Unusually this boot comes in two versions, the Mercury GTX with the almost ubiquitous Gore-Tex lining, and the Mercury LTH which is leather-lined and officially non-waterproof. To me the option of LTH is very welcome. There aren't enough leather-lined boots on the market. But why not? Being soft, durable and non-sweaty, leather is as suited to the inside of footwear as it is to the outside. Except in winter conditions or prolonged wet weather I'm not a huge fan of waterproof linings in walking boots. Lab tests say they are no sweatier, but my own experience suggests otherwise. In warm weather what's the advantage of a lightweight boot that's as hot and clammy as a heavy one? Boots like the Mercury are ideal for less hostile terrain and conditions, and in such situations a waterproof lining is often redundant.
"...To me the option of LTH is very welcome. There aren't enough leather-lined boots on the market. But why not?..."
Given my bias I naturally chose the Mercury LTH for this test. Did I live to regret it? Not at all. In a late heatwave on a round of Ennerdale I was particularly glad of their cool feel, and I imagine anyone heading to sunnier climes would agree. I have mostly used these boots for fair weather walking, but you can't always plan for the ground underfoot. The Back o' Skiddaw, for instance, was unexpectedly boggy (OK, I should have known). With all the peaty puddles and wet grass the surface of the nubuck uppers soon wetted out, but leather itself is not water-permeable (how often do you see a cow leaking?). Despite the uppers' elaborate detailing there aren't that may stitched seams through which water can penetrate, and it took some time for my socks to feel damp. A bit of Nikwax on the seams would have kept me drier longer still. Compared with a membrane lining the leather inners are quick drying, and I was back to comfortable dry feet before the long walk-out was over. All that said, if you intend to wear them a lot in rainy weather or on spongy terrain then the Mercury GTX would be a more sensible choice, particularly at this time of year when the ground is less likely to be dry and the cold cancels out any possible membrane sweatiness.
The Mercury LTH (not the GTX version) has something called 'Foot Mapping', which puts different materials next to different areas of the foot – those that sweat most, those that feel the cold and those most sensitive to pressure. To this end the lining comes in several panels, smooth leather at the cuff and heel, leather perforated with little holes on the tongue and mid-foot for ventilation, and something fleecy-feeling at the toe (I can't see what exactly). For me at least the fit is spot on – snug and supportive enough at the rear to eliminate any danger of heel lift, foot-hugging in the middle so there's no sliding around, and sufficiently roomy at the toe to accommodate my broad feet.
Mammut's 'Rolling Concept' creates underfoot support with areas of differential damping on the sole to help prevent fatigue and reduce the danger of a twisted ankle. My ankles are fairly twist-prone, and the Mercury's soft low-cut cuffs don't offer quite the level of stiff support that I'm used to in a more traditional leather boot. But so far I've had no turned ankles in these boots, so perhaps the Rolling Concept is doing its job. It's an unusually soft sole but still offers a degree of lateral stiffness, which combined with a pronounced forefoot flex gives a good combination of support and a lovely natural spring to the step. With their shock absorbing EVA wedge these soles soak up the impact when walking on hard surfaces rather better than my road running shoes. Given the Mercury's combination of lightness, flex and support it wouldn't actually be ridiculous to run in them, and I've done so once or twice. There aren't many boots I can say that about.
"...The Mercury LTH has 'Foot Mapping', which puts different materials next to different areas of the foot – those that sweat most, those that feel the cold and those most sensitive to pressure...
I'm less convinced by the claims made for the tread. The board-lasted 'HLX' (?!) sole has been developed in cooperation with Vibram, so I expected more than it delivered. The hexagonal lugs of this 'Frog Grip' sole are said to ensure optimum adhesion to all types of terrain. Apparently Mammut took its cue from the hexagonal surface texture found on frogs' feet – I'll have to take their word for that since I've never been on sufficiently friendly terms with an amphibian to see for myself. It sounds great, but in practise I haven't found the Mercury any more confidence inspiring on wet grass or slimy rock than other boots, which is to say not very. 'Frog Slip' might have been a better name. The sole has quite a complex tread pattern, but it's not particularly deep. There's also rather a lot of flat surface in contact with the ground. This is great for friction on dry rock, but won't help the sole bite securely into mud, for instance. I would not go scrambling in these boots unless I was confident of finding bone dry conditions underfoot. I suspect the pliable rubber lugs will wear down quickly too.
Back to the uppers. These are soft nubuck, stamped with a pattern of raised ridges around the mid-foot to give some minimalist structure. There's a tiny but adequate rand at the toe but very little protection elsewhere, meaning that on scree slopes your feet do get bashed. The nylon webbing lacing is smooth-running, which is great if you want an even tightness along the length of the foot, but not so great if you prefer to lace loosely at the toe and tighter towards the top. The lack of excess stitching on the uppers will help with durability, but combine the lack of rand with the softness of the nubuck and you've got a recipe for easy scuffing. After fairly light use my sample pair is already looking wrinkled and a little worn.
The Mammut Mercury does not offer the durability, support or protection from the elements found on more substantial leather boots, but then it isn't meant to. The tradeoff in these terms is offset by the weight saving. This is the epitome of light hiking footwear, ideally suited to gentler terrain and less challenging conditions, and fine on summer mountains too. If you're a fair-weather hiker or heading somewhere nice and dry abroad then consider the LTH version. For wetter winter walks below the snow line the GTX will have the edge. Neither model is likely to withstand years of hard use, and since more robust mountain-capable boots are available at this sort of price point the cost does look a little heavy for the length of service you can expect. But if you believe weight saving trumps money saving then either Mercury boot is a good choice.
More info on the Mammut website