January 1999, updated October, 1999