Singer-songwriter KT Tunstall is set to release the second part of a trilogy of albums. She talks to Andrew Arthur about her inspiration

KT TUNSTALL is a musician who is, quite literally, going places. Due to a scheduling issue, our interview takes place in the back of a taxi en route to the airport where she needs to catch a flight to France.

The well-travelled Scottish singer-songwriter is about to release WAX, the second part of a trilogy of albums, exploring the themes of spirit, body and mind - a concept that came to her in an American park.

"I didn't have the idea of making a trilogy until I was on tour with my last record, KIN. I was trying to meditate in a park in Nashville and I opened my eyes and I just had this solid vision that this is what I should do," Tunstall, 43, says.

So while KIN focused on spirit, WAX sees her take on the theme of the body - and she adopts an electric guitar-driven sound. "KIN was all about surviving some shit and coming out stronger. Divorce, my father dying, moving continents and having to start again. That's led to a place where I feel free and want to make rock 'n' roll. Electric guitar is the most physical instrument for me."

But despite the physical theme of WAX, Tunstall insists there's a more spiritual edge to its lyrics. "It's all tied into emotions turning into physical problems. The body image stuff is a very superficial part of this record," she explains. "We are these transcendent, conscious souls experiencing the physical world and we basically have to drive a meat car. We're totally fallible."

WAX features the playing of former Ash guitarist Charlotte Hatherley and for her upcoming tour Tunstall will have an all-female band - but she acknowledges the next generation of female rock musicians do not have enough role models to pick from.

"I think women are ruling it in pop and in urban. There are great female artists that are at the top of the charts regularly. But there is a massive hole when it comes to rock music. I don't really know why. It might be that rock isn't quite as fashionable at the moment. But in my age group there is myself, PJ Harvey and Shirley Manson. They're a bit older than me and I'm looking around thinking, 'Where are all the women?' There have got to be role models for kids to see, for them to think they'd really like to do something."

Despite wanting to see more women in successful bands, Tunstall stops short of supporting initiatives which would see festivals ensuring 50-50 representation of the sexes on bills.

"I don't think there is a need to put a number on it. A band should be on a festival line-up because they're good. I don't want to go to a festival where there's 50% guy bands who are great and 35% of bands that aren't good because the festival hasn't taken the time to find good female artists. It's going to be solved by every single level of the music business supporting female musicians to become successful. There's just been a status quo of a massive imbalance of male acts across the board."

Tunstall's own musical abilities were evident from an early age. She had been placed for adoption when she was 18 days old and, though her adoptive parents were not musical, they were hugely supportive of her talent.

She talks warmly of both her adoptive mum and biological mum, saying: "Now I'm older, I have a really good relationship with my biological mum. She's in Spain and there is a lot of love. It can be difficult, especially as I'm well-known. The Scottish tabloid press were more interested in her than they were in me! We've got a very nice relationship and I'm very close to my adoptive mum. It's a beautiful thing to give a kid a great upbringing when they are in a vulnerable situation. There are a lot of kids who need a family.

"The Government has got to make sure that policy is in place so kids can get adopted as quickly as possible. For kids being kept in orphanages or adoption centres until they are two or three, it's just too late.

"I wasn't going to inherit any hereditary skills from my adoptive parents. Although they didn't play instruments or listen to music, right from the get-go that's what I was into. I was begging them for a piano when I was four. I was like a little Fife Mozart! I got 149 out of 150 on my grade one. I just saw my piano teacher and she told me that I bought my colouring book instead of my music to the exam. I just went in and smashed it. There was a downward trajectory from the age of four to the age of 16..."

Tunstall now lives in the US. Her song Suddenly I See was used by Hillary Clinton's campaign when she ran for the Democratic nomination for President in 2008. Her music has also been played at an event for the man currently in the White House.

"I think it should be against the law for politicians to use music without permission. I wasn't asked but I was quite happy for Hillary Clinton to use my song," she says.

"I'm not a US citizen and I would certainly like to see a Democrat government in the US. But the very difficult thing about someone using your music for their campaign is what if they become president and start a war? And you're the anthem?

"Donald Trump apparently used one of my songs at a rally. I'm having a f****** spiritual shower, absolutely disgusted. We work really hard to make meaningful art. To have someone commandeer it for a political agenda that we vehemently disagree with is not fair at all."

As our journey draws to a close, I ask Tunstall if she has ever had the chance to thank rapper Nas. He inadvertently gave her that big break by pulling out of an episode of Later... With Jools Holland in 2004.

"Nas' father was playing trumpet on his single and he got sick," she remembers. "I got a call with 24 hours' notice asking if I would like to be on the programme. Much love to Nas. I've never met him. I should buy him a beer really."

* KT Tunstall's new album WAX is out now.