Water Sensitive Limestone
Limestone due to the fact that the problem I am discussing is relevant to both of these stones which can more accurately be described under this banner. On these trips I have seen numerous jobs where the colour of the limestone was “bleached out” as well as the polished destroyed. In almost all of these cases the prognosis by locals was that the stone was stained and therefore the problem could be cleaned. Obviously this is not the case as I will show. However the problem of loss of colour and loss of polish although related are caused by different mechanisms. Loss of Colour-This is perhaps the more simple and common of the two problems. Most of us have experienced a stone that has lost colour once subjected to usually moisture. Green Marble for example is well documented in the way it will warp (excessive moisture expansion) and also bleach. The reason for the bleaching effect is less well understood but is simple to explain. The colours we see in stones are created by different minerals. For example green is created mainly by Mica, Chlorite or Silicates, red and browns by iron oxides such as Hematite. The problem of colour stability arises when some of the minerals are soluble and when the stone gets wet the soluble minerals are dissolved creating a change in colour. The white streaks are where minerals have been dissolved resulting in a bleaching effect. These cannot be cleaned. They are now part of the stone. In most of these cases re-wetting of the stone does not recreate the colour because the colour creating minerals have now gone or changed. This is the reason why Aqua Mix Enrich N Seal will also not work in these cases.
Loss of Polish- The answer to this problem is related to the one above, moisture and soluble minerals in the stone. In both cases the stone not only had loss of polish but also an accumulation of efflorescence.
The loss of polish can be attributed to two things, firstly the dissolution of several soluble minerals and secondly the evaporation of soluble salts. A stone shines because the surface has been rendered flat enabling it to reflect light in parallel. Anything that roughens the surface will scatter the light and in so doing create a matt or unpolished look to the stone. In the cases I inspected the surfaces had become rough due to both phenomenon. Soluble components had been dissolved there was no doubt; however the biggest contributor was efflorescence. As the stone dried out (both installations were interior) the salts were evaporated to the surface. These came from the stone but mainly from the thick sand and cement mortar beds laid under the stone. In most of Asia sand is used unwashed hence it has large quantities of soluble salts.
The moist highly alkaline salt not only helps to dissolve other soluble minerals but also manifests itself as a crystal that can both deflect light and increase surface expansion which inturn creates fissures in the surface. All of these mechanisms result in a roughening of the surface which is visible as a loss of polish. In the cases in question they were made even worse by the on and off operation of the air conditioning.
The markets that see the highest proportion of these types of problems are the ones with high temperatures and high relative humidity’s due to the fact that these problems require water and drying cycles as catalysts. The on/off operation of the air conditioning simply created a great environment for drying cycles as well as allowing excessive condensation to occur and in doing so exaggerating the problem.
Can Sealers Solve the Problem?
Generally we are asked to solve these problems firstly in terms of restoration and secondly in terms of future prevention. The answer for restoration is easy. For the bleaching effect the answer is we can’t change this at all if it is caused by dissolution of colour creating minerals. These stones should simply never be used in wet or humid areas.
As for the loss of polish, restoration is actually quite easy in that you need to repolish the surface. However you also need to address the efflorescence and this can be done by applying Aqua Mix non-acidic EFF-EX to the surface of the stone to remove exiting salt. Then apply several applications (depending on the assessed quantity of salt in the system) of EFF-EX to the stone allowing it to dwell so that it can penetrate, inhibit and minimise further efflorescence migration. The application of a sealer is also recommended. It will not guarantee the problem will not reoccur however it will dramatically reduce the catalysing water that can get into the stone and in doing so reduce the risk of future problems.