unknown, bowl, 1600 / 1700

10 × 13.6 × 11.8 cm, 569 (KN&V)
COLLECTION Museum Boijmans Van Beuningen. Bequest of Dr E. van Rijckevorsel.

creation

making process

Material colourless and coloured clear glass
Technique à la façon de Venise
Hot working optic blown, incalmo, appliqué
Blown on the blowpipe, transferred to a pontil, then opened and shaped by hand

• making techniques bowl, 1600 / 1700

(art) history

In the Venetian lagoon, the city of Venice became an important centre of glass production, attracting glassblowers who shared formulas and developed new techniques. The glassblowers' guild was also established there. From the late 13th century onwards, glassblowers were permitted to work only on the neighbouring island of Murano because Venice was built on wooden piles and highly vulnerable to fire. Venetian glass was enormously popular, and the city guarded its formulas jealously. Foreign glassblowers were unwelcome, and Venetian glassblowers who left risked punishment. Nevertheless, glassmakers elsewhere attempted to replicate it, producing what became known as à la façon de Venise, French for ‘in the style of Venice’.

conservation

research & diagnosis

technical analysis

visual examination

b.2 crack(s), b.5 dust, dirt or stains c.1 cloudy appearance, c.4 possible colour alteration, d.2.2 full-blown crizzling

Thin walls, crack around the base of the handles, large cracks, (micro)cracks. Possible colour change in the colourless glass. The craquelure pattern in the colourless glass appears to differ from that in the blue glass. The presence of a slippery surface varied over time: sometimes it was observed, sometimes not.

IC analysis of ion concentrations on the surface

Relatively high sodium concentrations, consistent with the visual characteristics observed during examination. Sodium is present in higher concentrations than potassium. When both are present in unstable glass, one is usually dominant.

• Ion Chromatography (IC) Form

pXRF analysis of the composition

Both colours of glass are probably potash-lime glass (they might also be classified as mixed-alkali glass), but the potassium-to-calcium ratio is higher in the blue glass than in the colourless glass. The blue glass is coloured with copper and also contains more iron than the colourless glass. It is likely that two different batches of base glass were used, rather than colouring part of the colourless batch by adding a copper compound to the molten glass. Lead was probably introduced together with the copper, although it may also have entered the glass in another way, for example through the inclusion of recycled lead-containing glass in the raw materials.
The glass also contains manganese, and here too the proportion differs between the blue and colourless glass. The proportion of tin and antimony is also higher in the blue glass.

• XRF Form

Previous treatments

The object was cleaned in 2013 with demineralised water and ethanol in a 4:1 ratio. No before-and-after photographs exist, and no information is available about the cleaning method used.

Condition diagnosis

Condition (visual) very poor
Condition (IC) likely unstable

Although the incalmo technique has been used in several unstable objects, there is currently no reason to assume a relationship between glass instability and the incalmo technique. The possibility will, however, continue to be monitored. Glass composition and the shape of the object may also play a role.
The fracture pattern of the crizzling differs markedly between the blue and colourless glass. This could be due to the glass composition but also to the shape of the object.
See also fractography.

(click on photos to enlarge)

advice & treatment

Suggestions for packaging, handling, storage and transport

Advice: record the vulnerabilities of this object in the (museum) documentation system and/or at the relevant location in the (museum) storage facility. Note that the glass is severely crizzled and must be handled with the utmost care. The object’s extremely fragile condition means that handling should be kept to an absolute minimum. It should therefore be placed in such a way that it can be moved and handled without direct contact with the object.

In such situations, care must be taken to ensure that there is as little contact as possible between any packaging materials and the glass (packaging materials may absorb droplets and create a microclimate that is not conducive to the conservation of the unstable glass object). If touching the object is needed, bear in mind that the object is badly fractured and has losses around the handles. Never handle the object by the handles! Always wear nitrile gloves, hold the object with both hands and support it properly. Place the object in a portable container and only handle it when absolutely necessary.

Keep the object in a separate cabinet with other unstable glass. Ensure the object is positioned so that its condition can be easily monitored and that it is readily accessible for inspection.

Suggestions for monitoring the environment and climate

Place the object in a stable environment with minimal fluctuations in relative humidity and temperature. It is highly advisable to monitor both the condition of the object and the climatic conditions regularly to detect changes at an early stage and to analyse any links with the environmental conditions. The object cannot be loaned.

Suggestions for treatment

Research shows that the object’s condition is deteriorating. Ion concentrations on the glass surface have begun to rise again following a past cleaning treatment. This deterioration is currently being studied to devise specific cleaning strategies.
Cleaning the object is advisable because droplets on the surface contribute to further degradation.

Only a glass conservator can determine whether cleaning is desirable and feasible, which cleaning method to use and how to closely monitor the condition. If undertaken, only a glass conservator should clean the glass.

Suggestions for further research

Further research into glass instability, particularly in relation to cleaning, and into the storage of unstable glass is urgently needed.
In this case, it would also be interesting to investigate further why the morphology of the crizzling differs so markedly between the colourless and blue parts of the object.