Nontoxic Printmaking, Safe Painting & Printed Art

Etching Copper and Brass:

The Edinburgh Etch


A new etching solution for copper and brass
Friedhard Kiekeben

special thanks to Keith Howard, Michael McCann, and Robert Adam who helped revitalize ferric chloride-based etching

(Above)                                                                                    Friedhard Kiekeben, Floral, brass frieze, 1997

Plate etched in Edinburgh Etch.                               
Made in collaboration with Eduardo Paolozzi, 1997                                                                                                            

note of caution: all safer etching methods involve more or less hazardous chemicals and by-products, they are not ‘nontoxic’. Safe handling, informed training, and various protective measures are essential requirements in metal etching.     

The History of Etching with Ferric

A solution of ferric chloride crystals has long been known to etchers as a highly controllable etchant with the most accurate bite for copper plates. It doesn't give off toxic vapors and is of low risk if it comes into occasional contact with the skin - although this should of course be avoided. While a bottle of nitric acid can generate explosive and fuming reactions when dropped, ferric is not readily volatile. In contrast, ferric spillages can be diluted, neutralized with soda ash, and disposed of safely and easily.

Why, then, hasn't ferric been used as the predominant mordant in etching?

The answer lies in the speed and ease of biting. The way that ferric was used in the past meant that etches of a reasonable depth took a very long time.
When metal plates are etched in ferric chloride, sediment gradually builds up in the bitten intaglio areas. If the crystalline residue is not continually removed by some means, it eventually clogs up the grooves and prevents the plate from etching further. Old handbooks recommended placing plates face down while rocking the bath: a slow, cumbersome and inefficient method.

This problem with ferric was eventually addressed by etching plates in vertical dip tanks, a reliable technology borrowed from the electronics industry and first applied to printmaking by Keith Howard. Plates are suspended in the upright position in the solution, allowing sediment particles to drop out of the etched grooves. Agitating and heating
the heavy iron salt solution also accelerates the bite and improves efficiency.

Dip Tanks for Etching
Due to its generous volume of solution and vigorous agitation, a dip tank represents the best facility for the mechanical removal of the crystalline sediments that ferric chloride generates as it etches. Larger tanks can be custom made by most acid unit manufacturers (or plastics fabricating companies), preferably from welded polypropylene. For home use and smaller scale work, even a square bucket or plastic crate will make a very serviceable etching tank, especially if fitted with an agitation pump.

                                                         Dip tank at RIT:
Many print studios have customized dip tanks for etching.
To agitate the etch solution; one or several tubes of fish tank airline can be fitted to a dip tank. The air outlets should point upward and be connected to a powerful fish tank aeration pump. The stream of bubbles rising on the sides of the tank produces a circular flow within the solution which activates the etch. Aquarium shops also supply small valves that regulate the air and therefore the strength of the flow. Some delicate resists such as aquatint can be etched with reduced flow. Ensure good ventilation if using fish tank aeration. Ideally use a pump or propeller system to agitate the solution mechanically as this minimizes corrosive fumes coming off the tank. 

The Discovery of the Edinburgh Etch

The innovations outlined above dramatically improved the performance of ferric chloride, but
I was convinced that the eroding power of this safer mordant could be extended still further. This thought motivated me to develop the Edinburgh Etch solution - a chemically enhanced formula in which the corrosive potential of ferric is fully realized as a superb etchant for copper and brass.

The Chemistry of Ferric Chloride
The corrosive properties of ferric become clear when you look at its chemical composition. A ferric chloride molecule consists of one atom of iron with three atoms of chlorine hooked onto it.

The bridge between iron and chlorine, which creates the chemical adhesion of the molecule, is made up of two electrons (negatively charged particles) for each chlorine atom.

Since the iron atom needs eight electrons to be in a stable condition, but has only six, it wants to
react (electrochemically) with the copper atoms of the etching plate to gain the missing two electrons. 

Citric Acid: A New Catalyst
In the course of my research, I approached the issue of how to activate the ferric etch from a new angle, searching for additives to ferric chloride which might be capable of dissolving the sediment as it is produced.
After systematically experimenting with a variety of possible substances (initially with mixed results) I tried a new kind of additive normally associated with the soft drinks industry rather than with etching: citric acid. It turned out that a citric acid solution mixed at a certain ratio with a ferric chloride solution not only speeds up the bite of ferric but also produces an entirely new kind of mordant which I christened The Edinburgh Etch.

On copper and brass plates the etch process takes place with the utmost precision and without the build-up of sediment typically associated with unmodified ferric chloride. In chemical terms, this is because the individual molecules of the metal salts are locked into the clamp-like carbon rings of citric acid atoms, thus keeping them dissolved. They are no longer allowed to solidify as the crystals which impede biting and result in a coarser etch. The main obstacle encountered with ferric chloride is literally being dissolved by the new mordant.

The white citric acid powder is synthetic lemon juice.
It acts as a catalyst when added to ferric chloride.

Inspecting a brass panel etched using Edinburgh Etch.

Edinburgh Etch Ingredients
CITRIC ACID (anhydrous) is now available from most major printmaking suppliers; it can also be obtained from suppliers to the food industry. A dust mask and goggles should be worn when dispensing the fine powder.

FERRIC CHLORIDE is available from most chemical suppliers either as yellow granules or as a saturated solution. If at all possible, the ready-made solution should be used which for its industrial applications comes in 25 liter or 5 gallon containers of about 42 to 48 BE (Baume) density; increasingly printmaking suppliers also stock smaller bottles. The density or weight of ferric can be measured with a hydrometer; but the Baume scale describing the specific weight of liquids is only a rough guide, and sometimes manufacturers give other specifications. The best thing to do is to ask the chemical supplier for a strong or a saturated ferric chloride solution, technical grade.

Remember that liquid ferric is a heavy solution of ferric (iron) chloride salt crystals in water. The solution would not normally go beyond a certain strength (about 48 BE) otherwise the crystals solidify. It is easy to dilute a strong solution with water to obtain a weaker strength but impracticable to strengthen a ferric solution which is too weak for a good etch from the start.

Even though ferric chloride is relatively safe to use, respect it as a corrosive chemical; eye protection and gloves must always be worn when handling or using the solution.



Making up the Edinburgh Etch

Refer to the INTAGLIO MANUAL for detailed instructions on PLATE PREPARATION, ETCHING THE PLATE and more.


        Products and equipment needed to make up the Edinburgh Etch:

  • supply of hot water
  • ferric chloride solution (purchase ready-made up)
  • anhydrous citric acid powder
  • goggles
  • heavy-duty gloves
  • measuring jug
  • buckets for mixing the solutions
  • implement for stirring solutions
  • tray, bucket or dip tank 


       (Above) Howard Pro-Vertical Etching Tank: designed by Keith Howard. 

       See for details.

        Make up the Edinburgh Etch as follows:

        The Edinburgh Etch solution consists of five parts.
        Four parts are a saturated ferric chloride solution.
        One part is a citric acid solution.


4 parts ferric chloride solution + 1 part citric acid solution = Edinburgh Etch


4 liters ferric chloride solution + 1 liter hot water mixed with 300ml citric acid powder (ratio 3:1)
= Edinburgh Etch

  1. Put on goggles and gloves.
  2. Start by measuring out 4 liters* (or the desired quantity) of the saturated ferric chloride solution into a bucket.
  3. Now make up the citric acid solution using the ratio of 3 parts water to 1 part powder. Fill a bucket with 1 liter* of hot water. Gradually add 300ml* of citric acid powder. Stir continuously.
  4. When the citric acid powder has completely dissolved, gradually pour this citric acid solution into the ferric chloride solution. Keep stirring until you have produced a uniform liquid. This solution is the Edinburgh Etch.
  5. Pour your Edinburgh Etch mixture into an etching tank, tray, or bucket. 
  6. The Edinburgh Etch can be used straight away but it is better to run the aeration pump for a few hours first to obtain a perfectly uniform mix.

          *These are sample quantities: use less or more according to the ratios provided.

(Left) Ferric Pump: The exemplary etching facility at Western Michigan University includes a large etching tank filled with Edinburgh Etch which uses a professional pump system rather than a fish tank air pump. It provides excellent agitation of the solution and is easy to maintain. The polypropylene tanks have tight fitting lids so that etching can take place in a sealed environment with the minimum emission of corrosive vapors.

The Edinburgh Etch mixture is a universal mordant that can be used in flat trays and upright tanks.

Temperature: The Edinburgh Etch works best in warm conditions (from 18 to 30 degrees C).
A warmer temperature improves etching times and the responsiveness of the mordant.

During etching Edinburgh Etch trays or tanks should be covered with a lid to prevent evaporation. When not in use, etching solutions should be stored in clearly labeled containers, stating the composition of the solution and the kind of metal etched in it. It is also good practice to place safety notices, such as CORROSIVE - WEAR EYE PROTECTION to all containers and work areas.

If no citric acid is available a saturated ferric chloride solution will do the job, but the etch will be slower and somewhat less precise.


Edinburgh Etch for Brass                                                    
Brass is a superbly suitable material for intaglio etching and printing.
The Edinburgh Etch method allows this noble metal amalgam to be etched as easily as a sheet of copper. Brass can be etched in the same solution as the one described for the Edinburgh Etch for copper.

See the section on METALS in the INTAGLIO MANUAL for more about the suitability of Brass for printmaking.

Friedhard Kiekeben
etched brass and intaglio prints

Chloe Randall

Paper Assemblage
made using ferric based dye

The strength of an Edinburgh Etch or ferric chloride solution can be tested by dipping a paper strip in the bath. A bright orange color indicates a fresh solution. If the solution is the color of black olives this indicates that the solution is exhausted and requires disposal and replacement.

Testing Freshness: Color Code
The Edinburgh Etch solution is exceptionally long lasting; a single tank-full, used daily and occasionally topped up to compensate for loss, has been known to remain active for several years without significant drop in its biting properties.

The freshness of the bath can be easily tested: dip a strip of white paper into the bath to assess its color: due to its dominantly iron-based content, a fresh Edinburgh Etch solution has a bright orange or rusty color. A used solution turns green as cupric compounds accumulate in the bath; eventually the solution acquires a dark black/green olive color and a thicker consistency; it is then near exhaustion and requires disposal and replacement.

                         BRIGHT ORANGE                                         Solution is FRESH                                       

                         BLUE/GREEN                                                 Solution is still GOOD                                  

                         DARK GREEN / BLACK                                Solution should be REPLACED                

Neutralization and Disposal
An Edinburgh Etch solution can remain active for several years. The citric acid component in the solution allows for literally dozens of kilograms of metal to be dissolved without causing any precipitation. The bound, chelated copper compounds in the solution are considered a much lower hazard to aquatic life than pure copper ions, but precautions should be taken not to allow a concentrated solution to go down the drain. Once the Edinburgh Etch solution is of an oily consistency and of a black olive color, its ability to etch suddenly drops; and it is time for replacement. A simple gravity-fed siphon pump is ideal for transferring the solution from the tank into plastic storage containers.

The ideal way to dispose of spent etching solutions is to take them to a chemical disposal company, such as the ones dealing with waste photo-chemicals or ceramics waste. If a ferric chloride or Edinburgh Etch solution has been properly neutralized with a suitable agent such as sodium carbonate and highly diluted, disposal in the drain may be permissible by some authorities. However, copper ion concentrations in water should normally not exceed about 1 part per million. The Edinburgh Etch contains chelated, non-reactive, copper ions so a higher ratio may be permissible.

To neutralize an Edinburgh Etch or ferric chloride solution, gradually add a strong sodium carbonate solution to it. In a harmless fizzing reaction, carbon dioxide is produced. Allow this to settle before adding more sodium carbonate. Once the solution no longer fizzes when soda ash is added, and neutralization is complete. This can be confirmed by a pH test showing a value of seven. After that, dilute with plenty of water.

A professional etching facility should also have a catchment tank filled with an alkaline neutralizing agent. Manufacturers can provide this ready built into an acid unit.

The Small Business section of your local Environmental Protection Agency offers free advice on the safe disposal of exhausted metal salts and liquid ferric-based etching solutions.


A Guide to the Safe Use of the Metal Salt Etching System

The Metal Salt Etching system is safe to use if the warm and the cold colored metals are etched in their respective warm and cold colored solutions.

Use the rust/orange colored Edinburgh Etch  (ferric based) for etching the reddish metals: copper or brass.

Use the blue/green colored Saline Sulfate Etch (copper sulfate based) for etching the silvery metals: mild steel, zinc, and aluminum.

DO NOT use the Edinburgh Etch or a ferric bath to etch zinc or aluminum, as this may cause a hazardous chemical reaction.

It is recommended that print studios using metal salt etching systems display a notice such as the one below.

Also read and follow etching precautions given on The New Etching Chemistry page

NOTE: Although considerably safer, the new systems still utilize harmful corrosive chemicals and their reactive properties, and it is very important to follow all safety instructions in their use. Medical science suggests that as an extra precaution women should not engage in extensive etching practice of any kind (acid or salt based) during pregnancy to safeguard against the possibility of reproductive damage.

Metal Salt Etching Safety Precautions. Strong long sleeved gloves, goggles, and protective clothing (e.g. plastic lab apron) should be worn when handling metal salts and metal salt solutions. Use a dust mask when handling copper sulfate or etching residues. In professional shops ideally there should be an emergency shower and eyewash fountain in the etching space, in an artists personal studio make sure you have clean running water or an eye wash bottle nearby to use in case of splashes. Any splashes in the eyes must be rinsed immediately with plenty of water. Immediately wash off any splashes on skin. Provide supplies of sodium carbonate for neutralizing any accidental spills and run-offs in the etching space. Frequently clean the etching spaces with warm water and mild detergent to prevent build up of salt residues. Keep all solutions and etching residues in closed containers, and keep etching supplies in locked non-metal containers and cabinets.


Warm Colors

Copper and Brass: Etch in Edinburgh Etch
(reddish-brown solution)

Cold Colors

Zinc, Steel, Aluminum: Etch in Saline Sulfate Etch
(blue-green solution)

Safety Warning



The Chemistry of Etching without Acid

Art Meets Science
Dr Paul Craig, PhD & Dr Paul Rosenberg, PhD
First published in The Contemporary Printmaker, Keith Howard, Write-Cross Press, NY, 2003


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