Edinburgh etch

04/19/2016 - Technical focus



An etching solution for copper

A solution of ferric chloride crystals is the best etchant with the most accurate bite for copper plates. It doesn't give off toxic vapors but even if it allows fine lines and controlled bite, etching deep areas takes a long time.

When copper plates are etched in ferric chloride, sediment builds up in the bitten areas. If the crystalline residue is not removed, it prevents the plate from etching further. It is advised to place plates face down in the tank.

Citric acid is an additive to ferric chloride which is capable of dissolving the sediment as it is produced. Then it is possible to place plates face up in the etching tray. In addition, a citric acid solution mixed with a ferric chloride solution speeds up the bite of ferric. This new kind of mordant is called The Edinburgh Etch, by its inventor Friedhard Kiekeben (artist and researcher at the Edinburgh Printmakers Workshop).



Citric acid (anhydrous) is available in as a fine powder.

Ferric chloride is available as yellow granules or as a solution of 43°Bé (baumé) also labeled 40%. In case of using granules, dissolve 1kg of ferric chloride into 500ml of water. The density of ferric can be measured with a hydrometer (43°Bé means 1,42g/cm3).

Always dissolve the granules of ferric outside wearing eye protection and gloves since the process creates chlorine fumes!



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

4 liters ferric chloride solution + 1 liter hot water mixed with 300grs citric acid powder.

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.

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 to prevent evaporation. When not in use, etching solutions should be stored in containers.



The Edinburgh Etch solution is long lasting: used daily and occasionally it remains active for several years.

The freshness of the bath can be easily tested: dip a strip of white paper into the bath to assess its color: a fresh Edinburgh Etch solution has a bright orange color. A used solution turns green. The solution acquires a dark black/green olive color and a thicker consistency when requiring disposal and replacement.

L’état de fraîcheur de la solution peut facilement être vérifiée: plonger une bande de papier blanc dans le bain et observer la couleur. Une solution fraîche donnera une couleur orange vif. Une solution usagée donnera une couleur verte. Si la solution de consistance épaisse donne une couleur noir sombre ou vert olive il faut la remplacer. 



The ideal way to dispose of spent etching solutions is to take them to a chemical disposal company. 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.

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, then neutralization is complete.