Real Flame Effects on stage

[Malcolm Gordon]

The current National Theatre production of The Voysey Inheritance has an excellent set which includes two real-flame gas fires, both installed on trucks with chimney breasts above, but of a limited height. What is involved in providing an adequate flue for them and a safe gas supply? One of the fires sent an occasional spark up the chimney, like a wood fire. What would be introduced to achieve this? It then slowly died down very realistically.

 

Before someone says "if you have to ask, don't do it", I am only asking for interest, hoping to get an answer from someone who actually knows.