Charcoal Cavity Test

Some salts on heating with fusion mixture (Na₂CO₃ + K₂CO₃) in the charcoal vanity decompose to form their corresponding oxides which appear as colored incrustation or residue as :

ZnSO₄ + Na₂CO₃     →     ZnCO₃ + Na₂SO₄
                ZnCO₃       →     ZnO + CO₂

In the above reaction, zinc oxide is yellow when hot and white when cold.
In some cases. the metallic oxide thus formed gets reduced by carbon to metallic bead or scale as

Pb(NO₃)₂ + Na₂CO₃    →   PbCO₃ + 2NaNO₃
                       PbCO₃     →      PbO + CO₂
                                                Yellow

PbO + C    →     Pb          +  CO
                           Metallic bead