Na2 is a restrainer used in palladium printing to increase the contrast of the print. It acts by inhibiting the sensitivity of the solution. It affects highlight areas more severely than shadows and thus increases the resulting print contrast. Varying the amount of Na2 solution you add to your basic palladium-ferrric oxalate mix will allow you to fine tune the contrast. The downside to this method is that it uses a platinum compound that can significantly cool the tone of a palladium print. It is also useful only for pure palladium prints. The benefit of this method is that it does not cause graininess in the print like some other restrainers can when used in the larger quantities.