Politiken has just announced that the main gable wall of the stock exchange building that faces towards Christiansborg Slotsplads, and the wall and ornate gables along the street - along Børsgade - collapsed late this afternoon.

Fire services have been monitoring the building throughout the day to ensure that fire could not break out again in the smouldering ruin. Police have reported that no one was injured when the outer walls collapsed.

This is, without any doubt, a tragedy.

Built for Christian IV in the early 17th century, Børsen was one of the most important historic buildings in the city from that period and only Rundetaarn and the Trinitatis Kirke and the palace of Rosenborg and it's gardens are of comparable architectural status and importance.

Børsen had, and hopefully still has, a significant place in the history of the city and of the country as a monument to and as evidence for 400 years of Danish trade and Danish history.

Investigations are ongoing to determine how and when and where the fire started but there has to be a major and formal assessment of how recent restoration work was planned and organised.

The whole building was encased in scaffolding and sheeting as external brickwork was being replaced - to make good inappropriate restoration undertaken in the late 19th century - but also, and probably much more significant, copper covering the roof was being replaced.

Work is progressing on a major project to restore the famous Nyboder houses in Copenhagen. These long terraced rows, with cobbled streets and narrow yards between the rows, cover a large block in the north-east part of the historic centre of the city within the old city defences … so south and west of Kastellet - the fortress or citadel - and close to Østerport railway station that is on the site of the old east gate.

The first of the rows were built in the 1630s and 1640s at the instigation of Christian IV for naval personnel and were single storey but with attic rooms. More rows - the two-storey terraces - were constructed in the middle of the 18th century and the last houses, in grey brick, date from the very end of the 19th century.

The houses from the 17th and 18th centuries were built in pale-yellow brick but given an external wash of lime with deep ochre pigment but this wash has just been removed so the brickwork can be repaired and, where necessary, repointed.

Stripped of this ochre wash, the facades reveal important archaeological evidence to show clearly how window and door openings were constructed. There were no obvious lintels but lines of headers above the windows so, almost certainly, the timber frame of the floor structure would have been set out to take the weight and outward thrust of the roof to prevent the outer walls bowing out or the ground-floor openings failing under the weight of the brickwork and roof above.

In contrast, more precise coursing in historic brickwork can be seen in a house on Wilders Plads in Christianshavn with alternate rows of different colours of brick so that brickwork was clearly built to be exposed and left without render. The Nyboder brickwork is not of the same quality so the houses must have been covered and protected with was of lime and pigment from the start.

It is also clear that by using lime wash - rather than a thicker and smooth coat of render - the final surface is not just more resilient - as each thin layer is applied, the lime oxidises and bonds to the stone or brick it covers - but the visible and slightly irregular brickwork gives a texture to the surface that seems to make the colour deeper … modern brickwork, in contrast, seems mechanical and flat or, if anything, dull.

more images and historic map of Nyboder