Inside the world of Alasdair Gray

Nevertheless, I brace myself as I set off to pick him up at Glasgow Print Studio, where I find him adding colour to a digital print of his 1982 title page design for Lanark Book 2 ("stolen", as he puts it, from the title page of Vesalius's 16th-century De Humani Corporis Fabrica). This, plus five other seminal works in the series, will become screenprints for sale at the upcoming major retrospective season of his visual work.

To my relief, all is calm - apart from the whining, rhythmic swing of a large laser printer. Doesn't it drive him mad? "I don't mind it," he responds amiably. "I feel it's saying 'hee-haw'. I used to think it was saying, 'spirit pie'. I don't really know why."

In a taxi bound for his flat in the west end of Glasgow, I ask how he feels about the idea of colouring his hand-drawn oeuvres (though the black Indian ink originals, which he began in 1979, are safely with the National Library of Scotland). "I like it," he says, "though there are bits that should not have colour because the brush work is so detailed."

He describes the painstaking process of creating the ceramic tile mural at Hillhead Subway, and how the viewpoints and perspectives of the buildings in it, such as the Boyd Orr at Glasgow University, had to be made to look convincing. He sings the praises of muralist Nichol Wheatley and assistant Nick Harrington. He tells me about his son Andrew, a physiotherapist in Connecticut, and how his grand-daughter Alexandra, coming up for 20, is a student of art and feminist studies there, and recently visited Glasgow to undertake an internship with her grandfather. "She's really nice," he says, and I'm struck by the quiet intensity of his demeanour for a third time. And by his kindness: after so many interviews and accounts of his life so far, he must surely feel there is nothing new to say. But he responds in cheerful challenge: "I've never heard a question I can't answer."

When we arrive at his home, the Victorian living room cum studio is well-heated and well-ordered. On the desk in the bay window clean brushes are neatly stuffed into jars; glass pots of mixed paint are all to hand. Books line one wall, numerous unfinished portraits from the 1960s and 1970s posing neatly above them. Jean Redpath tapes sit snugly in a rack close to his armchair; a pair of grey woollen slippers sits in the hearth. A single lightbulb hangs from the central rose in the vast scarlet ceiling - a symbol, perhaps, of his new existence. Gray lives here alone now, since the untimely death from cancer of his wife of 23 years, Morag, in May.

And then. On an easel to one side of the bay, a large unfinished canvas featuring a brown-clad central figure standing by a river and a representation of Glasgow Cathedral. The painting's state of undress, as it were, scotches any assumption that Gray's working life is over: it certainly won't be ready for the retrospective, as originally hoped. "It's a new commission for the Museum of Religion - a view of St Mungo, Glasgow's patron saint," he explains brightly. Does he mind me seeing it at such an early stage? "I don't mind people seeing what a mess I'm making as I struggle to get things right." He was commissioned in 2013, but Morag's death left him unable to continue. It's now scheduled to be hung in the St Mungo Museum next spring.

"I'm brooding on it," he says. "I've been mucking about with bits of it. I hope to have it finished next year." The internal wheels of his artistic mind are turning; he finds it difficult to find words at this agonising stage. What he does say, though, is a tantalising insight into the lucid mind of a working modern master. "When painting my first mural in a church, Six Days Of Creation, in 1959 [at Greenhead Church of Scotland in Bridgeton, now demolished], I was keen to expand it all over the walls of the church, like Giotto's Life Of St Francis frescoes at Assisi. Now I am revisiting that idea."

But St Mungo is not so straightforward. "The problem is the lack of miracles associated with his life; he's not an internationally-known saint. The only other work of art he's depicted in is a stained-glass window in Cologne Cathedral. The legend of the fish and the ring, given to him by a lover and represented in the Glasgow coat of arms, is attached to more than one saint." On the other hand, he says, there is no doubt St Mungo founded the church in the sixth century while living in a small cell beside the Molendinar Burn where he preached.

Gray is also referencing Gaugin's 1888 masterpiece The Vision After The Sermon, where a group of women see Jacob wrestling with an angel after hearing a sermon at Mass. Without any visuals to hand, the artist offers his own description. "It's a vigorous piece, a movement towards something like Christianity; the head of a tonsured priest is among the women," he says. "My immediate thought was that he is near to a tomb by the side of a rock, and that the Glasgow Necropolis has several tombs cut into the rock, so I could fit one in here. But how to reconcile this with a sixth-century saint? It will take a miracle, a miraculous vision!"

At the moment he's translating the Garden of Gethsemane (where Jesus prayed before his crucifixion) to the Necropolis, with Jesus as the priest/gardener and St Mungo kneeling after his sermon. "I decided I wanted to show St Mungo preaching beside the Molendinar, preaching on the theme of Jesus's resurrection at 32 or 33 years of age, which any Christian missionary would have had to preach at the time." But it will be "as if living in modern Glasgow, in which we'll see the old Catholic Cathedral of Glasgow and the 19th-century tombs".

Won't it take a long time to complete? "If I'm spared I'll be able to finish it."

We sit drinking tea as Gray's thoughts continue to unfold in a kind of magical vision to fill the vast space around him. I wonder if he's lonely. "Yes, I'm lonely," he replies. "I have no reason to be, as I have friends and folk I like and get on with. But I have never lived as close to anyone as closely as I lived with Morag for 24 years." He breaks off to stare into space. Morag McAlpine, a librarian and crossword expert, was his second wife; Inge Sorensen, a Danish nurse whom he married in 1961, was Andrew's mother; she died, also of cancer, in 2000. They had separated many years earlier, though they remained friends. Andrew and his wife Patty are attending the exhibition with him, and coming over for his 80th birthday in December.

The room is very quiet. Does he feel Morag is with him? "I regret I have no sense Morag is with me," he says. "I'd like to feel she was, but I doubt it. She was afraid of dying. I'm not, unless it's sore. But she was afraid of me dying. I'd hoped I'd die first, as she was 15 years younger." Then, for the first time, he assumes the lofty voice of an imaginary third person. "The thing to remember," he says, "is that anything that's in the past can't be destroyed. The past contains everything that's ever been. So when I die you will not have lost me, as I will be with you as much as I was in the past." Then he adds, "Now she is dead I certainly feel she is not there for me."

On the table between us is a Perpetual Mass card for Morag, given to him by a waitress at Mario's Plaice on Byres Road, a fish and chip restaurant the couple often frequented. "A month ago the waitress asked me how Morag was, as we were quite friendly, and she was quite shocked to find she was dead. She subequently gave me this. I'm not a Catholic but I find it very moving."

Though much of his work depicts religious subjects, Gray has often said he is not a believer himself. "Jesus Christ set an example of heroic behaviour that I haven't had the courage to imitate," he says. "Morag wasn't religious at all, and we never went to church. There are other ways of talking and thinking about things good in life, and how we should live."

I spot a large black notebook lying on the sofa, neatly inscribed by hand: "Paradise, begun 4 August 2014." The whimsical, crossed-out and corrected lettering inside is surely a work of art in itself, though it too is obviously a work in progress. This time he's less willing to share, and holds it in his own hands.

"I can't say I'm translating Dante's Divine Comedy; I'm paraphrasing it," he begins. "I have six different translations in English, and every one produces a different aspect of the original, so I thought I'd produce my own version. I could not express all the slight variations; all I could do was take the main strains of thought." He began Inferno in November 2012, completed Purgatory, and has just reached chapter four of the 33 in Paradise.

It turns out Dante is helping him through his bereavement. "It was the one job I was capable of continuing to do." He does not seem to be in a hurry to finish it.

From early childhood Gray has been afflicted with asthma-eczema syndrome, and he tells me he spent much of his youth slathered in "nippy" coal tar oil, and enduring various dietary regimes of naturopathy and vegetarianism. Nothing helped. In desperation his mother Amy spent "a few guineas" on a private appointment with the chief consultant in skin diseases at the Royal Infirmary in Glasgow. He recommended over-the-counter Vaseline, and when he suggested that an expensive serum could help Gray develop immunity to asthma, his mother "came to the conclusion that he was a charlatan". Gray's worst asthma attack came after her death in 1952 while he was studying at Glasgow School of Art, and was so severe it put him in Stobhill hospital for two months. When I ask him about this devastating reaction, he dismisses it as having been caused by depression after "the woman on whom I had set my heart just wanted me as a friend, and I wanted more and could not get it".

In his 2010 autobiography A Life In Pictures he states that Amy was not given to much cuddling or caressing, and posits the Freudian theory that asthma starts "with struggles to draw breath while screaming hopelessly for a mother's attention, in a state of rage and horror".

Was the attack triggered by the final, primal loss? Munching now on a box of dates, he says, "Oh, the psychological stuff about Mum not giving me enough attention as a baby … Well, she was very attentive, though my sister agrees that she didn't cuddle us very much. She never laid a hand on us.

"But there was no doubt that our parents loved us. I don't remember wishing Mum was more affectionate. Once on holiday, when my sister Mora was seven and I was 10, we asked if we could call her Amy instead of Mum, and she said that was all right. But it didn't seem right that she was just a pal." He's had no major attack since, and says his asthma only returns when he has a cold.

He regrets that he has no picture of his mother, "because I never tried to draw or paint her". A good sketch he did of his father Alec appears in A Life In Pictures, though it will not be in the exhibition. There are many of Inge and Andrew, and quite a few of Morag.

Earlier this year, Gray had cataracts removed from both eyes after a routine eye scan for diabetes (he was also diagnosed with type-2 diabetes, which he calls "the easy type that thankfully does not require insulin"). The result is that after a lifetime of wearing glasses - a habit acquired after having them prescribed to correct a squint - he no longer needs them except for reading or writing. He certainly looks different. How does it feel? "I quite like looking around without spectacles. I see things over two feet beyond my face as clearly without glasses as before I needed glasses to see them."

However he puts it, it seems his artistic vision remains crystal-clear. As he unlocks his front door to shoo me out, I get the impression that here is the artist on top of his game, and relishing the task of putting his house in order. n

Alasdair Gray Season: From The Personal To The Universal is at Kelvingrove Art Gallery and Museum, Glasgow, from Friday until February 22, 2015. Alasdair Gray: A Life In Print And Posters is at the Glasgow Print Studio, Trongate 103, Glasgow, from October 11 until November 16. Alasdair Gray: A Life In Print is at Taigh Chearsabhagh, Lochmaddy, North Uist, until November 1. Alasdair Gray: Spheres Of Influence I is at the Gallery of Modern Art, Glasgow, from November 21 until May 25, 2015. Visit glasgowmuseums.com. See tomorrow's Sunday Herald for Phil Miller's account of sitting for an Alasdair Gray portrait.