Today there are so many slab distributors, not to mention the huge selection of slabs available. All too often I hear nightmare stories about slabs that are flawed and cannot be used. Hopefully the following guide will help avoid these problems.
Size
All other things equal (finish, consistency, and imperfections), size becomes very important. It can be the determining factor of whether you get the job. First of all, if the slab is too small to make some sections in one piece, then the client may or will find someone with larger slabs. Secondly, size can determine your waste. If there is a lot of waste then your cost goes up, and when your cost goes up then either your prices go up or your profit goes down. If you are bringing slabs in for stock, then you must anticipate its most probable use and determine your needs and waste from that.
Finish
Polished is probably the most popular finish for stone fabricated for commercial and residential use. Other finishes include honed, thermal (flamed), sawn, bush hammered (pitched), sandblasted, etceteras. There are different degrees within most of these finishes.
If you are fortunate enough to own a reflective light or gloss meter then this factor is easier to determine. Some quarries, factories, brokers, importers, or distributors may alter a slab to make it appear better than it really is. This practice can cost you in the long run: time, money, aggravation, and reputation. Most people try to darken the typically dark stones or apply waxes, coatings or dyes to make the polish appear better. I am not aware of a wax or coating to enhance the polish that will last as long as the natural polish of most marbles, granites, and onyx. If you suspect that a stone has been altered then you should not accept it or have the supplier prove that it has not. One way to test a material is to rub your finger across it to check for smudges. If the slab smudges then use a solvent, such as acetone, mineral spirits, or neutral (pH = 7) wax or sealant strippers to determine what the slab really looks like. This practice may also help to determine the proper color of the stone. If you have the capability to face polish the stone, then another method would be to polish the face along the edge, back, or other inconspicuous area. The best method for avoiding these problems is to deal with a knowledgeable and reputable supplier.
Consistency
Consistency, for this application, is synonymous with balance. If a slab has good balance, then you will have less trouble getting a two-piece countertop to match at the joint or a fireplace header and legs to look right. If you cover most of the slab with a piece of plywood, which has randomly cut out sections in it, then you could more easily determine its consistency. Unfortunately, sometimes you don’t know the consistency until the pieces are cut and being membered up with the piece that will adjoin it after installation. It is always a good idea to look at the consistency as the pieces are being cut and before doing edge work, cutouts or taking them to the job site. Because stone is a product of nature, very few slabs will be as consistent as you would like.
Imperfections
Beauty is in the eye of the beholder. What some people call an imperfection others may consider desirable. Although some imperfections are just that¼an imperfection, no matter how you look at it. The following is a list of some of the major imperfections I have found over the years:
fissures, cracks, veins that look like cracks, scratches (of all origins), large brecciated sections (in either a veined slab or one with typically small brecciations), holes, fill, pits, shift veining (can be good quality also), inconsistent veining, too small, irregular shape, discoloration, weathered polish, dry or mud veins (can be typical in some material), mineral deposits (can be a good quality also), and inconsistent thickness or gauging problems.
Two machines that would do well in this industry are the stone stretcher and the stone planer. You need a stretcher for every slab that is too small and for every piece that is cut too short. The planer would be useful to ensure that all adjoining pieces are the exact same thickness. These machines do not exist, so you must take great care to reduce your risk of needing them.
Soundness
Members of the Marble Institute of America (M.I.A.) are probably familiar with “A, B, C, and D” ratings for marble. “A” is the most sound and “D” is the least. Part of the “A, B, C, and D” definition includes slab size restrictions and is now, because of new technologies, considered out if date. The rating of a slab is very important to fabricators and installers. If you are unfamiliar with a material specified on a project you can look up its M.I.A. rating. If it is a group “D” material you will need to estimate extra time for patching (filling voids with glues, shellac or other filling material), penetrating (strengthening the marble with a penetrating adhesive), sticking (gluing together pieces that have come completely apart), rodding (cutting grooves in the back or unseen section of the stone to insert a flat bar metal rod), and face polishing (grinding the face smooth where the marble has been patched or stuck back together). A group “A” material should mean that your fabrication time will be less than with the other groups. There are exceptions to most rules: I have seen group A material that had holes in it. This rating system only applies to Marble, not Granite.
Limestone is normally less sound than most marble of the same thickness. Granite, on the other hand, is normally more sound than most marble. Each stone has its own unique advantages and disadvantages when compared to other stones. There are granites that will break if handled incorrectly, so never overestimate the soundness of a material.
Suitability
You need to know the limits of the stone selected for the job. I lost a job because I would not bid it as specified. The architect specified polished ¾” Negro Marquina Marble for a conference tabletop and a credenza top (also used as a dry bar). I priced the job using polished ¾” Black Absolute Zimbabwe Granite. This job was priced back in the late 1980’s when the cost of the granite was much higher than the marble. As I mentioned before we did not get the job, but we earned the respect of the architect, which helped us get many jobs afterward. I knew the limits of the specified stone. It was soft, acid sensitive and black. This meant that it would etch when they spilled fruit juices on it, scratch easily, and both would be very obvious. The person who did the job would not help the architect resolve the problems they were having and was excluded from their future bid lists.
To determine suitability you must rely on your past experience with the stone, or obtain the A.S.T.M. (American Society for Testing Material) values for abrasion resistance, absorption, flexural strength, compressive strength, and density. It is also very important to know the mineral composition of the stone. Once you know the value for abrasion resistance, this helps you determine approximately how long the polish will last or if the material will last long if installed in a high traffic area in 3/8” tile. Absorption will give you an idea about how porous or how stain resistant the stone is. Flexural strength is a value to help you decide how much support an island top or stair tread may need and how thick it should be if the customer is unwilling to add the support needed. If you plan to install a load-bearing structural column on a pass-through top, then you should know the compressive strength of the stone and the anticipated load of the column. Many people ask if the weight of 1 ¼” granite requires reinforced cabinetry and floor joists. Density measurements are taken and available for some of the more popular stones. This measurement will reveal the weight of the stone at one cubic foot. To obtain the weight at 1 ¼” divide the value by 1728 (12x12x12=1728), then multiply by 1.25 (or the exact thickness of the stone). I tell people that most well made cabinets will take the weight of the 1 ¼” granite, but if there is any question, then give the weight per square foot to their cabinet supplier for verification. The mineral composition can prevent some stones from being used outside. If a stone has pyrite, for instance, then it will most likely leak iron oxide or rust, which can become unsightly. If you are not sure of the performance of a stone for a certain application, then contact your supplier to get more information.