In spite of heavy investments in modern production lines and good quality resin, many stone processors are still dealing with the incomplete filling of defects. Thin cracks running deep into the thicknesses, holes and crevices remaining on the surface polished to a mirror finish, and large full-thickness pits are the most common problems. Let’s examine causes and consequences of each of these cases in detail.
It is a fact that seemingly sturdy slabs can contain fine narrow cracks that have not been completely saturated with resin and hence have not been stabilized. These are almost always naturally-occurring fissures which have split, and the stone fabricators know very well that they should be regarded as “active cracks” with the potential to undermine the integrity of the slabs, and namely cause the material to chip or break during the manufacture of fireplaces, vanities, tables, kitchen countertops, stairs, windowsills, skirtings, wall panels and so on. There are several factors which can hinder the resin penetration into the finest cracks:
1) The slabs are not honed prior to being resin coated.
2) The technique of stone drying is ineffective.
3) The surface of the slabs is not been meticulously swept using an appropriate brush.
4) The resin product is too thick.
5) The resin product sets too quickly.
6) The cure temperature is too high.
It is noteworthy to mention that most often the incomplete filling of thin and deep cracks is caused by a combination of factors (especially the points 4, 5 and 6) occurring simultaneously.
Notwithstanding that the blocks extracted from a certain quarry feature an optimum grain size and wonderful colours, the slabs machined down from them will never be sold at full market price if there are holes and crevices on the polished surfaces. There are a number of potential reasons and combinations of reasons why slab producers may find it difficult to block up cavities which are deeper than 3 or 4 mm:
1) Workers do not apply sufficient amounts of resin to the slabs.
2) Workers do not apply the resin carefully enough.
3) Workers miss to carry out the “retouch”.
4) Workers do the retouch carelessly or wrongly.
5) Workers should carry out various retouches.
6) The time separating the initial resin coating from the final retouch is too short.
7) The resin product is too runny.
8) The voids should be filled with mixtures of high viscosity resin and stone crushed into chips and powders.
9) Surface filling should comprise of two resin treatments interrupted by honing.
Unfortunately, it has become common practice among inexperienced stone processors to fill holes and cracks with coloured “epoxy gels”. Anybody would find it easy to block up holes with gel products indeed; and yet the gel fillings are rather awful-looking and not really long lasting. The point is that the epoxy gels were never devised for surface restoration and enhancement (as we explained in our previous editorial titled “Important aspects of the repair and restoration of marble which are often overlooked and underestimated”).
Owners of “coloured” or “breccia” marble and onyx quarries interested in cutting their blocks on an industrial scale have been long asking themselves what to do with the slabs that have huge full thickness pits. We must in all honesty say that it is impossible to learn the proper methods for restoration of fragile stone without a teacher. For example, only a few experts know how to fill and repair onyx slabs with “patches” made of resin and pieces of stone that can hardly be distinguished from natural waving at first sight. To seal cracks which go all the way through the slabs and though are not so wide it is enough to follow the instructions provided in the afore mentioned article: “Stone processors needing to treat marble damaged with fractures and holes which are so deep to pass through the entire thickness of the slabs, should always block the cavities up before the material is loaded on the plant, to ensure they are thoroughly filled with the resin that is applied to attach the fibreglass mesh. The resin would otherwise seep through the cracks without filling them indeed, and as a consequence the desired strengthening would never take place. In summary, dedicated stone processors should close the major faults of the slabs on the side of surface – the one which will be then resin coated and polished – to make sure that the voids are saturated with resin at the time of mesh backing. Factory workers can seal holes and fractures with either quick setting mastic or adhesive tape, or even both these products, depending on the amount, extent and depth of cracking and holing. Regardless of which product workers choose, the operation has to be carried out with the slabs held vertically by a robot loader or a clamp connected to the hoist of a jib crane”.