The Mora Poultice

The Mora Poultice

An interesting system of cleaning limestone and marble on a chelating agent was developed by Mora. The chelating agent is ethylene diamonotetraacetic acid (EDTA). This weak acid facilitates the dissolution of calcium salts by complex formation. Complexing or sequestering agents separate or are cut off. The Mora poultice has been used with considerable success on marble and travertine which have had moderate soiling. The poultice contains 60 g ammonium bicarbonate, 60 g sodium bicarbonate, 25 g EDTA, 10 g surfactant disinfectant and 60 g carboxymethylcellulose (CMC) in 1000 ml of water. The ammonium and sodium bicarbonate give a slightly basic mixture of pH 7.5 and facilitate the dissolution of some salts.

The poultice, in the form of a clear jelly, is applied to a pre-wetted surface by spatula or by brush to a thickness of 3-4 mm, and is covered at once with a thin polyethylene film to prevent drying out. The film is of utmost importance as the cellulose body of the poultice is very difficult to remove if it dries and hardens. The contact period may be twenty-four hours, and intermediate lifting and reapplication may be necessary. After cleaning and removal of all poultice material by the use of small rowels or spatulas the surface should be washed thoroughly with clean water.

The advantages and attractions of this system are principally that it is safe chemically and avoids any excessive use of abrasion or water. It cannot, however, be applied to friable or flaking surfaces (any more than any other poultice) without removing surface material. Surprisingly, large areas can be cleaned relatively economical when the surface is not too detailed.

Source:

Cleaning masonry buildings

John Ashurst

In Conservation of Building and decorative Stone

Eds. John Ashurst and Francis G. Dimes

Butterworth Heinemann Great Britain

1999

P. 134