The main characteristics of the cellular structure of cork from Quercus suber L. are reviewed and complt;mented with new observations of virgin and reproduction cork by scanning electron microscopy. Particular emphasis is given to cell geometry and topology and to the corrugations that are observed in the cell walls. The effect of the growth season in these features is described. Large variations in cell size, wall thickness and corrugations are reported.

Highly porous silica xerogels were synthesized by the sol−gel process under atmospheric conditions. The silica alcogels were prepared by a two-step acid/base-catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS), with a water:TEOS molar ratio of 4, in 2-propanol. The catalysts used in the two steps were HCl and NH3, respectively, and the catalytic conditions were varied by changing the molar ratios HCl/TEOS (in the first step) and NH3/HCl (in the second step). After aging in an appropriate solution, the alcogels were washed and subcritically dried from 2-propanol, under atmospheric pressure. The resulting silica xerogels were characterized by the volume shrinkage upon drying, envelope density determinations, scanning electron microscopy, and nitrogen sorption isotherms. It is shown that by chemical control of the hydrolysis and condensation steps, through pH variation only, it is possible to obtain monolithic materials with properties similar to aerogels, stable under atmospheric conditions. Densities as low as 0.37 g cm-3, corresponding to porosities of 80%, were obtained for xerogels with specific surface areas of ∼940 m2 g-1 and a bimodal pore structure, with a narrow mesopore size distribution (average pore diameter of 8 nm).

The compression properties of cork were studied on samples obtained from cork planks of two commercial quality classes (good and poor quality), with densities ranging from 0.12-0.20g cm -3 and porosities from 0.5 to 22.0%. The stress-strain curves were characterized by an elastic region up to approximately 5% strain, followed by a large plateau up to 60% strain caused by the progressive buckling of cell walls, and a steep stress increase for higher strains corresponding to cell collapse. The direction of compression was a highly significant factor of variation, with cork showing higher strength for the radial compression.

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