Enamel glass paintings were at the height of their popularity in the eighteenth-century. After the Reformation tastes had changed and the production of stained-glass windows in the traditional, medieval style had declined. Members of the Royal Academy considered painting, especially oil painting, the height of artistic expression. So, techniques developed to transfer the style of oil painting onto glass and enamels were perfect for this.
In this ‘traditional’ 1471 stained glass panel most new colours have to be introduced on a new piece of glass, separated by lead (Holy Trinity Goodramgate, York) 
Here subtle shades and various colours can be achieved in one piece of glass (Steeple Bumpstead)
General background
Glazier’s had started using enamels paints on glass during the mid sixteenth century. Initially they were just an addition to the glazier’s repertoire, adding extra and more subtle colours to their palette. Other bonuses of this medium included the ability to blend colours together and use multiple colours on one piece of glass. This was great for complex designs, heraldry and skin tones, which had previously been difficult to achieve.
Glaziers began to use enamels more frequently and in greater quantities. Eventually, due to changes in tastes and the development of the technique, artists used enamels solely to create pictures on glass. And so the enamel glass painting was born!
Just to note, artists still use enamel paints in glass decoration in a variety of ways.
What are enamels?
Stained glass is quite a difficult medium to break-down and explain. But I’m going to give it a try anyway. One of the main problems is that we often use the terms ‘stained glass’ and ‘painted glass’ interchangeably to describe anything that falls into the category of decorated glass. But enamel paints, grisaille paint and stains are all used to decorate glass (clear or coloured). Even glass that is not actually stained is called ‘stained glass’!
Stains, grisaille and enamel paints all come in powder form. They mixed with liquid to make a paste, painted onto a piece of glass and fired in place in a kiln. Paints and enamels form a glassy layer that bonds to the surface of the base glass while stains just stain its surface. Their composition is built around silica, which also forms one of the main elements used to make the base glass. It is the silica that creates the strong glass bond. Metal oxides are used to make change the colour of the enamels, while grisaille glass paint is usually black, brown or grey. Examples include iron oxide which makes green and cobalt oxide which makes blue. (Curious about more colours and chemical compositions? Click here for a handy website).
Enamel paints are generally the weakest method of decorating glass. It has a thin translucent layer that can flake off after just a few years. Too much or too little of any ingredient in the composition cause this failure, as well as impurities or incorrect firing temperatures. Grisaille paints will behave in this way too if they have compositional faults. If the composition is right however, they can survive for centuries. (This brilliant article contains more information on the complex science behind enamels paints and their composition.)
Style
The enamel glass painting can be difficult to pull-off. At their worst; an ugly grid of lead-lines breaks up the image, while colours can sometimes be wishy-washy rather than vibrant and powerful. Additionally, mending leads, positioned to secure cracks, sit intrusively on the surface of the window and distort its design further. However, skilled artists have created wonderful artworks using this style.
Here are a couple of artists who aimed to achieve greatness with this technique:
Sir Joshua Reynolds
Sir Joshua Reynolds was a portrait painter and produced hundreds of works capturing the rich, famous and elite of eighteenth-century society. He was the first president of the Royal Academy and his annual lectures called Discourses in Art, had a lasting impact on the development of British Art.
His ambitious design for the west window in New College Chapel, Oxford are probably the most famous of all enamel glass paintings. Executed by glass artist Thomas Jervais, it controversially replaced all of the medieval glass that was already there!
In the top half of the window is a Nativity scene spreading across all six lights. Full-length personifications of Virtues fill the bottom half. From left to right, these are Temperance, Fortitude, Faith, Charity, Hope, Justice and Prudence. These figures are dressed in long-flowing drapery, reminiscent of Italian Renaissance art.
William Peckitt
William Peckitt was a stained glass pioneer in the eighteenth century. He was based in York and was the son of a glove maker. He taught himself how to make stained glass by experimenting with different painting techniques and new recipes for glass colours. Despite his limited background in the arts, he landed some prestigious commissions including York Minster, Audley End House and Lincoln Cathedral.
Peckitt’s enamel painted light is sandwiched between two medieval stained glass lights (c.1330s) and the contrast between the two styles is striking 
The personification of the Resurrection by William Peckitt in St. Martin-cum-Gregory, York. The image is separated by a heavy grid of lead-lines and the blue enamel on the head band is deteriorating.
In 1792 Peckitt produced this figure for the centre light of a three-light window, in the church of St Martin-cum-Gregory in York. She symbolises the resurrection and points to heaven, holding a page from a book that reads “I know my redeemer liveth. Job, Chap: XIX, Verse 25”. The skull beneath the book signifies victory over death.
This window also contains medieval stained glass dating from the fourteenth-century which presents a really striking contrast of styles.
Unfortunately, mending leads and the grid of glass panes have broken up the design of this panel. In addition to this the blue enamel in the headband of the Resurrection figure has started to flake off.
Later, Peckitt ventured away from this ‘painterly’ style of glass decoration and made efforts to revive the earlier style of stained glass. Traditional stained glass techniques had been lost in the seventeenth century, but Peckitt played a pivotal role in their revival. Members of the Royal Academy, including Joshua Reynolds, criticised him for this move. They considered stained glass as more of a craft than an art-form and continued with the oil-painting style enamel glass paintings.
A few more examples
Here are a few more examples of enamel glass paintings:
An angel – Churchgate Street, (Old Harlow) Essex 
Another Angel this time playing a musical instrument – Churchgate Street (Old Harlow), Essex 
Moses, a crucifixion scene and depictions of the church at Allerton Mauleverer (North Yorks) within which this window is housed, by William Peckitt
Survival
Sadly enamel glass paintings have not survived all that well for two main reasons. Firstly because of the structural issues already mentioned above. The enamels sit as a layer on the top of the glass and if the bond is fragile then the layer flakes off.
But also, Victorians artists and architects restored a lot of churches during the gothic revival. In doing so they replaced these painted-glass windows with traditional stained glass windows. Luckily restoration like this does not happen anymore. But conservators have their work cut out when it comes to securing enamel paintings on glass. Find out more about the conservation of this fascinating form of glass art right here.