For the second part of my ‘six-sided sealing’ theme we will look at issues relating to the back of the stone. Take a look at the picture, what do you see? Well, it is a granite facade; quite a porous one in fact, the light grey areas are correct and indeed have the desired appearance. However there are a couple of noticeable issues.Perhaps the most obvious issue is that dark staining. This has come from behind the stone; as you can see, the stone adorns more than just a simple external façade, it is also a planter the staining is mainly due to the presence of water in the soil of the planter, and as such if the water dries out the intensity of the stain will likely reduce. However, water is very rarely pure and untouched by contamination, especially when it emanates from what is essentially a great big plant pot: the water having passed though soil and compost may have any number of dissolved minerals which, even if the water were to recede, will likely be left crystallized for eternity in the stone.Now look at the two golden-brown spots; naturally occurring phenomena or a preventable blemish? This granite, like many stones can contain soluble minerals like iron sulphide, such as Pyrite. This source of iron reacts with water and oxygen – basically it rusts when it gets wet and so those spots are rust, growing from the back to the front and cannot be removed.
Would sealing the back and sides of the stone have prevented either of these? – Well it certainly would have helped, although to totally prevent such issues proper thought should be given to the design of the planters and adequate provision for drainage ought to have been designed in and a water-proof membrane could have been used on the inside of the planter to effectively tank it. However, with the proper planning and design, there is still the initial construction moisture to consider – in other words the temporary but considerable amounts of water in the concrete and the setting and grouting materials, sealing all six sides, along-side the correct design, would make a considerable contribution to the prevention of such problems.
Just reading about six sided sealing. One set of instructions still puts the onus of proper adhesion to the tiler rather than a purpose made ahesive for this situation. My experience with this situation was in the repair of someone elses shower recess where six sided sealing had been recommended to the tiler. Care was taken in the removal of the sandstone so as not to damage the membrane. This was not needed as the stone released from the adhesive with a small amount of pressure. How can adhesion be guaranteed by the manufacturers. I am only comfortable with five side sealing.
Hi Eddy,
Yes, this is still a fiercely debated topic, there are clearly some advantages to sealing he back of the stone (stopping various kinds of leaching etc) but it can only be done if the installer/client is confident that there will be no detriment to the bond strength.
Since writing that article I have seen independent test results on several sealers. The results were, to my mind, mixed. For example, one sealer showed that bond strength (the strength of the bond developed between the stone, the adhesive and the substrate) was reduced by the addition of a sealer to the back face of the stone. Conversely, another brand showed significantly improved adhesive bonding as a result of 6 sided sealing.
I am not sure why one sealer resulted in a weaker bond, perhaps it was to do with leaving residue of sealer on the face. However I can see the logic in a properly applied sealer (i.e no residue left behind that may act a s a potential bond breaker) acting in much the same way as a primer on porous surfaces; reducing suction and allowing the adhesive to cure more slowly, thus develop a stronger bond.
I would personally look at this issue on a project by project basis and seek verification from the sealer and adhesive company manufacturers involved at the time.
Ian