Resin treatment of slabs is not a new process. It has been part of global processing lines for many years. Reputable stone adhesive manufacturers have studied the importance of strengthening various types of stones, some in heavy demand, before they can be sold and used as building materials. These manufacturers use their sophisticated Research and Development (R&D) Departments to develop products specifically for this purpose, setting strict quality-control standards throughout the process with special attention given to the integrity of the finished product or slab. As a matter of fact, there are and have been automated resin-treatment systems that are now an integral part of many automated processing lines. For slab distributors and fabricators, this can mean lower costs for slabs because of a higher quarry yield, or in some cases, turning an unproductive quarry into a productive one.

Understanding geology

To understand why a stone would benefit from resin treatment, we need to understand a little about the composition of natural stone. Marble and slate are metamorphic stones. That means that they have been exposed to high temperatures, causing the minerals to go through a molten phase, creating the “bands.” All marbles have calcite and/or dolomite. Marble contains calcium carbonate, which reacts to acid, prohibiting the use of acidic cleaners, and in general, it is softer than granite.

Granite, an igneous stone, ranges from six and higher on the Mohs Scale of Hardness. These rocks are formed from the solidification of magma deep within the earth. Granite contains 45 to 66% silica (quartz), feldspar, mica and iron ores. Limestones and sandstones are sedimentary stones, produced from the erosion of other rocks as well as from compression and underground water erosion. They consist of calcite, but they are mixed with other minerals such as magnesium carbonate (dolomite). Slate has been metamorphosed from shale and consists of clay-like material. Slate will rarely have a high polish unless it has been coated with a barrier sealer. It is best treated with a silicone penetrating impregnator.

Resin-treating versus filling

When addressing these issues, we need to clarify the distinction between resin-treated slabs and filled slabs. The process of filling holes and small fissures - and strengthening areas of marble, travertine and other stones - has been a fundamental part of stone production lines for years. Polyester - color matched and sometimes with added components to mimic the texture of the stone - or epoxy is applied to the surface before the final polishing is done. UV lamps can speed the curing time to seconds, further increasing production rates.

The more recent process of resin-treating slabs, however, goes beyond filling. Resin treatment began as a technological advance to increase yields in quarries, to allow for the use of stones that were previously unsuitable in certain applications, and to provide a slab that is stronger in general. Today, machinery companies work closely with adhesive manufacturers to produce a better quality product with the added value of higher yields for the quarry owners and a better product for the stone fabricator.

Weighing the benefits and problems

Resin treatment of slabs can address micro-fractures caused by the natural formation of the stone. They can also consolidate and strengthen the stone, making it usable without altering the natural characteristics of the stone. A third effect would be enhancing the stone's color and other characteristics for a desired look. A clear, non-color-enhancing, UV-resistant resin applied before polishing can bring out more depth and give the stone a more uniform appearance. Stones previously undesirable for certain applications are now aesthetically appropriate for more applications.

Exactly what type(s) of resins are being used in the three situations above? The majority are very viscous, penetrating clear epoxies, with a UV-resistant agent added to prevent yellowing. These epoxies are the slower curing type to permit deeper penetration into the material. Pre-colored epoxies with the same characteristics and properties of the clear types are used when appropriate. These epoxies are very different than the quicker setting, faster curing epoxies used by the stoneworker for bonding and laminating. As a matter of fact, reducing the curing time of an epoxy can compromise the bond, as the penetrating action of an epoxy is the essence of its strength.

However, problems occur when stone fabricators are working with low-quality slabs that have been altered with a cheap resin-type product. These products may have been applied to the surface of the slab during processing to hide inherent flaws in the low-grade material so it can be sold as a “first-grade” slab. The edges do not match the color of the top of the finished slab, and the top of the finished product can look unnatural, even plastic-like.

It is easy to tell whether a slab has been resin-treated, because the resin is easily seen along the top of the edges (provided the slab hasn't been trimmed before delivery). If the slab has been filled properly, you should not see any staining on the top of the edges. Remember that polyester fillers are used to fill small holes and/or micro-fissures on the surface of the stone, and the slab is then polished. Proper resin treatment uses a viscous epoxy that will penetrate through the entire stone. But if you detect a coating on the top of the stone - penetrating only millimeters into the surface - it is likely that the process was done improperly.

Many manufacturers of resin products offer problem-solving enhancers to “cover up” the flaws, and many of these products offer permanent solutions, having been designed to legitimately enhance natural stone. Most are manufactured by the same companies that manufacture the original resin epoxies used in the processing plants.

However, some fabricators are using very creative homegrown remedies (some better than others), but the results are temporary and will not guarantee a satisfied customer a year from now.

The need for quality processing

Who is responsible for ensuring the quality of resin-treated slabs? Ultimately, this falls upon the stone producers, who not only need to invest in quality resin products, but they also need to make sure the products and processes are right for the specific materials being sold. The top stone producers have their raw material (stone) analyzed and the subsequent finished processed product tested for quality. They can and will tell you what you are purchasing and how it has been processed. They conduct ongoing tests for different locations in their quarry and alter the chemical composition of the resin they are using to work appropriately with the varying mineral contents in the stone. Besides knowing what you are buying, you are also able to determine how to care for the stone after installation. Most of these quarries welcome visits to their plants by prospective buyers.

The disreputable processing plants, on the other hand, are just capitalizing on a new process to cover up below-standard material, hoping you won't know the difference or worse, dumping this sub-standard material through third party exporters, hiding behind anonymity. They have found a cheap alternative to costly epoxies and often don't know or care what this product is made of. They know they can get away with this because they export to people who don't understand the resin-treatment process; they only know that “something” has been done to the slab.

North American quarries have very diverse opinions regarding the resin-treatment process. Some are marketing the fact that their material is of good quality and does not need to be improved upon in any way, dismissing the entire process as undesirable for any reason. They promote the fact that you always know what you are buying from them. Other North American quarries have worked with resin manufacturers to develop a suitable, food-safe resin to use on their material for the purposes of improving the final product as well as to increase their yield with no perceptible affect on the quality of their finished product.

The fact remains that there are limited quarries in North America and limited selections of material. You will continue to import stone from around the world, and it is on your shoulders to become involved in setting the standards for what you import.

The consensus among those of us involved in solving problems and finding solutions for the slab distributors - and ultimately the stone fabricator - is that we need to become educated. When we hear about a new trend in our industry or we happen upon something that doesn't feel right, we need to question and do the research immediately or we lose our edge. Even properly resin-treated slabs may cause more wear and tear on cutting blades and polishing discs.

In general, we need to become more educated regarding resins and slabs. We need to learn about the resin products - and more importantly, to learn about the resin-treatment process as a whole. It is in your best interest to ask your suppliers about what resin products they use and how the slabs are processed. You will gain an understanding of all of the pros and cons involved, and more importantly, you will be able to convey this knowledge to your customers.