Slipknot make the kind of gloriously heavy metal only a nine-piece featuring two guitarists, three drummers, a DJ, sampler, bassist and vocalist could. A blitzkrieg of riffs, thrash and heavy, heavy beats.

Slipknot have risen through the ranks and are now the biggest modern metal band in the world. Recent album We Are Not Your Kind finds them at the top of their game, still bristling with righteous fury and refusing to compromise. It was also their first UK number one album since 2001's Iowa.

They are now so huge that they even have their own global touring festival, Knotfest, hitting the UK for the first time in August this year. A full day of metal (line-up tbc), the infamous Slipknot museum and the Iowan nonet promising a suitably brutal headline set.

Even if you only saw a photo of Slipknot you could probably guess what they sound like. The fright masks and matching boiler suits perfectly encapsulating their brand of noise terrorism and sonic violence. Percussionist Shawn Crahan (aka Clown) was there from the start and as one of the original masterminds behind the madness, the masks were his idea. As we sit down with Clown backstage at the SSE Hydro just before Slipknot prepare to tear Glasgow to pieces, he seems surprisingly chilled and relaxed seeing as he is about to spend the next 90 minutes atop a drum stack towering above the stage or battering out rhythms on a beer keg suspended from the ceiling with a flaming baseball bat.

Where did the idea of the masks come from?
Well it's interesting, you've caught me on a very interesting learning curve in my life. I'm not ashamed to say I have a therapist and a psychiatrist. I work on my mental health. Sometimes it's every day, sometimes once a week and sometimes I take a couple of months off to figure out what I've learnt. When I've told the story of when and how the masks went on, I've always followed it up by saying "I don't know why I did it." I actually know why I did it now: it voices the truth.

And does it make it easier to tap into another persona and other aspect of yourself?
I'm not able to explain what I feel but this thing I've designed for myself [the mask] offers redemption and salvation. When that entity comes out it's highly elevated, it's intense, it's dangerous, it's suspicious …

Slipknot are famous for their ferocious live shows. How do you keep up that level of intensity night after night?
If I'm in a song and the burning starts and I start thinking "I'm not into this, I'm really fucking tired, my fucking neck hurts", all these voices, I immediately go "shut up, you're fucking playing a show, you fucking love it, get into it." I force myself, but there is a physical attribute that has nothing to do with Clown, it's called the human body.

My band plays 90 [minutes] a night, it's hell: cold hot, this that, no sleep, bad food, weird food, these sort of things. But I want to let you know there is nothing like Slipknot and there will be nothing like Slipknot. What we've created, it's never been faked. It's reality. I don't like to talk about death, but that's where I live [on stage].

We've done three shows in a row, but we've got a day off tomorrow, so it's all about the good eats, tonnes of sleep, maybe even three showers. I'm 50 now, this band's fucking intense.

Slipknot deal with anger, despair, isolation and many of the darkest aspects of our lives. What do you think your fans take from your music?
The truth. Everyone else lies to them, they can always look to Slipknot. We're very lucky we were able to tap into human souls that feel identical to us, I saw those kids, man, I was one of them. I represent the ones that never get represented.

How did you approach recording We Are Not Your Kind?
This album was conceived very differently. After .5: The Gray Chapter I made a decision I was going to start talking to everyone, to start writing early, to meet up in LA several times before we even started recording. So, for almost three years that's what we did.

When we got done with the final show on the Gray Chapter [tour] Jim [Root] and Jay [Weinberg] started that process. Jay laid down some live drum tracks and Jim added guitar to some tracks we'd written out on the road. Then we took a month off, then the process started. That really was the difference, at this time in our career it really felt like a necessity to have the time to hash out our ideas.

I wanted to search out my soul for what each beat means. When we come back [to the studio] I want us to agree that's the right sound or add to it or demanufacture it. That was really the whole thing, let's take time, let's take a deep breath, let's not take on any pressure. I believe we created a masterpiece.

Let's talk Knotfest. So, what led you to set up your own festival?
It started a long time ago, being a band fortunate enough to fly over here [to the UK] and going from doing a show like [legendary, but now closed, London venue] the Astoria to us going to stuff like Reading and Leeds. And those blew our minds. I was really intrigued by the unity and culture at festivals. I always wanted to try and give that back to the American kids.

So, take what kids want and add to it what we can bring creatively. Let's see if we can make something for the kind of people we love to be around. It's all for the fans really.

You must be proud that Slipknot have reached a level where you can host your own festival.
I think if you spoke to most people close to me, they'd tell you I'm never happy and get bored real quick, I'm always searching for something. So, I wanted to make a festival for the kids, I get embarrassed if it's too corporate, the beer's too expensive. I want to bring it all together, the band coming together, playing a set, creating a sort of cult for the day.

Are you looking forward to the first UK Knotfest?
It's going to be interesting to bring it here because we've been fortunate enough to play Knebworth, Reading and Leeds, Download – so many wonderful festivals. My mind got blown by Download, but that's their deal. We are bringing our deal, making it more personal, as you know we have a museum and things like that that bring it more into the philosophy of Slipknot. We love music and try to make it a wonderful day that people will remember for a very long time.

Slipknot headline Knotfest UK, Milton Keynes Bowl, Sat 22 Aug.