About 10.0 g of soil samples from Londrina, Floraí,
and Tibagi and clays, bentonite, montmorillonite, kaolinite,
were put into porcelain crucibles, heated for 15 min at
either 280, or 400, or 650 °C in a muffle furnace, and
cooled to room temperature. Samples were kept in a
desiccator until use. The samples were split in two
portions and one was treated with GPS.