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20 May 2011

Thursday 19th May 2011; Salts

Week 6 of 12 
Module 4: Deterioration Mechanisms; Methods of Survey & Analysis

Alison Heritage is the Conservation Research Specialist at ICCROM and is a specialist in salt weathering which was the topic of this morning's lectures and this afternoon's practical laboratory class. Alison started her lecture by introducing us to the basic chemistry necessary to understand salts. Salts are ionic crystalline solids formed by the electrostatic attraction of positively charged cations and negatively charged anions. Cations (+) of sodium, calcium, magnesium and/or potassium combine with anions (-) of sulphate, nitrate, chloride, carbonate and/or phosphate to create a salt. Our common table salt is a combination of sodium and chloride. Salts can reach our masonry structures from many different sources including salts which are already present in the construction materials (autochthonous salts), from the ground water, biogenic sources, atmospheric salts and marine salts. 

But are all salts damaging? No. Alison identified a few examples of salts which are not damaging to historic buildings; calcium carbonate (a.k.a. limestone or lime putty) and copper chloride (a 'salt-green' pigment). Salt damage is caused by salts that are soluble, or sparingly soluble in water. If salt crystals develop slowly inside the pores in a stone they will grow through the pores and result in little damage. However, it is when the salt solution inside a pore becomes super-saturated and rapid crystallistion occurs when the solution reaches 'critical saturation' that some of the pressure exerted by the crystal growth will be exerted on the pore walls and not simply through the connecting pores. The greater the super-saturation, the more pressure crystal growth can exert on the stone. 

Most of us will have carried out an experiment at school of heating up a salt solution to see the water evaporate and the crystalline salt remaining. Different forms of salt require different amounts of water to stay in a soluble state, from potassium nitrate which will crystallise if humidity falls below 93% to lithium chloride which will stay in a soluble state until humidity drops below 10%. It is not just humidity that affects salt crystallisation, but many salts are also affected by variations in temperature. The speed of the fluctuations (rapid drying or rapid cooling) are the factors that create the environment for super-saturation and therefore damage to occur. If the environment remains above or below the point of crystallisation of a particular salt then it will be stable and will not damage a stone, it is where the environmental factors fluctuate and repeatedly cross the relative humidity and temperatures at which a phase change occurs that damage will be an issue. In addition, in the majority of cases there will not always be a singular salt present, but may be several and therefore how do we identify the appropriate stable environmental conditions?

As highlighted by Ippolito Massari in the last two days of lectures (see Day 24 (17.05.2011) and Day 25 (18.05.2011)), understanding the source of your problem is essential before you attempt to take remedial action. In Scotland we commonly see the tell-tale signs of a wavering mark a metre or two up the side of a wall where rising damp has been able to reach and evaporated leaving its deposits of salt. In some cases this may appear to be damp, but to understand if the rising damp is a current issue we need to identify whether the wall below is damp and to identify the form of salt present. Certain forms of salt are hygroscopic (they attract and hold water molecules) and therefore it may be that the problem has been fixed and that the dampness is due to hygroscopic salts.

Some basic salt demonstrations were carried out in this morning's lectures; the image shows a brick which has been left in a salt solution.

This afternoon we were split into small groups in the laboratory and were given 4 bricks which had been saturated with a known salt solution and then dried leaving visible efflorescence on the surface. We were introduced to numerous different techniques from simple flame and pH tests to optical microscopy for identifying the different forms of salt present and it was our task to identify the salts that were present in our brick samples.

This afternoon's laboratory session involved us using various analytical techniques to identify the types of salt present in bricks which had been saturated with various salt solutions.

pH and Merck Strip Test (for identifying presence of chlorite, nitrate and sulphate) results for our 4 samples.

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