Glenna Gardiner's father always used to boast that he owned an original work of art by the renowned Canadian painter Tom Thomson, but she never believed him.

Now the 71-year-old retired Edmonton nurse has egg on her face — and a sizeable cheque coming her way — after Heffel Fine Art Auction House determined it is an authentic Thomson sketch worth between $125,000 and $175,000.

Gardiner spoke to As It Happens host Carol Off about the artwork that was collecting dust in her basement for decades and will be on the auction block in May.

Here is part of their conversation.

Glenna, what did you think when you found out this painting that's been in your family for decades is actually painted by Tom Thomson?

Well you know it's sort of surreal disbelief. This kind of thing doesn't happen — to me! Other people might come across treasures, but I didn't expect it.

But you could have expected it. Didn't your dad tell you that it was by Tom Thomson?

I know, but he used to joke around a lot.

I used to think it was a hopeful thing for him to say on his part, not necessarily that it was the truth.

Sketch of a Lake in Algonquin Park by Tom Thomson is shown in this undated handout photo. (The Canadian Press)

And where was it when you finally decided that you should do something with it?

It was in my basement. I had moved from one place to another and I had a stack of paintings.

And my girlfriend was visiting and we were looking through these things and she says, "Oh Glenn, this might be really valuable. You should get it evaluated."

And I thought, "Oh, yes, sure."

I am going to take my girlfriend who got it evaluated and we're going to go for a trip to the Mediterranean. And we'll take her husband along to carry luggage. - Glenna Gardiner

I didn't know who to go to to do that sort of thing, but she persisted, and in other years she kept on after me.

And I guess it was October 2016, as a gag, I sent it to her as a birthday present.

She waited for about a year and she finally took it in to Heffel in Vancouver.

And what did you find?

They, she said, treated it with great respect. Where I was very casual with it, they were very delicate with it.

It didn't look like much of anything with coats of varnish and stuff on it. Sort of a dull picture. Not very exciting at all.

So you didn't like it?

Well, I liked it because it was part of my dad's joy, you know? But I wasn't really taken by the picture itself.

Gardiner says she kept a copy of the artwork to remember her father by. (Jason Franson/The Canadian Press)

So your friend took it in for evaluation. How did she how did she react when she got the news that it was a Tom Thomson?

[Chuckles] She was really excited, just really thrilled that she proved to right that it was something worth looking at.

And so what happened when she told you? How did you react?

Well, disbelief still. You know, when I see a cheque in my hand I might really believe it. But all of this seems sort of surreal. It isn't part of what I expected in my life.

When you learned [how much it's worth], how did how did you react?

It seemed like an awful lot of money, and what would I do with that?

But I am going to take my girlfriend who got it evaluated and we're going to go for a trip to the Mediterranean. And we'll take her husband along to carry luggage.

Tell us how your father came to own this sketch.

Well, he was at Emmanuel College and there was some man that was in the library that was trying to get a gallery of sorts going, but he ran out of money.

And the paintings that he had, he gave it to various students. And he gave one of them to their father, talked about his esteem for my father. And so dad just had it since 1938, I think it was.

And what it did mean to your dad, that painting?

He never really talked about what it meant to him, but I knew it was something special.

I think he related it to maybe that time he had at university, which he enjoyed.

Is there a part of you that is selling this with regret given what it meant to your dad? The connection it is to your father?

Yeah, there is some of that.

I don't know the true value of art, but I've got a good copy of it. And if I want to look at the copy, I've got it with me. So I'll be OK.

What would your dad think?

[Laughs] He must feel vindicated. My friend feels that he's laughing down at us and all the fun we're having with this.

Written by Sheena Goodyear. Interview produced by Sarah Jackson.