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At our last conservation workshop in Venice, we had the chance to visit one of saltiest buildings in the world - The Old Salt Storage of Venice. But before I dive into the challenges that building meant in terms of lime plastering let's recap quickly what problems salts can pose to a building in general.
Salts and Lime Plastering
Very few people realize that the amount of salts in the building fabric plays a significant role in how long a lime plaster will stay nice and presentable. The areas where a plaster tends to blow are the ones where the building fabric is the saltiest.
In a typical British listed building only certain parts of the building tend to accumulate salts. These are:
- The bottom 2-3 feet: in old buildings that don't have a damp proof course water-diluted salts are absorbed from the soil in the form of rising damp, which later crystallize inside the plaster or on the surface,
- Around fireplaces: burning wood and coal results in ashes and soot which are very rich in sulphates - a type of salt that when crystallized can break down the plaster.
- The facade of buildings: when exposed to sea spray.
Subsequently, these are the areas where the plaster will most often fail, often accompanied by a "white dust" on the walls - the crystallized salts.
Salt Problems in Venice
As mentioned in other articles, our plaster manufacturer, MGN, is the main supplier for the Venetian building conservation sector. In fact, the founder of the company, Mr Naldo Busato, is a third-generation Venetian Building Restoration Master - one of the last living members of a guild tradition.
We attend building conservation workshops in Venice several times a year, which gives us an opportunity to learn about how to overcome some difficult conservation challenges specific to Venice only. Venice can be regarded as a special testing ground where the erosion of bricks, the deterioration of plaster and the aging of building materials occur many times faster than anywhere else in the world. This is mainly due to the extreme salinity of the buildings, which are sitting in sea water - and rising damp is essentially rising sea water inside the building fabric.
Furthermore, buildings are constantly exposed to sea spray. The hot sun evaporates the water quickly, causing extreme salt crystallization. When salts turn into crystals, they expand about 10X in size, and the crystallization pressure generated by this expansion bursts the bricks, resulting in blown plaster and eroded buildings. This is just the everyday "norm" in Venice, but even there some buildings take this problem to a whole new level!
Lime Plastering the Saltiest Building in the World: The Old Venetian Salt Storage
As the name of the building suggests it served as a salt storage for five centuries. The earliest record about the building is a map by Jacopo de' Barbari from 1500, which shows nine storages in use for storing hundreds of tons of salt - which was the main commodity of trade for centuries.
Over the centuries, salt has migrated into the walls everywhere, and saturated the brickwork all the way up to the ceiling - as shown in the video below, a footage recorded by us during our last visit.
Plastering these walls seems a nearly impossible task. A normal plaster wouldn't last here more than a few months. A hard sand-and-cement plaster - apart from being non-building-friendly - would also soon detach and fall off in slabs,
The City Council of Venice has turned to our Italian partners MGN for a solution, due to their top-most expertise in lime, as well as their plasters being recognized as one of the best (if not THE best) in conservation circles throughout Italy. As a result they have devised a plastering schedule for re-plastering the saltiest building in the world: Magazzini del Sale - The Old Venetian Salt Storage!
The Base Plaster - A Traditional Salt-Resistant Roman Lime Mix
The solution they have recommended was based on an old Roman technology. Research by the Polytechnic University of Milano proved that the most durable and suitable solution would be an ancient Roman salt-resistant lime mix, described by Roman architect Vitruvius (Marcus Vitruvius Pollio, c.80 - 15 BC.) in his work De Architectura. Vitruvius a has described the type of lime mix that needs to be used for seaside applications, for the building of harbours, piers and seaside buildings, that is capable of withstanding the eroding effect of sea water and salts.
This type of lime plaster has been re-created in 1980 after nearly 2,000 years, with the joint effort of Mr. Naldo Busato, Venetian Building Restoration Master, and Milano University - produced and available under the name of MGN Rinzaffo. This mix has been used as the protective lime base coat on the Old Salt Storage building, as the only building friendly solution that is capable of coping with such salinity for an extended period.
The Main Plaster - Cocciopesto, an Ancient Phoenician Lime Plaster
The Cocciopesto lime mix has gained popularity due to its simplicity, pleasant colour, and its extremely good performance. Its durability, mechanical resistance, breathability and flexibility made it suitable for a number of specialist applications, from external facades to somewhat damp basements.
The Cocciopesto lime plaster recipe has been adopted by the Romans and has been widely used since. This is one of those very simple recipes that does not need to be improved, it performs great and has been used for millennia in its current form.
For the above reasons, it was the choice of main coat for The Old Salt Storage building as well. It did not need finishing nor painting.
An Italian conservation architect explained that an average plaster on an average Venetian building lasts about 6 months. A top quality lime plaster lasts about five years, after which it will need to be redone.
The Old Salt Storage has been restored in 2007, and remember this is not an average Venetian building, but one of the saltiest in the world!
In 2020, most of the plaster is still nice and presentable. However, at the back of the building, where the storage was full with salts most of the time (the saltiest part of the saltiest building in the world), the plaster started to come off.
It is worth mentioning that, as shown on the video, the face of the brickwork did not come off with it, it did not chip pieces off the building fabric either. The plaster came off cleanly leaving nice and undamaged bricks behind.
12 years under such conditions, in such a conservation friendly way, is a great success!