Historically (mostly) plastic double boots had a lot of pros: they were warm, rigid and could be dried out easily by simply removing the foam inner boot and sharing your sleeping bag with it. There were plenty of downsides too, plastic boots tended to be heavy, clumsy, not particularly breathable and horribly bulky.
As a result, for lower altitude mountaineering, they've tended to be replaced by lighter, more nimble, single boots. Not as warm or easy to dry out, but lighter, more comfortable and generally neater and more precise on technical terrain.
But what if you could make a boot with all the traditional double boot virtues, but with the same sort of weight and precision as a lightweight single mountaineering boot? That's pretty much what Arc'teryx has done with the new Acrux AR.
Outwardly it looks very similar to a lightweight single boot with an integral gaiter like the Scarpa Phantom series, but the difference is that inside the Gore-Tex covered is a Gore-Tex-based - again - Adaptive Fit liner that can be removed for quick drying and to be used as a hut slipper.
Despite that double construction, the new boot is at 960g per foot (size 43) lighter than an original Scarpa Phantom Guide and pretty much identical in terms of size and bulk.
Want more details? The boot has three distinct layers: on the outside there's that Gore-Tex integrated gaiter with lightweight rand put together using Arc'teryx laminating technology and bonded to a rigid Vibram sole incorporating reinforced ledges for clip-on crampon attachment.
The gaiter uses a waterproof T-ZIP, same as the Scarpa, with a press-stud closure up top. Next layer in is an insulated shell boot made from dense PE foam with a combination of conventional laces and a Velcro-fastened top strap. It feels supportive, but also surprisingly flexible in the ankle.
Finally, the inner boot, like those in other Arc'teryx footwear, is made from a stretchy Adaptive Fit, Gore-Tex and foam liner that moulds to the foot. Look carefully and you'll see tabs at the front and rear of the ankle cuff as well as one at the base of the heel. They're necessary because that inner is quite a snug affair and can be tricky to take on and off, which may get interesting on an alpine start with frozen fingers...
The inner boot also has its own lightweight rubber sole, so you can use it as a tent or hut slipper as well as just removing it to dry it out. And yes, you got that right, there are two separate Gore-Tex layers in one boot, one in the shape of the integrated gaiter and the other forming the Adaptive Fit inner.
Overall it looks like a genius concept: speaking as someone who's climbing partner once got mild frostbite on a south-facing route in the Andes due to his preference for precise, lightweight single boots over clumsy plastics, the idea of a neat, precise, low-volume double boot with better insulation value makes a lot of sense.
At the boot's press introduction last summer, Arc'teryx told us that it should work in temperatures of down to approximately -20˚C and up to around 5,000m or so, which makes it Alps and, up to a point, Andes friendly.
But here's the thing, we can't see any particular reason why it wouldn't also work in Scottish winter conditions where the removable, fast-drying inner would be brilliant for multi-day mountaineering trips and the snug, supportive fit would be a boon on technical stuff.
How durable will it be? Time will tell; boots using integrated gaiters like the Scarpa Phantom range, have tended to be less tough than traditional leather or plastic designs, but as you'd expect from Arc'teryx, the brand has embraced top-end materials and components and put them together with an obsessive emphasis on detail and design, they are impressive things just in terms of pure engineering.
Yes, the £500 price-tag reflects all that, but our first impressions are that these really do offer something that's markedly different from anything else out there. If you're looking for a technical mountaineering boot that offers many of the advantages of plastic double boots, but without the downsides, the Acrux AR could be exactly what you're after.