In the Gossage process for the preparation of caustic soda the chemical added to washing soda is

Gossage process (or) soda lime process
Dilute Na₂CO₃ (washing soda) on reaction with hot.milk of lime [Ca(OH)₂] gives caustic soda (NaOH) is called Gossage process

${\mathrm{Na}}_2{\mathrm{CO}}_3+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\to \underset{\mathrm{ppt}.}{{\mathrm{CaCO}}_3}+2\underset{(\mathrm{caustic} \mathrm{soda})}{\mathrm{NaOH}}$