The description 'mantel clock' is just a generic term for almost any free-standing clock that is not known by another name (such as Bracket clock or Carriage clock). So it can be used to describe anything from a French four glass clock or a Swiss tortoiseshell and brass inlaid boulle clock, to a German Napoleon Hat Westminster chiming clock or an American steeple clock (though the Americans prefer to call theirs Shelf clocks).
The expression presumably originates from 'mantelpiece' though modern mantelpieces are too narrow for most of these and would be a wholly inappropriate place to position a clock anyway because of the extreme temperature variations and the propensity for attracting dirt and other harmful substances which slowly grind down the movement's pivots and pivot holes.
The commonest mantel clocks are probably French and include the ubiquitous French marble clock, which often conceals an eight-day one or two train movement of equal quality to much more valuable French clocks. If you have a marble mantel clock, get a copy of Nicholas Thorpe's book dedicated to them ('The French Marble Clock' published by NAG Press). It's informative, well illustrated and written in an easy to understand way, with lots of historical background. Much cheaper is a 32-page booklet called 'Marble Clocks' by Wotton and Oliver, if you just want a basic background.
Another popular French mantel clock is the four-glass (which looks like a large carriage clock with a pendulum), so-called because the sides, front and back are all glazed. Differentiating it from the carriage clock is the fact that it has a pendulum rather than a balance wheel to regulate time-keeping. Sometimes the pendulum incorporates two sealed glass phials containing mercury (or quicksilver as it was once known). This design improved timekeeping by compensating for temperature changes, which were more common overnight before the advent of central heating. In the cold nights, the contraction of a steel pendulum rod causes a clock to run slightly faster but for the same scientific reason the mercury in these clock pendulums also contracts, lowering the centre of the bob, having a counteractive effect.
American mantel clocks are much cheaper by comparison but be aware that US makers often copied French designs. So before buying a 'French' marble clock, check that the case isn't just painted wood or iron; check that the pendulum isn't comprised of steel tubes instead of mercury in glass; and check that when it strikes it sounds healthy and not like a bag of spanners!
When it comes to repair and overhaul, the good news is that the cost of servicing a mantel clock with a pendulum is the same for most pendulum clocks; £125 for a timepiece (with ONE winding hole) and £175 for a striker (with TWO winding holes). (If yours has THREE winding holes, go to my page on Cleaning services).
If there is no pendulum, it probably has a lever or cylinder escapement mounted on a separate platform, or a floating balance wheel, add £25 for the servicing of that, too. I'll dismantle the movement and clean each part separately, checking for wear and the proper engagement of wheels, pinions and pallets. It will be properly oiled and set up for a level surface. I can replace broken glass, bezels, and hands but these would be at additional cost as would any restoration work on the case.
All paid for work's guaranteed for a year.