Flashed glass is where two or more layers of colour are created in one sheet of glass. This is made by gathering one blob of molten glass on a blow pipe and dipping it into a crucible of a different coloured molten glass.
The glass is then made into a sheet and the process of glass blowing spreads the second colour into a thin layer over the first colour. This means that you end up with a piece of glass with two colours overlaid and the upper layer can be etched or abraded to reveal the colour underneath.
The relative thickness of each layer and even the shape of the initial gather will affect the final colour intensity and whether the colour is even or varied.
Flashing glass allowed extra artistic techniques to become possible. Methods were developed to remove the coloured layer of glass to reveal the layer below and create more than one colour using one piece of glass without having to lead together separate pieces of glass. In the example shown in the photograph, the red layer has been removed to reveal the clear glass below. Over time different colours have been produced in flash, so you can also use base colours other than clear. Removing the layer of glass was a very skilled process as the thickness of each colour layer would vary between batches and across individual pieces of glass as a result of the handmade process.
Layers of red and blue glass together appear as purple but you can remove the blue to show red, or remove the red to show blue – all on the same piece of glass. You can also remove only some of the layer thickness to create different depths of colour. You can also use silver stain or more than two layers of glass! The methods used to remove the layers of glass varied over time. Originally it would have been painstakingly ground away by hand, but later, Hydrofluoric acid was used to dissolve the glass. This gave greater control over how much was removed and considerably speeded up the process.
The Annunciation was a favourite subject for artistic representation and as you would expect, the Burrell Collection has a wonderful collection of examples in a range of media.
The Boppard panel is a classic design. God is up in the top left of the panel and the angel Gabriel in a red robe with yellow wings is at the lower left. He indicates towards Mary with his left hand and holds the end of a scroll with his words, “Hail Mary, mother of God”, in the other hand. Mary, in her blue robe and with her head covered, fills the centre right of the panel. She has been kneeling at a lectern on her right, reading from the Old Testament. She turns towards the angel with her hands raised in surprise. There is a curtain on a rail framing the right hand edge of the panel, suggesting we are in a private place. There is a plant in a blue pot at the bottom right which would normally be a lily indicating the purity of Mary, but in this case looks like a little tree. This could be a reference to the Tree of Jesse, the lineage of Christ. A canopy at the top reinforces Mary’s high status. A beam of light emanates from God down towards Mary, changing into a dove that signifies the Holy Spirit, which in this case represents Jesus entering Mary.
Another example of the Annunciation at the Burrell Collection is from Hampton Court and dates to 1400-1430, just before the Boppard windows.
Apart from the very clear vase of lilies, the composition is remarkably similar. Yet another similar composition can be seen in one of the Nottingham alabasters, beautifully carved and originally brightly painted, with traces of paint still visible.
Nottingham alabasters were quarried and carved, mainly in Nottingham but also in London, York and Burton-on-Trent. They were carved in large quantities from the 14th century until the early 16th century and Henry VIII’s creation of the Church of England. They would be in sets telling stories from, for example, the Life of the Virgin.
The Annunciation is also in a superb painting by the Netherlandish painter, Master of the Prado Adoration of the Magi, a catchy name used when an identified body of work cannot be attributed to a named artist. The Adoration of the Magi is his greatest work and is in the Museo del Prado in Madrid. His painting of the Annunciation is part of a triptych dated to between 1470 and 1480.
This is the left wing of a triptych, with the right hand wing also in the Burrell Collection. Where is the angel Gabriel? He was originally on the left, but has been sawn off at some time in the past, possibly due to water or worm damage to the painting. The composition is essentially the same as the Boppard panel, with lots of iconography: God was probably represented by the light shining through the open window and the bed provides the canopy for royal status. The rug Mary kneels on also indicates her royal status. On the bedside cabinet are a closed book, possibly representing the New Testament yet to be written and a candle not yet lit; the glass bottle with water next to the wine ewer perhaps suggests the miracles Jesus will perform during his life and the trefoil and quatrefoil carvings on the lectern may represent the Holy Trinity and the four gospels.
John, who has been helping with the blog, had heard about coloured pot-metal glass, so he called in on Marie and Megan and asked, “Can you show me some examples of pot-metal glass on the Boppard panels?” Instead of the expected, “there’s a little bit here and a section over there”, Marie pointed out that ALL the glass that was coloured, was pot-metal glass. Not what John had expected, but very enlightening!
Glass can have colour applied to it through a variety of methods which we will discuss in later blogs; such as glass painting, enamelling or silver stain, but all the glass that is coloured in itself has had metallic oxides added to the molten crucible (or pot) of glass. Up until the end of the Middle Ages the range of coloured glass available for use in stained glass remained basically the same. To create colour in glass metallic oxides or salts were added to the crucible or pot of molten glass and mixed in, colouring the glass throughout its thickness. Glass made in this way therefore became known as “pot-metal” glass. The metal is not dissolved into the glass but is dispersed evenly throughout the glass at a microscopic level, creating a uniform colour.
Producing colour in glass was a complex process since it not only depended on the metal oxides added, but on the temperature of the kiln, levels of oxygen in the kiln, and the length of time the glass remained at the highest temperature.
Controlling the kiln was particularly important as it affected both the temperature and the amount of oxygen being consumed by the fuel. Fresh fuel added to the kiln uses more oxygen and creates a “reducing kiln” in which oxygen is drawn out of the furnace atmosphere which in turn affects the metal oxide.. The changes in oxygen levels within the kiln can alter the metal oxide to create a different range of colours; for example if iron is added in the form of ferrous oxide this produces blue glass, but if oxygen is added ferric oxide is formed which produces yellow or brown colours in the glass.
Glass workers often kept their recipes secret, so little is known about medieval glass making techniques.
However some information survives thanks to the work of Theophilus, a 12th century monk who wrote about the process of making coloured glass in his treatise, De Diversis Artibus “The Diverse Arts”. He described for example, how the same mix used to produce flesh coloured glass could also be used to create purple glass. Once the sheets of flesh coloured glass had been made the remaining mixture would be returned to the kiln for another 2-3 hours to create a light purple or 3-6 hours for a deep purple. This change was probably a result of the time in the kiln and the changes in levels of oxygen.
Early colourants that were used include:
Blues – Cobalt Oxide
Violet and purple – Magnesium Oxide
Red and ruby but also turquoise – Copper Oxide
Greens and yellows but also reds – Iron Oxide
Other colourants introduced over time include:
Dark green to black – Chromium
Rich greens – Chromium, tin oxide and arsenic
Deep yellow – Cadmium salts
Yellowish brown – Titanium
Purple – Nickel salts
Burgundy red – Gold
Yellowish orange to red – Silver salts
Green or lilac red – Didymium
Whites – Tin oxide and antimony
Ruby glass was problematic, as a lot of copper oxide was needed and the resultant glass was so dark it was almost black. To overcome this problem, streaky glass was made with red and clear glass running through the body, to keep the colour bright. Quite how this was achieved is under question.
The examples, therefore, have been chosen to show the distinctive streaky ruby in early glass.
The Prophet Jeremiah window dates to about 1145 and was installed under the direction of Abbot Suger into the Abbey of St.Denis near Paris.
Arms of Somery
The Arms of Sir John de Somery, of Dudley Castle, Worcs., dates to the early 14th
Century and uses a range of wonderful richly coloured glass.
Marriage at Cana
The Marriage at Cana is a French window dating to before 1236 and was purchased by Sir William Burrell from the collection of William Randolph Hearst. The streaked ruby red has been used to excellent effect enhancing the flame effect of the cook’s fire.
 Information taken from “On Divers Arts” by Theophilus, J.G. Hawthorne and C.S. Smith ed. ; “Conservation of Glass” R. Newton and S. Davison.