When Roger and I wrote this article, my assumption was that a canister which labelled its contents as "Butane" did actually contain >99% butane, and a canister that was labelled "30% Propane 70% Butane" (like Coleman canisters) did contain just that.
But I was wrong. I should not be surprised of course; why would a manufacturer go to the expense of filling canister with a high purity gas which is only going to be burned in a camping stove? No, they are just going to use the cheapest fraction of gas they can get from a petroleum refinery.
So, what is the actual composition of a "Butane" canister?
The Campingaz/Coleman MSDS tabulates the contents of all their canisters under three headings: Butane, Super Butane and Butane-Propane Mix. It then describes each of these as:
Butane: composition in compliance with French decree of 3/9/79
Super Butane: mixture of butanes, butenes and propane (approx. 20%)
Butane-Propane mix : mixture of butanes, butenes and propane (approx. 30 %)
The Comite Francais du Butane et du Propane proscribes the properties of commercial Butane which it defines (in French) as "a mix of hydrocarbons made up mainly of butanes and butenes and containing less than 19% by volume of propane and propene". It also requires the vapour pressure of the mixture to be less than 6.9 bar at 50C.
So, "Butane" is a mixture too: Butanes (n-butane and iso-butane), butenes (4 possibilities, boiling points from -6.9C to 3.7C), propane and propene (less than 19% combined).
The MSDS gives some further information on the atmospheric boiling point of each of these mixtures:
Super Butane: -20C
Butane-Propane Mix: -25C
Pure butane boils at close to 0C, so clearly the other components of the "Butane" mixture are significantly depressing the boiling point.
Older versions of the MSDS also listed the PowerMax canisters containing Butane-Propane (40%) mix with a boiling point of -26C.
Conclusion? Depending on where you hike, "Butane" may be adequate for 3-season use, other brands may vary.