An Excerpt from the Dimension Stone Design Manual, Version VIII (May 2016)
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Restoration and Maintenance An excerpt from the Dimension Stone Design Manual, Version VIII (May 2016) Produced and Published by the Marble Institute of America 380 East Lorain St. Oberlin, Ohio 44074 Telephone: 440-250-9222 Fax: 440-774-9222 www.naturalstoneinstitute.org © 2016 All rights reserved. No part of this document may be reproduced or transmitted in any form or by means electronic or mechanical, including photocopy, recording, or by an information storage and retrieval system, without written permission from the Natural Stone Institute. Restoration and study those sections thoroughly to gain a greater understanding of the subject matter. Maintenance – Stone geology 2.0 STONE FORMATION 1.0 INTRODUCTION 2.1 Thousands of stone deposits exist 1.1 One cannot effectively participate in throughout the world. Yet all stones have been the field of stone restoration without at least a formed by one of three methods, and therefore rudimentary understanding of the geological all stones can be classified into one of three sciences. Stone composition is a far more groups: Sedimentary, Metamorphic, and complex issue than is commonly thought, and Igneous. the in-service performance and behavior of the stone can be significantly influenced by even 2.2 The term Sedimentary comes from minor constituents of its composition. This the Latin word sedimentum, which means issue is further complicated by the fact that “sinking” or “settling.” It is used to describe while geologists and petrographers identify stone deposits that are formed when sediment hundreds of different rock types, the stone is collected over geological periods of time, industry uses much broader definitions of stone causing individual grains, or “clasts,” to be types than the scientific community. In doing cemented together by another agent. Common so, the industry includes similar materials into cementing agents, in descending order of various stone groups despite the fact that the preference, include silica, carbonate, iron stone does not technically and scientifically oxides, and clay. Limestone and sandstone are belong to that group. As practical examples, examples of sedimentary rocks. popular stones such as Crema Marfil and Rojo Alicante are commercially identified as 2.2.1 There also exist sedimentary stones “marble,” even though they are not geologically which are not created from the settling of marble, and would be scientifically classified as clasts, but are chemical sedimentary “hard, compact, dense limestone.” Paradiso stones. Onyx is an example of a chemical and Uba Tuba are commercially considered sedimentary stone. granite, yet they are actually “gneiss” and “charnockite,” respectively. The differences in 2.3 Metamorphic is a term used to composition, behavior, and performance describe stones that have undergone a change between these rocks and those rocks that in structure. The term originates from two scientifically belong in these classifications are Greek words: meta-, meaning change, and generally slight, and negligible in most morphic, meaning structure. Perhaps the best commercial issues. But to the restoration known example of a metamorphic rock is professional, such differences can be marble, which has changed its structure from a significant. Reference is made to the Marble sedimentary limestone to a recrystallized rock Institute of America’s Dimension Stone Design fabric known as marble due to intense heat and Manual Chapter 1 and Chapters 5 through 12. pressure. While a brief introduction of stone geology is offered here, a far more comprehensive 2.4 The term Igneous comes from the discussion of the subject can be found in those Latin word ignis, meaning fire. This term chapters. The reader is strongly encouraged to describes stones that were melted deep within 2016 Marble Institute of America Stone Geology • Page 21-1 the earth by thermal energy released from the challenge to the restoration professional decay of radioactive minerals within the earth’s attempting to refinish it. The hardness of the core. These stones, eventually cooled and granite requires aggressive abrasives to solidified, have been harvested for a variety of effectively work the material, yet these same uses. Commercially, the most common of abrasives can create uncontrollable dishing and these rock types used is granite, yet many other gouging in the much softer travertine portions igneous rock types exist in commercial trade. of the floor. Variability in mineral hardness can also exist within one stone. Such variability can create the same challenges in achieving a 3.0 STONE MINERALOGY uniform level of grinding, and can also prevent a uniform level of gloss from being achieved due to the difference in the light reflectivity of 3.1 Siliceous and Calcareous Stones. the two portions of the stone. Geologists have identified roughly 3,500 different minerals found in the earth’s 3.2.1 Mineral hardness has long been composition, yet only about a dozen of these measured by means of the Mohs Scale of minerals are commonly found in Relative Hardness. In 1822, Friedrich commercially-used stone varieties. As Mohs, an Austrian mineralogist, published a previously stated, however, even minerals paper on mineral hardness based on the scratch found in trace levels of a rock fabric can resistance of each of 10 minerals when tested influence the behavior and performance of the against the other 9 minerals in his study. His stone. Minerals that make up stone are published paper listed the 10 minerals in order commonly divided into two groups: siliceous, of their relative scratch resistance. As simple or silicate minerals, and calcareous, or calcium as this study appears, almost two centuries carbonate minerals. Stones that are comprised later, we are still using Mohs’ research and of siliceous minerals include granite, slate, and findings as the basic rank of mineral hardness. serpentine, while stones that are comprised of The ten stones included in Mohs’ scale include: calcium carbonate components include marble, limestone, and travertine. Hardness Mineral 1 Talc or Mica 3.2 Mineral hardness is an extremely 2 Gypsum important consideration for the restoration 3 Calcite professional. Mineral hardness determines 4 Fluorite how the stone will perform in service when 5 Apatite subjected to abrasion, such as stones used for 6 Orthoclase walking surfaces. Mineral hardness is also an 7 Quartz important measurement in predicting how 8 Topaz easily a stone can be resurfaced, as the grinding 9 Corundum operation can be significantly slowed when 10 Diamond hard minerals make up the stone fabric. In addition to the absolute hardness of a mineral, 3.2.2 There are two common one must take into account the variability of misconceptions regarding the Mohs scale. The mineral hardness within an installation. A first is that many do not clearly understand that patterned floor, using granite bands and the scale is relative, not absolute. travertine fields, can pose an extreme Page 21-2 • Stone Geology 2016 Marble Institute of America Relative simply means that “4” on the scale is a stone, but acidic chemicals are the cause of harder than “3,” and both of them are harder most frequent concern. Acidic chemicals are than “2.” The scale offers no absolute commonly found in both commercial and information regarding how much harder, or residential settings, through food and whether the interval between 4 and 3 is greater beverages, as well as many common cleaners or less than the interval between 3 and 2. A and detergents. One of the most frequent mineral with a hardness of 4 IS NOT twice as inadvertent acid exposures encountered in hard as a mineral with a hardness of 2. It is both residential and commercial settings is more than 10 times harder! Today, we have the urine, which is quite acidic and has been instrumentation available to measure absolute responsible for the attack of many stone floors. hardness. If the Mohs scale reported absolute hardness, it would look like the table below: 3.3.1 Acidic content is measured with the pH scale. The pH scale ranges from 0 to 14, Mineral Relative Absolute although there are some exceptionally acidic Diamond 10 1600 substances that can actually be below zero on the pH scale. A value of 7 is neutral; anything Corundum 9 400 lower than 7 is acidic, while anything above 7 Topaz 8 200 is alkaline. The pH scale is not a linear scale, Quartz 7 100 but a logarithmic scale. This means that a Orthoclase 6 72 substance with a pH of 5 is 10 times more Apatite 5 48 acidic than a substance with a pH of 6. Fluorite 4 21 Calcite 3 9 3.3.2 Generally speaking, all calcareous Gypsum 2 2 stones are subject to acid attack, even from the mild acids found in household Talc 1 1 settings. The most common result of acid 3.2.3 The second area of confusion exposure is etching, which is generally visible regarding the Mohs scale is that it is a as a dull spot in an otherwise high gloss hardness scale of minerals, not stones. polished surface. Depending on the acidity of Since nearly all stones are polymineralic, each the attacking agent, the dwell time that the mineral within the stone’s composition has its attacking agent was allowed on the stone own unique properties, including hardness. surface, the vulnerability of the specific stone Oftentimes we see marketing literature for a to acid reaction, and the number of repetitions certain stone that lists a “Mohs Hardness” for allowed to occur, the result can be much more the stone. This is a technical inaccuracy, since severe than a simple dull spot. the stone is comprised of a variety of minerals and each mineral has a unique hardness value. 3.3.3 Siliceous stones are far more acid To ask, “What is the Mohs’ hardness of this resistant than their calcareous stone?” is the equivalent of asking, “What is the counterparts.