CATRIONA McKay and Chris Stout are on a rollercoaster. One day the harp and fiddle duo might be preparing for a concert with the Scottish Ensemble. The next it’s a recording with a Norwegian folk singer or a traditional musician from Mozambique, an Irish accordionist or in Stout’s case, a gig at a jazz festival with guitarist Graeme Stephen.

Their recent album, Bare Knuckle features compositions that were inspired by trips to Russia and time spent in Brazil, two more in a series of adventures that neither of them foresaw back when they were learning to play their respective instruments in Dundee (McKay) and Shetland (Stout).

Next Wednesday, they play the closing concert at Edinburgh International Harp Festival, an event they’ve featured in numerous times in the twenty or so years they’ve been making music together, and McKay teases Stout by remembering that when they first met he didn’t know what a clarsach, the Scottish small harp that McKay has taken as far musically as geographically, was. Stout counters that, in Shetland with its rich fiddle legacy, they didn’t need so many strings to play tunes.

That first meeting, at the then Royal Scottish Academy of Music and Drama in Glasgow, didn’t bode well. McKay had been advised to seek out “this fiddle player from Shetland” because he knew lots of tunes that might benefit a classically trained musician who wanted to explore the tradition. What Stout didn’t know at the time, but would soon appreciate, was that McKay’s preparatory research in the Academy’s library had led her to a quote from a music lecturer that described the harp as a limited instrument.

“That immediately got me going,” says McKay. “I thought, there’s no way someone’s going to represent my instrument like that, and what’s become clearer over the years is that the only limitations are in your mind. The thing about the harp is that it’s a very simple instrument whose body is actually its mechanism. Anything is possible and sometimes when I play I hear things that are not on the instrument. Because when you play with other people, from other countries or other genres, sometimes sounds they’ve made stay in the harp – or at least, that’s the way I see it.”

Stout, it turned out, had similar feelings about his own instrument and thinks this might be why their partnership developed and continues to grow.

“Catriona and I both love colour,” he says. “In music, colour can trigger emotion even before you choose notes or harmony. If you touch someone from another culture with colour you can draw them into your music and tell them a story.”

This is something that they’ve done time without number now as they travel the world. In Mozambique a few weeks ago they arrived not knowing a word of the local language but they still thrived. It’s been said before that making music with strangers is like conversing without the need for a translator and McKay and Stout have found this to be true.

“When you play music in another country you settle very quickly because you’re communicating at a fundamental level,” says McKay. “People can tell if you’re being honest in your music, even if – especially if – they haven’t heard you play before and don’t know anything about the culture you come from. They’ll trust you immediately because of that. To arrive in a strange place and have someone listen to you closely is a beautiful interaction.”

Bare Knuckle was intended to convey the idea of honesty. They’d reached a stage in their music where they wanted to bring out more of an edge, they say. This is quite a statement from a duo whose musical interaction has for some time appeared to be the equivalent of walking a tightrope without a safety net.

“A heavy metal fan might laugh at this but we were looking for more grittiness,” says Stout. “When you make an album you really want to create something you don’t already have, otherwise it’s not new, I suppose. We’d made albums that had, we hoped, sounded good and were maybe quite polished but I’m not sure that they captured what we do onstage. We wanted something a bit more raw, a bit more bold because I think that’s the way we are as people anyway. Well, Catriona is!”

McKay adds that the album is a fair reflection of where they are as a duo and perhaps contrary to expectation, when they work with a classical group such as the Scottish Ensemble they don’t have to bend too much to accommodate the other players’ style. Quite the reverse, in fact. Stout, who is a classically trained violinist as well as a traditional fiddle player who took McKay into the band he formed at school, Fiddlers’ Bid, laughs as he tells of leading the Scottish Ensemble astray by teaching them tunes the traditional way, by ear.

“I think when they learn one of our tunes that way and play it they sound like a folk band,” he says, adding with admiration that the ensemble can move so naturally back and forth between written Bach or Bartok pieces and a Shetland-style dance tune. “It’s interesting for us because what’s grown over the years is the realisation of how important this duo is for both of us and the importance of us keeping a sense of identity as a unit. At the same time, we want to keep growing, keep trying new things.”

In the beginning, they agree, they were looking for a sound and using largely traditional music as a launching pad for their playing, to see where they could take that material. With the confidence gained through working together over time, and winning awards such as the Herald Angel they were presented with at the Edinburgh Festival Fringe in 2014 (they’re also shortlisted in the Best Duo category of this year’s BBC Radio 2 Folk Awards), they now play almost exclusively original compositions, although these are written in the style of traditional tunes.

“Every experience we have stays with us,” says McKay. “It might register as just a sound at some point in a tune or it might lead to the tune itself being written. It’s clear that what we do as a duo is open enough to embrace other genres but what we find when we go somewhere to collaborate, be it a concert with an orchestra in Russia or a workshop with children in Mozambique, is that the people there want us to be ourselves.”

Bare Knuckle might be said to be the ultimate example, so far, of the duo being themselves and if it marked a bold step, as Stout says, it has been rewarded with high praise in reviews.

“You can’t try to second guess people’s responses when you go into the studio,” he says. “You can only make the recording you set out to make and release it. But it’s good, just the same, when you maybe go out on a limb and the people who are listening to your music really get it.”

Edinburgh International Harp Festival runs from Friday, March 30 to Wednesday, April 4. For further information, log onto www.harpfestival.co.uk

Catriona McKay & Chris Stout play at Merchiston Castle School, Edinburgh April 4; Perth Theatre, April 16; Glenuig Hall, April 21; An Lanntair, Stornoway, April 26; and Tolbooth, Stirling April 28.