What is acid etching?
Acid etching on flashed glass is used in stained glass to create pieces of glass showing more than one colour. Flashed glass is a mouth blown ‘antique’ glass which has a clear base and a thin layer of coloured glass on top.
Acid etching is a process where hydrofluoric acid is used to dissolve away all or part of the coloured layer of flashed glass to achieve a variety of tones from the full deep colour all the way to completely clear.
Hydrofluoric acid is very dangerous and rigorous safety procedures must be followed. Various resists can be applied to the glass to protect the areas that are not to be etched, for example Fablon (sticky-back plastic) beeswax or bitumen paint. To achieve the desired tonal variation, a piece of glass might have several applications of resist and baths in the acid, for example the deep turquoise colour in ‘The Abyss’.
Because the top layer of glass is being removed, it becomes textured and is an interesting and sometimes challenging surface to paint on. If you gently run your hand over the hand on ‘Saint Elizabeth’ you can feel the texture where the acid has eaten away at the surface of the glass.
After all the etching has been completed and the resist scrupulously cleaned off, the glass can then be painted on to create lines and add different colours.
Sometimes two layers of etched glass are used; this is called double plating. This allows a greater degree of colour variation when the colours overlap. For example, using red and blue glass together allows you to achieve red, blue, purple and white in the same ‘cell’ of a stained glass design. If you consider the addition of silver stain (which adds yellow and orange tones) you can then also achieve yellow, orange, green- pretty much any colour you like, making this an extremely versatile but highly labour intensive technique.