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Etching
Last Updated: 06/01/2009
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etching
Etching is a technique used to remove metal through chemical rather than mechanical processes. Specifically, mordants are used to remove metal by deliberately corroding it. Most often, resists (acid-proof materials) are applied to the metal to control which parts are exposed to the corrosive agent. This allows images (lines, shapes, patterns and textures) to be cut into the metal's surface wherever the bare metal comes in contact with the corrosive agent. Mordants
Mordants are chemicals (usually diluted acids or salts) used to deliberately corrode metal. Since each metal has a different chemistry, different mordants are used depending on what element makes up the majority of that metal's composition. See the metals/mordants chart for more details.
Materials and Equipment
tape
nylon string or thread
styrofoam blocks
mordant resist-prepared metal
resist-prepared sample of same metal
neutralizing bath
solvent for resist
Procedure:
Place your resisted-prepared metal and sample into the prepared mordant bath. Rubber-coated or nylon tongs can be used for this purpose, but it is better to avoid handling your metal with anything that can damage your resist. It is often better to tape nylon string or thread to the back of your piece for taking it in and out of the bath. When using acid mordants such as nitric acid, the piece should be placed face up, because as the metal is dissolved, the reaction produces bubbles that need to be able to escape. When using salt mordants such as ferric chloride, the work should be suspended just beneath the surface of the bath, face down. With this type of mordant, as the metal is dissolved, it produces a precipitate that needs to be able to fall away from the metal surface. An easy way to do this is to tape the piece of metal to a chunk of styrofoam and float it face down on the bath's surface.
After 5-10 minutes, lift the sample metal out of the bath, neutralize it until it stops fizzing, rinse it thoroughly, and test the depth etched so far. Testing the depth is easiest to do by using the tip of a scribe to scratch away the resist at the edge of one of the exposed areas. If there is a noticeable change in depth, you can use this to assess how long to continue etching your piece of metal. Return the sample to the bath.
Continue etching, checking the etched depth at regular intervals. For a cleaner etch, the bubbles that form with acid mordants should be genltly brushed off the metal with a feather. Also, if a mordant is ocasionally agitated (stirred or rocked) the metal will etch cleaner and faster.
When the etch is complete, thoroughly neutralize the pieces of metal and the materials used to handle the metal, and rinse well. Avoid etching for too long, or in too strong a mordant, as this will cause under-cutting. If using ferric chloride, neutralizing and rinsing the metal will not completely stop the reaction. The metal will also have to be scrubbed with an ammonia solution.
Remove the tape, string and foam. Remove the resist with the appropriate solvent, or burn it off with a torch.
Safety:
When mixing mordants, always add acids to water to avoid explosive reactions.
When etching or handling mordants, always wear eye protection, rubber gloves and protective clothing.
Always use acids and ammonia in a well-ventilated area, and have neutralizing agents handy.
Never mix different acids and/or salts together. Never mix ammonia and chlorine
Metal Mordant
Copper-based Metals:
copper, brass, bronze,
NuGold, nickel fast etch
one part nitric acid and two parts water
slow etch
one part nitric acid and five parts water
clean etch
ferric chloride
Silver-based Metals:
fine silver, sterling silver fast etch
two parts nitric acid and one part water
slow etch
one part nitric acid and three to five parts water
clean etch
ferric nitrate
Ferrous Metals:
iron, steel standard etch
two parts hydrochloric acid and one part water
deep etch
one part nitric acid and one part water
slow etch
one part nitric acid and four to eight parts water
Gold and Gold Alloys Dilute Aqua Regia
one part nitric acid, three parts hydrochloric or sulfuric acid, forty to fifty parts water.
Aluminum 1.5 fl.oz. ammonia, 5g copper sulfate, 14 oz. sodium hydroxide, 2 gal. water.
Etching Silver:
The following recipe seems to work well (taking up to 2 to 4 hours to etch deeply enough).
400 ml/14oz distilled water
300 g ferric nitrate
1 tsp. old nitrate acid solution (1: 5 solution) to 'kick start' the solution.
Add the crystals to the water * contained in a Pyrex or plastic container and mix with a plastic spoon.
SAFETY PRECAUTIONS:
Mix under a fume hood. No fumes seem to be given off when the etchant is working, but it is always wise to work under a hood or in a well-ventilated space.
* ALWAYS ADD THE FERRIC NITRATE TO THE WATER - the same as mixing acids.
Use rubber gloves and protective clothing. Ferric Nitrate will make a persistant stain on skin. Neutralize ferric nitrate with Baking Soda [also see Ferric Chloride notes]
RESISTS:
See notes on Ferric Chloride etching. Read them first. Ferric nitrate works much the same way, only takes longer to etch. Safety precautions are much the same as for Ferric Chloride.
Permanent markers (follow instructions in Ferric Chloride etching notes). Because metal remains in the solution for such a long time, the ink can lift off. Make certain the metal is absolutely clean, with the appropriate finish.
Tape - Brown or clear cello type tape eg. packing tape(see Ferric Chloride notes).
PnP Blue Film - Photocopy resists (see notes on PnP Blue and Ferric Chloride) This seems the most reliable method to use with ferric nitrate as it stands up better than pen ink to the long immersion in the solution.
Liquid Floor Polish ("Future") - this makes a good resist for silver as it does not lift off in the solution. It can be painted on with a brush, or flushed across the entire surface of the metal, and scratched through with a scriber - wait about 20 minutes before scratching designs.
Nail Polish and waterproof paints.
ETCHING PROCESS:
Put etchant in glass or plastic container. Read the label for safety precautions. Submerge piece into etchant with the design facing down (to allow particles of silver to fall away and not interfere with the etching process). It is best to suspend the piece in the solution, so it is clear of the bottom of the container. Either hang the piece from a wire strung across the container (you can make a tab of tape to attach to the wire) or tape onto pieces of plastic foam (e.g. Styrofoam) to float the metal above the bottom. Agitate the solution frequently by gently knocking the edge of the container or placing it on top a small motor that vibrates - like a bubbler for an aquarium.
IMPORTANT: When the piece is etched to the desired depth, remove the piece and wash immediately with water and scrub with an old toothbrush or brass brush and baking soda (or ammonia solution). The baking soda will neutralize the etchant and stop the etching action. If the etchant is not completely neutralized, it will continue its etching action.
Remove the ink with alcohol:
Remove Staedtler inks with methyl hydrate [a.k.a. denatured alcohol] Wear rubber or latex gloves and use in well ventilated area.
TIME:
My early experiments have taken about 4 hours to achieve the depth of etch I wished. Older solutions can take up to seven hours which makes it more important to use a strong resist such as PnP Blue. A newer solution may work in 2 hours. You must check the depth of etch regularly - say every hour or so. If it is possible to warm the solution, that may cause it to work more quickly. Agitation may help. You may also add a small quantity of citric acid which may enhance the etching (I am presently experimenting with this and will add notes, when the experiment is completed).
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