If you don’t absolutely have to DO NOT SOLDER ANYTHING ON A CLOCK. I say this because I have seen thousands of clocks with blobs of solder and acid flux all over them, many of them destroyed because of it. For example, simply tack soldering gear teeth on a gear to fix the gear is something one should never do. This is an insult to the owner, the maker, and the repair persona. What usually happens is it holds long enough for the owner to wind it up a few times then it lets go and destroys more that just the gear. Or worse yet, it may hold for several years, and then let go. This opinion is because of many bad experiences with clocks other persons have ruined. It is very discouraging to open up a pristine looking french clock, only to find it has been butchered with a punch and blobs of solder. Because of this, it is important to realize, that other repair persons may not be as negative about solder as I am; and it is true that it could have been repaired during the war when no brass was available for repairs, however, a good repair person can tell just by looking whether a clock has been repaired recently or not. How, you might ask? You look at the oil ( or lack of it ) on the verge, you look at wear spots with an eye loop, you look at the condition of the pivot ends, you look at the lustre of the entire mechanism, you look at the inside of the cabinet, you look for traces of dust, you listen very carefully to the mainsprings and pay close attention to their feel as you let them down. When you have done this many times on many clocks, you will be able to tell if a clock has been worked on recently or not; then you will know the reason why there may have been soldering where there was not an immediate necessity.
If you must solder a part on a clock, silver solder is ok but be careful because heating brass makes it very soft. You can ruin a gear or a barrell if you are not careful. If you see the brass get red hot, you have already ruined it. Right before the brass gets red hot you have already ruined it. Electronic solder will not work. It is not strong enough. There is solder available that requires only slightly more heat than a soldering iron, and is much sronger. It is available from most clock repair parts suppliers. It is possible to solder parts with this solder without ruining them, but you still must be very careful. Do not overheat the brass. Try a heating a piece of scrap brass and then hammering it and working with it before you ever try any soldering on a clock. This is the best way to learn for yourself how brass reacts to heat. Do keep in mind, however, that the size and shape of a piece of brass will affect how it reacts to heat.
Soldering in itself is somewhat of an art. If you have worked in electronics or plumbing and understand how solder works, then you have a head start, but you are not there yet. Brass does not react the same way as copper, or tinned copper wire, it is not as forgiving. Don’t even attempt any kind of soldering on clocks until you fully understand the basics of it first.