conservation

advice & treatment

cleaning glass

Few people have never cleaned a glass object. However, the approach to cleaning objects of cultural and historical value is different. The primary difference lies in the diagnostics carried out beforehand. An understanding of the object’s making processes, knowledge of degradation phenomena, expertise in the field of instability, and the ability to visually identify the smallest cracks or old restorations mean that every glass object is unique and can therefore undergo a specific cleaning process. A conservator can identify the vulnerabilities, the type of soiling and any possible cleaning strategies.

A conservator will always first seek to contextualise the ‘dirt’ and ask the following questions:
– Why is the object soiled or why does it have certain stains?
– How might the dirt have accumulated?
– Does the dirt contribute to our understanding of the object (for example by telling us something about the making process, the conditions in which the glass has been kept, the context of any degradation or how the object was used)?
– What negative effects does the dirt have on the condition of the glass object?
– What cleaning treatments can be considered, what results might the various options produce and are there any risks involved?

It is possible that some 'stains' are related to the making process or have even been deliberately applied by the artist. The following factors must be weighed: dirt as a source of information, dirt as a source of degradation, and the possibilities and limitations of cleaning.


Particulate soiling in the deeper cavities of a glass surface.   PHOTO M. Slager

In addition, a conservator is familiar with conservation cleaning techniques. In many cases, the conservator will use dry or minimally wet methods. Wet cleaning is often carried out using (demineralised) water, although other solvents such as acetone or ethanol may sometimes be added. The choice of solvent and concentration depends on the object’s characteristics. Common techniques range from the careful dry removal of surface dirt with a paintbrush or soft bristle brush to immersing the object in water. Commercial glass-cleaning products are never used. During the cleaning process, a conservator will monitor progress and effectiveness using a loupe or microscope.

 
Sometimes it is necessary to remove dust immediately using a vacuum cleaner. It is safer if the vacuum cleaner can be worn on the back.

Cleaning is always preceded by a careful assessment of the type of glass and its vulnerabilities. Lead glass, for example, can be scratched when a brush or cotton swab is used to remove dust. It is also important to determine what type of dirt is present.

Special materials and techniques are used to clean enamelled glass, mirrors, chandeliers, archaeological glass, cold-painted glass, reverse glass paintings, architectural glass, glass mosaics, glass featuring special intermediate cooling techniques, glass in public spaces, glass with old restorations, glass with particularly fragile or almost sealed forms, gilded glass and glass combined with other materials.

Cleaning is important for the preservation of glass objects, but how often an object needs cleaning depends on its specific context. Many museum objects are cleaned at least once during their ‘museum life’, usually when they first arrive as new acquisitions.

Shown below are special homemade ‘sleeves’ made from various materials and in various shapes that can be slipped over a long cotton swab or a flexible brush. Some have a long thin ‘tongue’ for removing dirt from bottles or a pocket into which cotton wool or other soft material can be inserted for absorption or to provide extra protection for delicate surfaces.