CONTENT | SEARCH
Using Cupric Chloride for
Etching Copper and Other Metals
by Nik Semenoff, 2013
1/ SaniFlush and copper sulfate: With these three materials, one can produce cuprous chloride for etching aluminium, zinc and iron. I have heard that SaniFlush is hard to find and I have been recommending a purer form of sodium bisulfate one can find at stores dealing with swimming pools and hot tubs. Use coarse salt that does not contain calcium silicate, which will form Plaster of Paris in the bath.
The reaction between copper and other metal depends on the electrochemical reduction/reaction potential of voltage between the metals concerned. With copper at +0.34 volts and aluminum at -1.66 volts, there is a potential difference of 2 volts, thus aluminium should etch faster than zinc with a difference of only 1.1 volt. With the presence of weak hydrochloric acid, it reacts with both of these metals to produce hydrogen gas in very a small amount to lift the resulting copper particles out of the etched line to float on the surface as long as it is not disturbed. There is no need for feathering, which actually breaks the bubbles and allows the copper particles to sink.
Plate in bath: Fine hydrogen bubbles from the reaction of weak hydrochloric acid and the metal will lift the copper particles to the surface and do not have to be feathered or removed while the plate is being etched. The color of the solution will change from green to a clearer liquid and will stop etching when all the color is gone. More chemicals can be added or the bath regenerated by supplying oxygen in some way.
While many combinations of chemicals work, I found that one kilogram of copper sulfate added to 10-20 liters of water will make a good mordant, depending on how strong you want your bath. To this add salt until the solution turns green, indicating the presence of cuprous chloride. I have since found out that it is best to use pickling salt rather than common table salt that contains calcium silicate to prevent clumping in damp conditions. When table salt is used, insoluble calcium sulfate is formed, which is known as Plaster of Paris, used in casting projects. Technically pure salt can be purchased for softening water in the household and sodium bisulfate can be found at stores dealing with swimming pools and hot tubs. These compounds produce hydrochloric acid with a pH under 1.0.
Natural aquatinting on aluminium: Due to the nature of the chemical reaction, the grain of aluminium is exposed as coarse crystalline texture, depending on the depth of the etch. On some plates, the grain may show the direction in which the sheet was rolled while reducing its' thickness. This depends on the manufacturing of the sheet metal.
For etching both aluminium and zinc, I recommend using the original formula because of the hydrogen produced to keep the plate textures free for etching. Cupric chloride with copper plates produces no gas and the heavier dark compound sinks, making a vertical tank with bubbling air good for two reasons. One is to keep the plate surface free of the dark compound and the introduction of oxygen that helps regenerate the bath. Since many etchers prefer copper to the other metals, cupric chloride is the perfect answer for their studio. The commercial electronic industry has mostly turned to using cupric chloride over ferric chloride for a number of reasons. First, they don’t have to constantly purchase the ferric chloride and pay for its disposal; the regeneration of cupric chloride with air not only saves money, but with the increase of strength of the mordant, chemical companies are willing to buy the surplus produced on a daily basis. If it is respectable enough for these companies, it certainly should be looked at seriously by schools and
large print shops dealing with etching.
Simple vertical tank: For my research, I found that a vertical tank with air bubbling from the bottom is the best method of introducing oxygen to the solution and keep things circulating. A cheap aquarium pump can be left on all the time to regenerate the mordant.
Hydrogen peroxide: I purchase my 35% hydrogen peroxide from health food store and dilute it if necessary to regenerate cuprous chloride. When regenerating cupric chloride, the stronger 35% can be used when etching copper platesSodium bisulfate: Sodium bisulfate can be purchased at stores catering to owners of swimming pools and hot tubs. This white crystal chemical is much purer than SaniFlush and better to use to produce a weak sulfuric acid.
Small pH meter: An inexpensive pH meter can be ordered online and will greatly help in formulating the best bath for your images
Toner image on copper plate: I have developed a process to use toner in the same way as 'sugar-lift' is used for images. This one is done on copper and is unusual as the metal does not produce a natural grain like aluminium and zinc, which produce better toner images. Diluted dyed shellac with propylene glycol used as a retarder, which allows one to roll the ground over the toner already heat boned to the plate. After etching, the ground will come off when put into an alkaline solution containing borax, washing soda or sodium metasilicate.