Inspired by a feature in Jo Connell’s book ‘The Potter’s Guide to Ceramic Surfaces’, we (Sarah and Sandie from the Tuesday Morning class) decided to experiment to see if we could manage similar exciting effects – with fascinating and sometimes dramatic outcomes.
The Mocha technique was developed in 19th century England and is so called from its similarity to the mossy patterning of the semi-precious stone Moss Agate.
The effect relies on the reaction between a very wet surface coating of slip and a strong alkali solution mixed with a very small quantity of oxide (the juice). The juice is dripped onto very wet slip, and immediately migrates across the slip, forming minute channels that are filled by the oxide. The whole process must be done very quickly on freshly applied slip applied to leather-hard clay for the mocha effect to work. One use of the technique is to grow trees on a piece, by dripping the juice onto the edge of the slip and holding the item upside down to enable the tree to grow – as seen on Sandie’s amazing countryside scene and her tile of trees and reflections. Other uses include simply decorated, tiles, dishes and vessels, like that done by Sarah.
With much encouragement and practical help from Julia, our class tutor, our first experiments had dramatic looking results and immediately lots of people were quickly making jars, jugs, tiles and dishes, trying to get them leather-hard to have a go themselves. However, having proudly biscuit-fired then put on a transparent glaze, we realised what wasn’t quite right – firing at Stoneware temperatures resulted in some of the colours vanishing, and our trees were reduced to blobs! Thankfully Julia realised the problem, and from then on the pieces were Earthenware glazed and fired.
Sandie led a systematic series of tests with different juices (vinegar, strong instant coffee, lemon juice and boiled cigarette ends) – although a blog had recommended cider vinegar, we found that coffee and tobacco juice worked best. We didn’t try urine though . . . we’ll leave that one for someone else to try out! The oxides we used were Manganese, Red Iron Oxide, Chrome, Titanium, Cobalt and various pigments. Chrome made a very strong green pattern. Some of the colours changed depending whether a transparent or honey glaze was used. Sandie is continuing to experiment and grow increasingly larger trees on a series of countryside scenes and using different pigments. Her latest advice is that the juice needs to be recently prepared for the effect to work.
If anyone else at WSP is inspired to have a go, we’d be most happy talk about our experiences – even some of the failures!