Turpentine, ink and glass

Caldecote is probably one of the smallest villages in Hertfordshire. And its church, dedicated to St Mary Magdalene is likely to be one of the county’s smallest. The A1 traffic thunders less than a mile away, but the church sits in serene repose.

The diminutive St Mary Magdalene’s is a weather-beaten majesty. Externally, its walls are a patchwork of nibbled clunch, rounded brick, the odd flint nugget, and islands of lime render. Its design, however, incorporates embattled parapets, cinquefoil tracery, a buttressed tower and a rather regal south porch.

There are plenty of points of interest at St Mary Magdalene’s but for me, one of the southeast windows is the most intriguing. This window is made up of twelve panes of decorative roundels depicting scenes from the Life of Christ. But this isn’t stained glass or painted glass. It’s something I have never seen in a church before… glass transfer painting.

Glass transfer painting became fashionable in the 18th century as the printing industry boomed.

It involved imparting a print on to a sheet of glass by coating the glass with turpentine, pressing the print flat onto surface of the glass and smoothing with a rolling pin, the glass and with print is allowed to dry near a fire. When completely dry, the print is gently rubbed with a wet finger until the paper dissolves and the print ink remains bound in the turpentine varnish to the surface of the glass. The print can then be coloured in the oil paints. The process was popular with ladies of the aristocracy in the 18^th century, when prints were widely available and relatively cheap, and the colouring in required, well … less skill, but achieved attractive results.

I am most interested in the materials and processes here. First, let’s look at how 18^th-century ink was made; one of my favourite descriptions comes from Papillon, a French printmaker:

Place two pints of old nut oil in a kettle up to one third of its depth. Boil on a slow fire for two hours. Add an onion or bread crust to remove grease. Test for threading. Put into another pot half pound of turpentine and heat over a small fire. It is ready when it powders off a piece of paper dipped into it. If oil is new, pour in some turpentine whilst hot. Stir with an iron spatula and leave on a fire for a few minutes.
Linseed, rape, or hemp oils may be used, but these are in decreasing order of quality.

Papillon, Traité Historique et Pratique de la Gravure en Bois, Paris, 1766

So, in essence, the printer’s ink was composed of oil, resin and pigment – most commonly carbon black. In the glass transfer process, turpentine is applied to the glass, and the print placed face down. The turpentine will solubilise the oil-based ink layer, which as it dries becomes bound into the thin film of turpentine varnish. That’s pretty cool!

It’s unusual to find this transfer glass in windows, and this has led to the deterioration of the paintings at Caldecote church. As an organic varnish, turpentine will deteriorate over time – it can crack, yellow, chalk… Here at Caldecote, painting on to glass, the painting and varnish will experience condensation, varying relative humidities, sunlight (south facing), heating and cooling … None of which are good for paint (or glass) and all of which exacerbate the deterioration. Furthermore, I doubt the paint has been fired – though it may be fixed with a varnish – so this layer may be even more vulnerable.

If anyone reading this knows how we might conserve this painting in situ (no EPG, thanks!), please do get in touch.

The main source of information on glass transfer painting is Robert Dossie’s book, Handmaid to the Arts (1758): Of the taking of mezzotinto prints on glass, and painting upon them with oil, or varnish colours, Chapter XII, page 325