What is the Italian ‘Acido Psammico’ (Psammic acid?)
Actually, the meaning is: 

used to tan leather.


The acid used traditionally in tanneries is tannic acid, from there its name.

Tannic acid can be found in several plants one of which is the Sicilian Sumac (Rhus coriaria). This plant is also found in Sardegna (as per Flora Italiana), where the novel is placed.
The Italian name for this shrub is Sommacco siciliano.
The description of this plant is:

piccolo albero tipico di luoghi sassosi e rupestri, con fiori giallastri in      
pannocchie, le cui foglie e corteccia, ricche di tannino, si adoperano nella 
concia delle pelli (fam. Anacardiacee)


[the last part says: the leaves and bark, rich in tannin, are used in the tanning of leather.]

From sommacco to psammicco there is not too much distance.
But I agree that it is speculative.

Tannic acid is toxic, but the oral LD50 in rats is 2.26±0.083 g/kg body weight, so too much to hide in an ostia. 
However, that can be a writers license.

Tannic acid is a complex polyphenol with structure: