CLEANING ANCIENT AND ARCHAEOLOGICAL GLASS
Cleaning Ancient and Archaeological Glass
The most important question to consider before starting is should the glass object be cleaned at all? “Has it been washed previously? Is it weak? Does it show any flaws (manufacturing chill marks or bruises), cracks, or repairs? Does it have cold-painted decoration or a surface that could be damaged by wetting? Does it reveal a weathering layer (iridescence), damaged enamels, or lifting gilding? Answering yes to any of these questions should make you hesitant about cleaning the glass. Ancient or archaeological glass should not be washed or cleaned unless it is extremely sound, with no repairs and no weathering. Glass objects that can be safely washed (most objects in the collection except ancient and some modern and composite objects) are washed with tap water and a mild conservation-grade detergent (any mild detergent without dyes or perfumes would work), followed by thorough rinsing with de-ionized or distilled water. The glass is then either toweled dry with paper towels or air dried. For some objects, like bottles with narrow openings, the inside is rinsed with a small amount of acetone to help remove moisture. Old adhesives from previous repairs or labels are removed with solvents, mostly acetone, ethanol, or a petroleum distillate like naphtha” Stephen P. Koob, Chief Conservator (The Corning Museum of Glass) The major recommendation is to have, when desired, those cleanings executed by professional restorers and that only in case one wants to do the cleaning oneself the following procedures could be used.
Procedure for Washing Glass
This procedure does not purport to address all of the safety concerns, if any, associated with its use for the glass object or the user. It is the responsibility of the user of this procedure to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Partly fill a plastic tub with tepid water and small amount of mild liquid dish detergent. Gently wash one glass object at a time using a soft brush (plastic handle), soft cotton cloth or paper towel. Next the object is thoroughly rinsed, either under running water or with the use of a spray. The final rinse, if possible, should be with distilled or deionized water. On a plastic drain board lightly dry with a paper towel and then allow it to air dry. To remove moisture inside objects like bottles use small amount of acetone or ethanol.
Procedure for Removing Calcium Carbonates Accretions from Glass
This type of accretions form from evaporated water and sometime appear as white spots. It is hard to tell by just looking what the accretions are. It is recommended that if washing doesn’t clean the object to the level you want then proceed with this step next. Remember ancient and archaeological glass will never be as clean as new glass nor should it be. Calcium carbonate deposits can be best cleaned with dilute (3-5%) solution of nitric acid. Why nitric acid and not vinegar (acetic acid) or another weak acid is the evaporated salt of nitric acid is neutral. (Caution; wear rubber gloves and protective eyewear when handling acid and work on a plastic drain board or soft surface) Apply the dilute acid with a cotton swab. In the worst cases, immersion may be necessary. The deposits may effervesce (bubble) or fizz (CO2), and the deposits will dissolve in water or soften within one or two minutes. Any residues are then cleaned off with soap and water, followed by thorough rinsing. Other accretions may require mechanical cleaning, which is not recommended. Silicate deposits are rare on glass, but can result from burial.
Procedure for Removing Silicate Accretions from Glass
Silicate deposits have a similar appearance to those of carbonate deposits, but are not dissolve by acids. These deposits can usually be removed with a dilute alkali (caustic) solution such as 10-15% sodium hydroxide. (Caution; wear rubber gloves and protective eyewear when handling acid and work on a plastic drain board or soft surface) Sodium hydroxide solution should be applied only for short periods (10-30 seconds) using a synthetic brush or cotton swab. The treatment is to be followed by a thorough rinsing. (Caution; alkalis are extremely slippery, and the glass object should be held firmly in place, not hand-held if possible) Remember ancient and archaeological glass will never be as clean as new glass nor should it be.
The above three procedures can be can be repeated over again until you feel that the glass is as clean as it can be without mechanical cleaning. Cloudiness or the hazy appearance of a glass surface that cannot be removed by any of these methods may not be a stain or deposit but permanent deterioration of the glass surface which cannot be cleaned. Some more cautionary notes, phosphoric and hydrofluoric acid will dissolve silica and certainly attack the glass itself. Also don’t use commercial products recommended for cleaning glass surfaces (shower doors) on ancient or archaeological glass because you don’t know what is in it.
Koob-Conservation and Care of Glass Objects, Stephen Koob, The Corning Museum of Glass, 2006
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