The first results are presented from some experiments aimed to determine the porosity and different capacity to absorb humidity from soil of several pre-hispanic building materials covered with mural painting which are under restoration at present. The main destructive agents are not coming from external weather conditions, but from humidity originated in the soil with a great variety of dissolved salts. In the laboratory small prism shaped samples of the same raw materials utilized in ancient times have been tested and put in contact with a 1& Na2SO4 solution labeled with 22Na during fixed time periods. In this way, counts obtained divided by weight of each sample gives a figure approximately proportional to its porosity or capacity to absorb humidity from soil, making possible to compare the difference of this condition for several materials, and to test the efficiency of the proposed methods to control this problem. One suitable way to achieve it, seems to be the use of one solution of BaAc, plus thio-urea and methyl-methacrylate, in order to form a solid polymer. This method works reasonably well at laboratory level, and even when it has not been tested in the real scale, seems to be cheap and easy enough to slow down the increasing deterioration due to humidity and salts deposits coming from soil. However, even when this polymer is completely insoluble in water, the start of the reaction to form it requires the final addition of hydrogen peroxide, and temperature increase till the boiling point of the reagents, which may be too aggressive for ancient dyes and raw materials. Thus, the use of the so called French gelatin is proposed, and results at laboratory level are considered to chose the material better suitable at real scale with a minimum risk for the material.