AS ONE of the most remarkable churches in Edinburgh, St Patrick’s is not unaccustomed to revelations.

But sometimes, as its many congregations of differing denominations will have known, great talent can be hidden under a burning bushel.

Or, in the case of the mural currently being uncovered at St Patrick’s Catholic Church in the city's Cowgate, under five coats of paint and 200 years of both erasure and forgetfulness.

In an little-used apse of the church – which played a central role in the formation of Hibernian Football Club and is credited as being the childhood parish of Irish revolutionary James Connolly – restorers are this week uncovering a spectacular depiction of Christ’s Ascension.

The large mural, oil on white plaster, was painted by Alexander Runciman, one of the most important artistic figures of the Scottish Enlightenment, and has been described as “nothing less than the first significant public religious art in Scotland since the Reformation”.

The academic and art critic, Professor Duncan MacMillan, has been key to the discovery of the mural: he deduced the painting was in the church in the late 1960s.

He said: “I began to think about how we could recover it, 50 years ago, and it’s been a long haul, but now I think we are underway - we have the right skills, the modern materials and because it was painted out only 40 years into its life, so under all that paint, it is relatively clean.

“It will be his only surviving work on this scale, and it is unique.”

Special gels are now being used to remove the up to five coats of paint to assess how much of the mural has survived, and to begin a process – as yet uncosted – of renovating the entire work.

The history of the painting - celebrated, obscured, forgotten and now revived - is not one only of Church history, but one of societal change.

Edinburgh-born Runciman painted the work over several months in 1774.

His original commission had come from the Episcopalian Church, but when that congregation largely moved from the deteriorating Old Town of Edinburgh to the New Town, the building was taken over, in 1818, by the United Presbyterians.

Disapproving of idolatry, they painted over it - and it has lain under layers of whitewash and household paint since.

When the Catholic Church reconstituted the building in 1856, becoming a key religious and social centre for the large immigrant Irish community, they did not know about the art work.

The church itself was also re-orientated: when Runciman, who died in 1785, painted the Ascension, it was the spectacular artistic pediment to the altar, which faced east.

But by the late 19th century, the church now faced north with its Sanctuary at right angles to its former placing, and the painting was hidden to the side.

The exploratory work this week is being undertaken by Edinburgh-based conservator, Owen Davison, and Nicola Grimaldi, senior lecturer in the conservation of fine art at Northumbria University, Newcastle.

The work, to ascertain the scale, feasibility and cost of full restoration, is being led by the Runciman Apse Trust, which is chaired by the former Lord Advocate, Lord Andrew Hardie.

The efforts to restore the Ascension are taking place alongside with four of his subsidiary paintings: Christ and the Woman of Samaria; the Return of the Prodigal; Moses; and Elijah, which hang beneath it.

The academic and art critic, Professor Duncan MacMillan, has been key to the discovery of the mural: he deduced the painting was in the church in the late 1960s.

He said: "When I realised it had been painted over, I began to think about how we could recover it, 50 years ago, and its been a long haul, but now I think we are underway - we have the right skills, the modern materials and of course because it was painted out only 40 years into its life, so under all that paint, it is relatively clean.

"Where Owen has been working, the paint surface is clean and pretty, you can see how brilliant the paint is, but we don't know what the damage is."

Professor MacMillan added: "It will be his only surviving work on this scale, and is unique.

"We don't know what the Ascension looks like...he painted very quickly, with excitement, so there isn't a formal drawing of what it looks like, it is a mystery.

"It is going to be very exciting.

"We don't know how much it is going to cost, but we don't think it will be an astronomical amount, and it should take a year or so, if we get the money.

"After the Reformation, this is the first major piece of religious art since the 16th century, even in England there is not much that is comparable, not on this scale."

The church's priest, Mgr Philip Kerr, said: "The painting had really been forgotten about, but since the 1970s there has been an interest in doing something - but I think we have the momentum now to get this accomplished.

"I would very much like to see it recovered...it is a great and exciting opportunity.

"I used to come to Mass here as a child, but my parents did not talk about these things in a major way: and it is an Ascension, which is a key biblical theme."