By Elaine Davidson
People often talk about bleaching ceramics, first you have to ask - what is bleach how, does it workand is it necessary?
Restoring ceramics should be done to increase the structural or chemical stability of the object. Because Dirt is a source of deterioration, it is often necessary to remove and or bleach it. In ceramics Bleach usually means either Hydrogen peroxide H2O2 or Household Chlorine bleach Sodium hypochlorite
NaoCl. The bleaching action of both these agents is caused by the oxidation of the chemicals that make up the stain. Because the color in a stain is usually rich in electrons it is more susceptible to oxidation resulting in a loss of color. While this removes the color it does not remove the foreign matter (dirt etc.) causing the stain.
A prolonged soaking in water after bleaching will help to remove this dirt so the stain will not come back again.
The area used for washing and bleaching and should house two sinks and a drainage area. The sinks should be large enough to hold the ceramics that are being washed and the plastic containers used when bleaching or soaking an object. A Removable rubber mat should be placed on the bottom of the sinks to help cushion and prevent chipping and breaking of the ceramic object.
Surface cleaning of objects to remove loose dirt and dust (from dry surfaces only) using brushes, an airbrush or photographic cans of air, the object is then washed in warm water with a mixture of Orvus (sodium lauryl sulphate) and Calgon (sodium hexa meta phosphate) to increase wetability. If the mixture is prepared beforehand, it should be kept in a squeeze bottle and shaken well before using. Rubber gloves and protective aprons should be worn. An assortment of sponges, brushes, old toothbrushes are also used to apply the detergents and clean the object. Matte finishes such as bisque objects are better cleaned with hand rubbing or a soft sponge. If an object is extremely dirty, after testing, a liquid oven cleaner (sodium hydroxide) may be applied and the object again immediately scrubbed with a soft brush or toothbrush . It is thoroughly rinsed to remove all trace of the dirt and cleaner and the left to soak in water for 24 hours to remove any remaining traces of dirt or oven cleaner. The water should be changed frequently to remove the dirt and chemicals and prevent them being redeposited on the object.
If an object is still dirty or stained after its initial cleaning, it may require bleaching. If just a small area needs a spot bleaching, a paste of a chlorine type abrasive cleaning powder and water can be applied to the area for several minutes. The piece must be thoroughly rinsed and again soaked for 24 hours to remove all traces of the paste. If this does not work or if the whole object is to be bleached, it is painted with a poultice of 100 volume (35%) Hydrogen peroxide, ammonia (1 tsp. Per cup of peroxide) as a catalyst and flour (2 heaping tblsp. Per cup peroxide) to make a paste.If a stiffer paste is required more flour canbe added but it is much harder to apply. The paste is applied with a hair colorist's brush or a Rubber spatula. They are then wrapped in plastic (polyethylene) to keep the poultice moist and left for a week. After a week they are removed from the plastic and placed in a container of hot water with a small amount of chlorine bleach 1/2 teaspoon per gallon of water to aid in the removal of the poultice. They are left to soak for 1/2 - one hour. The objects are then removed and rinsed under warm running water using a soft cloth or brush to remove any remaining traces of the poultice If the piece is fragile or there are any concerns the piece should not be placed in the bleach but just soaked in water.
If the piece is needed faster, it can be boiled in peroxide with a few drops of ammonia as a catalyst. The container should be either enamel or ceramic and sufficient larger that the piece does not tough the sides. A cloth should be placed on the bottom of the container to prevent damage and the heat should be slowly raised to the boiling point of the peroxide and maintained a simmer, usually several hours, until the stains are removed. The ceramic should remain in the container until it has cooled to avoid damage from to quickly cooling it. A separate container of heated peroxide should be kept nearby to replace any peroxide lost to evaporation. A good ventilation system is also necessary to remove any fumes.
. They are then soaked in warm water for another 48 - 72 hours to make sure that all the residual chlorine bleach and dirt have been removed. This procedure may be repeated several times until all of the stains and dirt are gone. Both the colorists brush and the peroxide are available from a hairdressers supply shop. The object is then left to dry out for several days (usually twice the amount of time it has been in the poultice) before proceeding to the next step. Chlorine bleach is not recommended for porous ceramics because of the possibility of the absorption of the chlorine into the ceramic body, causing salt crystals to form under the glaze or in any cracks. Peroxide may also react with iron in earthenware ceramics and cause iron staining. It can also affect gilding and unfired decoration. Peroxide groups may also remain attached to the stain affecting further treatments.
Reducing agents are used to remove rust stains. Because the mineral is in am oxidized state the rust must be changed into a soluble form by the reducing action. A 5% solution of sodium hydrosulphite (sodium dithionite) is applied to the stain. It reacts with the rust making it more soluble. It may also bleach the stain. The artifact is then thoroughly rinsed in water and washed in a solution of DTPA (dethylene triamine pentacetic acid) dissolved in a 5% solution of sodium hydroxide (Ph of 5.5). The dissolved iron forms a chemical bond with the DTPA that can be washed out of the object. The object must then be washed and soaked in water until all traces of the chemicals are gone. Commercial rust remover, or muriatic acid may also be used.
Organ R. M., Studies in Conservation Volume 32 pp 41 - 46, Adaptable
Compact Modular Bench, 1987
Plenderleith, H. J., The Conservation of Antiquities and Works of Art,
Oxford University Press; London;1969
William's Nigel , Porcelain Repair and restoration, Colonade Books British
Museums publications Ltd., London; 1996
S Buys S oakley, The Conservation and Restoration of Ceramics, Butterworth Heinemann London 1993
i The Conservation & Restoration of Ceramics, S. Buys, V. Oakley, pp182