As it happens, I used some 30 year old meths in a Mamod this week. It was slow to catch light, but raised steam ok. It was in a sealed plastic bottle, and I assumed some higher fractions had migrated, though the blue colour was fine.

No, but alcohol absorbs water enthusiastically even through a sealed cap, and because it takes a lot of heat to turn water into steam the flame will be distinctly cooler. Wet coal and wood don't perform well either!

Dave

Edited By SillyOldDuffer on 12/03/2021 18:49:44

SOD , I interested to know how water is absorbed through a sealed cap -seriously.

Homebase stores are open and they stock it. So do B&Q.

Well the ones I’ve been in anyway.

It can be bought mail order.

We could test this! Leave the lid off and see if the level goes down or up

Rob

Sorry to disagree but Meths is mostly Ethanol with some (poisonous) Methanol, pyridine for colour and something to make it taste bitter.

MC

Same way moisture gets into hygroscopic brake fluid in a sealed system. Airborne humidity is sucked in mostly around non-perfect seals and even through not-perfectly impermeable rubber brake hoses or plastic meths bottles. Little by little over a long period it keeps accumulating . Here in the wet tropics with relative humidity 80 and 90 percent year round, brake systems corrode internally if not bled and flushed with new fluid annually.

As Hopper says, but with more detail.

Molecules are tiny compared with physical objects like joints and surfaces. Water has one of the smallest molecules found – about 3 picometers, or 10 billionths of an inch. In liquid form water molecules stick together into a larger body that can be sealed fairly easily, but water vapour is more difficult to contain.

A meths bottle is uusallly sealed with a screw-cap bearing on a waxed or aluminiumised cardboard disc. On the picometer scale even plastic threads that squish together are leaky, and the disc is essential. Cardboard is also full of holes, so the discs are sealed with wax, or aluminium. Aluminium is more effective than wax because its molecules form a tighter mesh, which is why aluminiumised plastic is used to package flavoursome foodstuffs and Helium Balloons.

Seals aren't really solid, they're more like a tight filter, ideally so tight nothing gets past but in practice small quantities often do. And the seal is even less perfect after the first opening damages it slightly.

Ethanol and Methanol molecules are more likely to be held successfully by the seal because their molecules are bigger, but air and water vapour can still penetrate. Once past the seal Meths has a strong affinity for water, so any that gets inside will be absorbed. This creates a slight vacuum encouraging more wet air to enter the bottle. Then day and night-time cooling create an effective suction pump. Air pressure inside rises whenever the bottle warms up forces some air out. When the bottle cools, the pressure inside drops and sucks in new wet air, which is promptly dried by the alcohol. Although the amount of water absorbed daily is tiny, it accumulates over time making it likely an old bottle of meths will collect enough water to reduce it's heat value.

Hopper mentioned hydraulics, but vapour escaping past seals is a common phenomenon. It was a major contributor to the Scott South Pole tragedy. The Scott expedition cached canned paraffin and food for use on the way back as they proceeded in stages towards the pole . They found most cans half empty on the return trip and not having enough fuel to melt snow for water weakened the men. Despite extreme cold paraffin vapour had escaped through the screw-cap and gradually emptied the can. Having to eating snow in the Antarctic didn't end well.

The blue colour of Meths is a dye. It's easily de-colourised by sunlight or a drop of household bleach.

Dave

My understanding is that they only harden in the absence of air enhanced by the presence of metal ions.

The presence of 'air' (oxygen) in storage is necessary to NOT harden.

They are (mostly) single component fluids of large molecular size – so low volatility.

The technicals appear to be endless.

FC

Some interesting points points raised in this thread. I have one of those Swedish "Trangia" camping stoves which is designed to run on methylated spirit. The user instructions say to add 10 to15% water to the fuel to avoid sooting. It still boils water very quickly for that outdoor cuppa and other culinary adventures, so thats fine by me. Unfortunately it still soots (more like a hard varnish really) the underside of the pans.

Zenstoves is a nice and simple US based website that looks at "backpacking" stoves and discusses, amongst other things, meths and other fuels. I found it quite informative and full of interesting rabbit holes to dive into. Some nice reading to occupy the mind until, blinking in the sunlight, we start to emege from the darkness of lockdowns and restrictions.

http://zenstoves.net/