The strength of the ferric solution can also be described as a
percentage, in which case say 40% ferric chloride (or 40% FeCl3)
simply denotes a ratio of 40% ferric crystals to 60% water.
Making up Ferric from Granules
If only the granules (or powdered form) are available these should be
handled very carefully as they are very corrosive and need some
preparation before they can be used for etching. The crystals should
be kept in sealed plastic containers to prevent any absorption of
moisture from the surrounding atmosphere.
The etch solution is made up by dissolving about one part of ferric
crystals in about two parts of warm water at approximately 40
degrees centigrade. If citric acid is also to be an ingredient of the
mordant, this can be added to the ferric crystals at this stage.
Once this mixture has gradually been added into the water content
sufficient heat will be generated to aid the dissolution of both.
The exothermic reaction may also generate some acidic gases, so
adequate ventilation and protection are strongly recommended.
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 (by freeing the ferric atoms from their weakening bond with
water) but produces an entirely new kind of mordant with outstanding
biting properties. Different kinds of metal require a different mix
of this mordant now known as the Edinburgh etch.
Edinburgh Etch for Copper and Brass
- 4/5 saturated ferric chloride solution (40%)
- 1/5 citric acid solution: i hot tap water to i anhydrous citric
- 16 oz ferric chloride
- 3 oz H 2 O (hot tap water)
- 1 oz citric acid powder (anhydrous)
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