Historical stringing evidence

Sources, including the epic Kalevala, suggest that the very earliest kantele were strung with horse hair. However, all surviving kanteles from the past four centuries are strung in wire. The earliest surviving dated kantele is from the end of the 17th century, so we can say for certain that stringing in wire occurred from at least that time, but how far back this stringing was used we cannot say for certain.

Surviving instruments in museums have variations of brass, iron, and steel strings, sometimes in combination and sometimes in exclusivity. We cannot say for certainty how many of these strings may be original and how many are modern replacements. This is further complicated by the fact that the traditional instrument of 5 to 13 courses was intended to be a playable, functional instrument rather than an art piece, so they no doubt had continuous service throughout their lifetime, and therefore were restrung as necessary throughout their lives.

String gauges according to scale length

First, the scaling of the instrument is critical to figuring out the appropriate pitch, and therefore the best strings to use. 

Strings can be all of one gauge if the pegs are arranged so that the string lengths can be mathematically proportional according to their pitch (as illustrated in the instrument in the bottom of the image above). For kanteles with less difference in string lengths (such as in the instrument in the top of the image above), a variety of gauges may be necessary. 

With regard to the pitch of the strings, finding a string close to its breaking pitch as one would for other instruments is not necessarily the best route, as the nature of the method of mounting the string on the varras by pulling the free end of the string through the string loop creates a natural stress point on the string where it touches the loop, making them slightly more prone to breakage in this area.

If one wishes to use brass or iron stringing, I recommend going with a string tuned well below its theoretical breaking pitch in order to decrease strain on the string in the critical area of where it crosses the loop at the varras. Contrarily, steel can be used with success at higher pitches, but in all likelihood its use would not be historical before the 19th century.