Stone Selection & Specifying
Originally Published in 1993
Selecting a stone for use requires that many questions are asked. “Who is doing the selection?” is one main question. When an architect/designer selects a stone he should be aware of the criterion for color, finish, application, quality, installation, and aesthetics for the job. When a contractor, owner, or individual selects a stone he will normally do it for price considerations and color only. For this article let us assume that we are selecting a stone for a commercial job.
Let us define dimension stone or building stone for commercial purposes as any decorative stone such as marble, limestone, travertine, slate, quartzite, and granites which has been cut to size for specific applications. These probably are most of the stones used in such jobs. More and more people are looking at shellstone, sandstone, and other natural fossil type stones that have a rustic look. People are always introducing other stones in hopes of finding a niche in the market for the rustic look that so many people are requiring.
Most often consideration should be given whether the stone is for the exterior or interior, or for vertical or horizontal surfaces (walls vs. floors or table tops-countertops vs. bathrooms). Do not forget to know whether stone in cubical form for columns, arches, or other applications is required. This may cause a different selection to be done due to price and structural strength of the stone which at this moment we are not considering. We will consider the selection as it applies to tile, or standard 3/4″ (2cm) or 1-3/16″ (3cm) thickness as this accounts for 95% of all stone purchased.
In considering a stone there are always lots of questions to ask about the job before looking at any stone. It is imperative to know whether the stone is to be used in freeze-thaw climate, wet conditions like around spas or in bathrooms, heavy traffic areas like shopping mall floors, or exterior cladding. Know what size panels will be required or how much quantity is estimated to be needed and when will it be required?
To select a stone we need to understand how stone is quarried and fabricated. Some stone comes from mountains, others from underground, and others from river beds. Slate is usually found in river beds or underground mines. Slate and Quartzite will have size limitations and finish limitations. Usually these can be used on the exterior and interior. Slate is derived from shale and is usually clad together as layers, thus allowing it to be split with irregular facing. Normally slate can be supplied in tiles of 1/4″ or thicker and is normally supplied with cleft faces or honed faces. The sizes are usually limited to tiles of no more than 18″ x 18″ x 1/4″ or slabs cut to size of no more than 28″ x 50″. There are always exceptions and some kinds of slates can be quarried in larger slabs of 4′ x 8′. Slate is good for flooring due to its natural cleft face which gives more slip resistance. It is also good for exterior and roofing applications in most cases. It comes in an abundance of colors from many countries.
Limestone or Travertine
(sandstone, coquina, dolomites, fossil or shellstone)
Limestone or Travertine is usually quarried underground. It often has size restrictions and usually will not take a polish finish. Sometimes you will need to specify whether you are needing a dolomitic or oolithic limestone. Normally buyers are selecting this stone with honed or rustic finishes for its texture and grain. This material may be used for exterior applications if it passes certain tests. You must be careful to question the absorption of the stone for its application. Normally limestone will have a high absorption rate and are not suitable for wet climates unless treated with sealers. Also many limestone’s are soft and thus wear easily on flooring applications. Some limestone’s are very hard and can be flamed. However, there are not as many of these available as those that do not take a flamed finish. There may also be size restrictions when selecting this stone. Many times you will find holes in this material which may be kept as natural or filled with synthetics such as epoxy or cement.
(greenstone, onyx, serpentine, some limestone’s)
Marble is defined two ways: as a stone which has been re-crystallized (the geological definition) or as any limestone that will take a polish (the commercial definition). As we are all finding out today, most marbles coming from Spain, for example, are really limestone’s and customs is charging a higher duty. Usually marbles will be found in mountainous areas. Therefore, the factors that apply to limestone’s apply here. In addition to the comments about limestone’s, consider that many colorful marbles have quantity restrictions as they are only a selection within the same quarry. Many marbles have tremendous color variation and could require 20 samples to show the full variation in the stone. However, due to the colors available you will usually have a much larger selection from which to choose. Some of these marbles may have holes or cracks and require reinforcement with fiberglass net backing or filling. Most marbles are used decoratively on interiors and very few are acceptable for exterior use unless you have special conditions. Most buyers want this material to be polished even though it is recommended to be honed, especially on flooring applications.
(gneiss, gabbro, diorite, basalt)
Granite, an igneous rock, is probably one of the most used commercial stones, especially in exteriors. It is very hard and durable, as are quartzite’s, and very suitable in most cases for severe weather conditions. Usually granites are found in mountainous areas and are monolithic in design. The size of the grains may vary from fine to large. The color may vary but is usually homogeneous. In some cases you can find granites with a lot of character, veining, and color variation. The granites are usually found in large deposits and large sizes. Many granites are being used on interiors, especially as flooring since it will hold a polished finish longer than marble and wears less than marble or limestone. Being one of the most dense stones and very good for abrasion resistance it lends itself to many textured finishes such as flaming, bushammering, sandblasting, acid, chiseling, etc.
The problem most buyers have in selecting a stone is that they only see a very small sample or only what the salesman shows them. The name is usually changed to confuse the buyers and maintain a lock on who can supply the stone. Usually the buyers do not get enough information on the stone to make a proper judgement. They select the stone simply for its color, only to find out later that the color varies more than they can accept or it is not suitable for the application intended, or there is not enough time to obtain the quantity they require. Therefore, it is wise to ask the following questions about the stone before making a final selection:
1) What color and texture do you want? There may be many stones that fulfill your requirements after much investigation. Once you have chosen several stones that fit your needs, then proceed to the next step in narrowing down whether the stone will fit your qualifications and standards.
2) What is the actual quarry name of the stone? Where is it quarried and is there more than one quarry? If you do go to bid with a fictitious named stone, you may have lots of problems getting competitive bids and others will offer alternate stones in lieu of what you want. Further, if you ever had to get more material at a later date, you may not be able to obtain it. What type of quarry is it? Are blocks available in large sizes? If so, what is the maximum size? How much quantity per month is available? In some cases, you may even wish to specify the layer in the quarry where the stone is located that meets your requirements, so ask if this is possible? Further, if the job is large enough ask for contiguous blocks from the same wall in the quarry. In random slabs, you may ask or specify alternating facing be polished to give book-matched slabs, and have them numbered sequentially.
3) What color variation can be expected in the stone? Are there veins, and if so, how do they change in size and color? Obtain enough samples of the stone to show the variation and understand what the problems are in selecting the color range acceptable to you. You may have to pay a little extra to get what you want but at least you will not be surprised at what you get. If the job is large enough, demand that the contractors, architects, owner etc. visit the quarry and factories and see other jobs where the stone has been supplied similar to the intent you are planning.
Be cautious, because what is variation to you may be nothing to a stone person. Once selection is made, have several sets of control samples prepared, signed off on, and kept by all parties concerned for control.
4) Find out about the integrity of the stone. Does the stone show holes which require filling, cracks or seams which require gluing, or other faults which require extra labor or care such as backing the stone for strength? These features may add to the cost or be a problem when it is time to grout the stone, seal the stone, or cause problems with failure later on, such as cracking, pitting or spalling. Some stones, such as Negro Marquina, spall. Other stones, such as green marbles commonly known as serpentines, warp when wet. Some stone effloresce, or lime powder, comes to the surface, and it looks milky.
5) Find out about the stone’s standards and require A.S.T.M. testing on the stone to make sure it passes the requirements the architect has set forth. Make sure the finish you have selected will wear well and that it will pass all codes and standards required for slip resistance. Make sure the absorption will satisfy your needs. Make sure the stone does not change color or fade in sunlight or have metallic contents or other minerals that will cause it to rust. Each type of stone may require different testing for the application intended. Get these tests done by qualified labs as it is your protection that it meet certain standards. In some cases, test the stones several times in production to make sure that it maintains these standards, as it can change in the quarry and from quarry to quarry.
6) Set standards of the finish you want and be specific. Not all polish stones have the same mirror reflection. Not all honed stones allow for the disappearance of cutting and honing marks on the face. Test each stone and see what is suitable and will satisfy your requirements and find out the type of finish and grit that is being used to accomplish this. Even today flamed finishes are accomplished by several methods and one may suit you over another.
Also, find out how the stone should be maintained and demand information on maintenance. Should the stone be sealed? If so, get recommendations of sealers or try some samples out with labs to determine the best sealers and how they may affect the color or look of the stone? Most sealers require a more porous finish such as honed to start with, so know this in the beginning.
7) Find out about the suppliers and have them qualified before bid date. (The same applies to contractors.) Give suppliers a questionnaire and ask them about job references where they have supplied similar jobs and quantities? Ask them for warranties or guarantees about their service, their quality control, and their delivery times. Ask how they will correct and take care of problems should any occur.
8) Get references of projects that either you can visit or you can call and check out the stone or the suppliers. Get pictures of the jobs showing the stone to see what a large quantity looks like.
9) Ask for recommendations of installation methods and thickness of stone for the application you are considering. Some stones require certain white portland cement, or epoxy to set. Other stones will stain easily with colored epoxy grouts. Part of the cost of selecting the stone is getting it installed properly and economically. The overall budget will be approved based on the stones delivered cost installed. Sometimes the installation method can cost more than the stone.
By finding out about the quarrier, the supplier, and the stone you will avoid problems down the road. You will alleviate the problems of surprise that so many buyers have when they open their containers.
Finally demand everything verified in writing. Find out how the stone is to be paid for, whether open account or with letters of credit. What will the supplier accept as a hold out for final payment until the stone is approved at the job site or warehouse? This will usually indicate to you whether you are working with reputable companies or those on a shoestring budget.
Once you have checked out all the variables on the stone, the contractor, the costs, and other details, than stay with your specification and make sure no one bids an alternate stone which has not been checked out and approved. Offering alternates is very common. In some states or jobs, the architect has no control. Once the job is turned over for bidding, the general contractor takes out the names of the specified sources of supply and in turn states “and/or equal or alternates allowed”. As soon as this is done, everyone takes risk to get the quality, price, and assurance of the supply of stone in question. Selecting stone requires that all parties concerned, the general contractor, the owner, the architect/designer, the contractor, and the supplier work as one to do a job that all will be satisfied with and that will last a long time. Cooperation and communication are important in any project if it is to work well and smoothly.