conservation

research & diagnosis

vulnerabilities in glass

atmospheric degradation

  characteristics

> droplets and/or crystals

> crizzling

> possible colour alteration

The deterioration of glass resulting from chemical instability and influenced by atmospheric conditions is a gradual process. The first sign is the dulling of the glass. Salts may begin to form on the surface and, depending on the climatic conditions, may appear as droplets or crystals. Sometimes the droplets are minuscule and can only be observed under a microscope, but they may coalesce to form larger droplets that can even run down the glass. At a more advanced stage, hairline cracks appear in the glass, which can become a network that spreads so extensively that the glass cracks completely and even falls apart.

Objects may also exhibit several symptoms simultaneously, such as having both cracks (crizzling) and droplets. Crizzled glass has a unique fracture pattern, although this can vary depending on the location and the object. It can be difficult for a non-specialist to distinguish between cracks caused by atmospheric degradation and those caused by stress or impact. (see also fracture and fracture pattern)

Visual and tactile identification of glass instability

The classification of symptoms of existing and potential glass instability forms part of the General Condition Form and is partly based on a system developed by glass conservator Stephen P. Koob. To facilitate international collaboration, the English terms used in research and literature have been adopted. (see General Condition Form)

droplets and crystals

Atmospheric degradation can make glass appear dull or feel slippery, which can be difficult to distinguish from glass with a dirty or greasy surface. When unstable glass is cleaned and stored in a clean environment, the dullness may return, sometimes quite quickly. Dullness and a slippery feel caused by glass instability result respectively from minuscule droplets and salts on the glass surface.

crizzling

Incipient crizzling   Crizzling caused by chemical instability and atmospheric degradation occurs in four stages. Initially, very fine hairline cracks form, sometimes barely visible to the naked eye.
Full blown crizzling   These cracks spread across large areas of the surface and become easier to see with the naked eye.
Advanced crizzling   Once the cracks have developed into an extensive network, the glass has reached an advanced stage of crizzling.
Fragmented stage   At the most severe stage, the glass loses its structural stability. Fragments may detach from the surface, or the object may break into shards.

See misleading characteristics.

possible colour alteration

In some cases, colourless glass contains Mn²⁺ cations which are susceptible to photo-oxidation and subsequently transform into pink-purple Mn³⁺ cations. This process is called solarization because it occurs under the influence of UV light. Unstable glass may also exhibit a pinkish glow. In such cases, the oxidation process occurs in conjunction with the degradation processes taking place within the glass object. A pink-purple colour in glass may also be intentional, such as to make the glass appear older, or result from small fragments of iron from the blowpipe becoming embedded in the glass and colouring it through reflection. Since the exact original glass colour is frequently unknown and any colour change has often not been monitored, an observed colour difference in such cases is referred to as a ‘possible colour alteration’.


Colourless glass that has turned pink  collection Museum Boijmans Van Beuningen   PHOTO M. Slager