Add the vinegar, the rest of the capers and shallots, then reduce over a hot flame until it has a sauce-like consistency.

Put your stock on the stove over a hot flame.

The blue flames had a property of being the most intensely hot flame, more so than normal red or orange flames.

The type of glass to use next to the hot flame was a problem eventually solved.

By vaporising the kerosene before it reached the burner, their construction had a sootless, smokeless, hot flame.