Stonework

The stone with which the Town Hall was built is popularly known as Anglesey Marble and is a white carboniferous limestone. It was originally quarried in north Wales, with the stone shipped 80 miles by sea and 100 miles by canal to Birmingham.

The master stonemasons have found that Anglesey stone in particular is extremely difficult to carve and work. It is so hard that the most up to date tungsten carbide chisels were being split by the stone as they worked it. As a consequence, most of the new carving has been completed using angle grinders. The masons are mystified as to how their counterparts in 1834 carved the original stone!

Investigatory work has found that there are fossils present in it, some of which are as large as the palm of your hand.

The masons have also found shards of mother of pearl (flat oyster shells) in the stone joints, which were used as spacers to keep the stones apart and level prior to the joints being pointed with lime mortar. This is not as strange as it would seem, as in the early 19th century, oysters were a staple food for everyday people.

Claimed by the buildings architects to “become whiter with age and (be) imperishable by time”, the industrial environment of central Birmingham has taken its toll: photographs taken prior to the cleaning of the external stonework in 1982 show the building to have been of a sooty hue.

Why has the stone deteriorated?

The stone has deteriorated for several reasons, chiefly because it has very fine natural fissures (splits) and natural impurities such as iron and salts.

This results in rainwater seeping into the stone along the fissures, taking the pollutants in the water with it. The water and pollutants then chemically react with the impurities in the stone, or expand when it freezes, causing the surface to break away. The problem is worse on the more ornate sections of stone.

How has the stone been repaired?

Prior to any restoration work taking place, a record and assessment of the stones and the best repair methods was undertaken.

A measured and photographic assessment was made of every stone, providing an invaluable ‘moment in time’ of stonework in the early 21st century and enabling architects to identify exactly what work is required on each stone.

The next step was the execution of trial repairs of particularly badly weathered stone, to see how well ‘new’ stone matched, how it weathered and to assess how it could be worked and carved by master masons.

The conservation and repair work to the external stonework used a variety of repair techniques to not only stem the decay to damaged stones, but also to safeguard the future of surrounding masonry.

The methods ranged from grouting to mortar repairs, piecing in repairs and stitching (which involves pinning and gluing cracked or dislodged stones) and wholesale stone replacement.