It all depends on quality

Does Maltese limestone weather well or does it erode badly, which makes it lose the shape it is given by the builders, as it is pitted by rain and wind? What can one do to preserve limestone?

In a bid to get answers to these questions, which often stretch the patience of anyone who wants to keep his house in good condition, The Times contacted Manwel Baldacchino, director of Limestone Heritage, the recently set up tourist attraction in Siggiewi.

Mr Baldacchino comes from a family of quarry owners. At the Limestone Heritage attraction, one can understand better how layers of limestone were formed over the millennia, apart from the skills, techniques and tools used traditionally by quarry workers.

"One can get various qualities of Malta stone or franka from the same quarry, ranging from the best quality to the worst," Mr Baldacchino said.

This fact is recorded in proverbs used in the trade, such as kull xiber iqalleb and kull difer jaqleb meaning in a generic manner that every layer in a quarry will reveal a different type of stone.

"Building contractors and architects worth their salt would know how to gauge the quality of stone," Mr Baldacchino said.

There is a variety of names given by quarrymen to limestone, each name related to the level of quality of the stone.

There are examples of good quality limestone on a house close to Dingli Street in Sliema, opposite to the tower. The British services in Malta used to make it a point to their contractors to use the best quality stone when they commissioned buildings.

The high level of preservation of these buildings is proof of this, Mr Baldacchino said.

But what makes one type of franka good quality and another type not so good?

"Good quality stone forms a patina that hardens with time and this layer protects the stone from erosion and weathering. In time, this kind of stone takes on a reddish colour.

"There are clients, however, who prefer the whiter stone, which is softer and, to them, more attractive. But this type has absorbent qualities, absorbing dust and fuel exhaust and eventually taking on a black hue.

"This stone is good for interior use," he said

This kind of stone was used extensively during the 1970s when stone balustrades were a popular architectural feature.

The stone used had to be soft as it had to be turned on lathes to be formed into balustrades. But most of these balustrades have now been eroded beyond shape.

Other types of limestone are known as gebel tal-qanneb, which is good quality stone replete with fossil remains, gebla tas-sadid, which contains impurities, and gebla mielha, also known as gebla kahla, because technically it is blue clay.

The gebla tas-soll is found above fossil layers which could date back to between 20 million and 25 million years ago.

First and foremost, limestone needs to be safeguarded against rising damp. Therefore, a good damp proofing layer is a must.

"You see damp rising in churches, for example, where damp limestone is covered by another material such as marble, but this only makes the damp rise higher.

"Rising damp needs a place where it can find easy release. Exhaust from vehicles and sea spray also leads to erosion so anyone who wants to leave the bare stone as it is, should seal it with a layer of linseed oil or clear spirit sealers," Mr Baldacchino said.

Asked whether a coat of lime would help, he said lime would eventually eat into the limestone. There is on the market a type of paint having 'airflow' qualities that allows the limestone to 'breathe' in order to curb damp.