The gas evolved at point W was likely carbon dioxide. When lead (II) oxide is heated with carbon, a redox reaction takes place where the carbon reduces the lead (II) oxide to form lead metal and carbon dioxide gas. The carbon dioxide gas produced is then ignited, causing it to burn for some time.
The overall reaction can be represented as:
PbO + C -> Pb + CO2
This reaction is commonly used in the extraction of lead metal from its ore, lead (II) oxide.