Devitrification

Devitrification is the process where a formerly crystal-free glassy material transforms into a crystalline structure, resulting in a loss of transparency and a hazy, white, or scummy appearance. It is sometimes called crizzling or sugaring and often occurs by a chemical imbalance in the glass itself, characterised by excess alkali and insufficient calcium oxide, which can be triggered and accelerated by moisture and the effects of fluctuating relative humidity. It is also known incorrectly as green glass disease, because it doesn't only effect green glass but other colours also.

"The phenomenon often referred to as ‘crizzling’ is usually associated with vessel glass, and its occurrence in stained glass has not been studied extensively to date. Historically, it was thought that in windows the problem only occurred with green glass, leading to the name ‘green glass disease’, but we now know that is also occurs in other forms of glass. Purple crizzled glass is found in the windows of the nineteenth-century church of St John the Evangelist in the small North Yorkshire hamlet of Howsham. The present author selected this church and its windows as the primary case study for her recent MA dissertation. Here she provides an introduction to crizzling, considers the case at Howsham (and the wider implications for our understanding of glass made in Britain in the mid-nineteenth century), and discusses the conservation issues around crizzling. Finally, she proposes a method for treating crizzled glass that would retain the material in situ whilst greatly decreasing its rate of deterioration." Merlyn Griffiths (York Glaziers Trust)

In the publication 'Conservation and Care of Glass Objects', by Stephen Koob (London, 2006) the process of deterioration is described in a number of stages: Firstly ‘sweating’ or ‘weeping’ caused by the appearance of alkali on the surface as droplets (in higher relative humidity) or as crystals (in lower relative humidity). The glass then becomes cloudy and opaque, as it develops a fine silvery network of cracks, known as ‘incipient crizzling’. The cracks become deeper and the surface of the glass begins to spall away, eventually leading to the fragmenting and disintegrating.

[edit] Related articles on Designing Buildings

• Broad glass.
• Crown glass.
• Cristallo.
• Curved glass.
• Cylinder glass.
• Decorative glass.
• Dichroic glass.
• Drawn glass.
• Environmental protective glazing.
• Façon de Venise.
• Flint glass.
• Forest glass.
• HLLA glass.
• Horticultural glass.
• Kiln-distorted glass.
• Kiln-formed glass.
• Lead glass.
• Leaded glass.
• Obsidian.
• Mixed-alkali glass.
• Potash glass.
• Soda-lime glass.
• Stained glass.

[edit] External links

https://www.vidimus.org/issue-98/feature-24/

http://www.yorkglazierstrust.org/