A process for the decontamination and cleaning of monumental marble works

The present invention concerns a process for the decontamination and cleaning of monumental marble works, based on the continuous abstraction from the liquid into which the pieces to be treated are immersed of the anions of the soluble salts, by means of exchanging resins.

In those countries with a cold or temperate climate the alteration of marble, or more generally of all those stones with a carbonate component, is nearly always characterized in the formation of a thin black crust, a few . millimeters thick, strongly adherent to the substratum. The outermost part of said crust, of about 1-2 millimeters thickness, mainly consists in calcium sulphate, with carbon deposits, traces of quartz, analcime, augite and most rare calcite. In the innermost part the carbon component increases with respect to the calcium sulphate.

The processes for decontamination and-cleaning used until now provide the application onto the pieces to be treated of a poultice of absorbing materials, like sea-foam : or attapulgite clay, or paper pulp free of soluble salts or cotton wool. Said processes are obviously long lasting, expensive and request highly skilled workmen.

Also other processes, e.g. saturation with water, are used for cleaning the monuments from their superficial crusts, but said methods are not only very slow, little , practical and rather expensive; they also cause the penetration of water - rich with dissolved salts - in the porosities and/or in the microfractures of the stone, so as to cause again, due to the following evaporation, the precipitation of a part of the soluble salts into the stone, thus supporting the further deterioration thereof.

It is the aim of the present invention to realize a process for the decontamination and cleaning of monumental marble works that eliminates the danger of a new salt deposit inside the stone, thus avoiding any degradation.

According to the present invention, this aim is reached immersing the works into deionized water, pushed by a pump for the circulation in a column containing ionic exchange resins for separating, as quickly as possible, the S0₄- ione from said solution, said ione deriving from the dissolving of the calcium sulphate so that the calcium bicarbonate formed following to the dissolving of the sulphate contained in the work immediately precipitates at values of PH = 7 - 7.5 in the form of undissolvable carbonate, determining the formation of a superficial crust that can be easily removed with a distilled water jet so as to leave the clean marble underneath.

A scheme of a circuit for the realization of above mentioned process is shown for exemplifying and not limiting purposes in the attached drawing.

Relating to the details of the drawing, 1 shows a basin full of deionized water into which the marble works are immersed; said basin 1 is connected to a centrifugal pump 2 that makes the water circulate through the columns-shaped container 3 of ionic exchange resin, provided with a filter 5, placed in parallel position to an auxiliary basin 4 from where the water returns to basin 1 closing the circuit, which is also provided with a PH-meter 6.

According to the present invention, the ionic exchange resin contained in 3 is preferably of the "GEL" kind, meanly basic and consisting in an acrylic copolymer with cross connections showing an - N(R) group as the functional group.

From the point of view of the iones to exchange, said resin may be provided in the form of free basis, i.e. able to exchange OH- groups, as well as in the bicarbonate form, able to exchange -HCO₃- iones.

The PH interval in which said resin determins the maximum efficacy in this process os from 0 to 7, but it is still efficient at PH = 7 ÷ 8.

The application of the process according to the present invention with the described means shows the advantage of being very quick and of determining the complete removing of the soluble salts and of the carbon components, also in the case of particularly carved works.