I am no Chemist, but I believe that "Nitre de soda" is Sodium nitrate sometimes referred to as "Chile Saltpeter" due to the large quantities in Chile. One of the many uses for it, is fertilizer, so I assume that it is quite available in Brazil. Sodium nitrate is a Oxidizer as is Potassium nitrite that is used in many hot bluing solutions. Sodium nitrate/nitrite and Potassium nitrate/nitrite are all forms of Oxidizers.
The common chemical in Celso Ari's hot bluing and other hot bluing is the Sodium hydroxide which I assume is the coloring portion of the bluing process and a oxidizer to help along with the process. Don't know which solution is better, but Celso's demo shows excellent results.