For years the stone industry denied that any granite needed sealer protection. However as more and more granite varieties found their place on work benches it become apparent that this was not the case. Many of the granite bench tops stained very easily with the lighter colours being the worst. There was however one granite variety that was still sold as an impenetrable product and in most cases still is today – Absolute Black (AB). AB is perceived as a rich black igneous stone that is not only extremely hard but is totally impervious and hence does not and cannot be sealed. The truth is however more complex than this. To find out if AB does require some protection you firstly need to understand more about the term AB itself.
By true academic definition AB granite is not actually granite. Granites are broadly stones that have a mineralogy consisting of alkali feldspar, quartz and plagioclase feldspars that together make up between 85 and 90% of the chemistry. Black granites on the other hand have very little quartz and instead have larger quantities of calcic-plagioclase feldspar as well as calcic pyroxene. So they are not quartz rich as are the other main granites. However the industry refers to these black stones as granites due to their other main similarities namely that they are both igneous rocks formed in a similar fashion having a similar degree of crystallisation. Their physical characteristics are similar making them different from the other main families of dimensional stone, limestone and marble. So AB is a high quality black stone that is very dense and hard. However if it is to be the low maintenance product it is marketed as then all AB stone must be created equal and this is not the case.
AB initially came from South Africa and given the Italian name Nero Assoluto. It had the quality high performance physical characteristics that set the precedent for AB. However today black granites referred to as AB come from many other countries – India, China, Brazil, USA, and Australia to name a few. In fact I know of a total of 12 countries that export “Absolute Black” and this is where the problem lies with the original statement that you cannot seal AB. Just like the ceramic world where technical definitions are becoming blurred due to competition so it is with the stone industry. Many black granites are being sold as AB however they are not AB.
One of the key features of AB is the very dense, rich, deep, intense black colour of the stone. This intense colour is due to the quantity and opacity of the plagioclase feldspar which in turn contains small quantities of very fine-grained inclusions of iron oxide. So just like in ceramics the black you are looking at is not really black it is a very, very dark brown. The intense colour is due to the quality and quantity of this feldspar and without it the colour is not as intense and the stone also has lower density, less hardness and is more porous. In other words it will not perform like Absolute Black. The market place and competition have prompted companies to take lesser black stone and artificially colour it with chemicals. The resulting modified stone looks like AB but does not perform like it. It is this situation that finally gives us our answer. If a stone is truly AB then it does not need to be sealed. However with the stone industry continually increasing the number of different stones that fit into this description more of more of the AB stone sold can be stained and hence can benefit from a sealer.
The type of sealers required to seal these “other” AB stones is also something that is slightly contentious. There are those that believe only solvent based sealers can penetrate such dense stone whereas others think water based is the way to go. The first thing to remember is that if it is true AB then no sealer will successfully penetrate the stone. So the categories that can be sealed are those “other” black stones that are marketed under the AB tag. These can technically be sealed with either a premium specialist water or solvent based stone. However you will find that the solvent sealers work better in that they are easier to apply to these dense surfaces.
If a water based sealer is used it struggles to get into the very dense surface. However if you allow enough dwell time it will eventually get in, cure and do a good job. Aqua Mix Stone Sealers Choice is a good example of this. A solvent based sealer has much smaller molecules getting into the dense stone more easily making it my choice for AB. I like to use and recommend products that are easy to install and hence are more tolerant to different application techniques guarantying better quality control. It must be noted that if you go to seal a more porous product such as limestone then the tables are turned. In this case the finer solvent dives way to deep whereas the water based sealer sits near the surface where it does a much better job of protection the stone. So if you think that one sealer type can achieve the best results on all stone – from sandstone to AB – then you are mistaken. A professional requires both water based and solvent based sealers to offer the best performance across the wide range of commercially available stone.