Rule Number 1 in dealing with historic stone surfaces is to stay within the Secretary of the Interior's Standards for rehabilitation. These standards are well known throughout the historic conservation community and should
be used as a guide when writing specifications or prescribing treatments for interior stone and tile surfaces.
The Secretary of the Interior's Standards for Rehabilitation
1. Every reasonable effort shall be made to provide a compatible use for a property which requires minimal alteration of the building, structure or site and its environment, or to use a property for its originally intended purpose.
2. The distinguishing original qualities or character of a building, structure or site and its environment shall not be destroyed. The removal or alteration of any historic material or distinctive architectural features should be avoided when possible.
3. All buildings, structures, and sites shall be recognized as products of their own time. Alterations that have no historical basis and which seek to create an earlier appearance shall be discouraged.
4. Changes which may have taken place in the course of time are evidence of the history and development of a building, structure or site and its environment. These changes may have acquired significance in their own right, and this significance shall be recognized and respected.
5. Distinctive stylistic features or examples of skilled craftsmanship which characterize a building, structure or site shall be treated with sensitivity.
6. Deteriorated architectural features shall be repaired rather than replaced whenever possible. In the event replacement is necessary, the new material should match the material being replaced in composition, design, color, texture and other visual qualities. Repair or replacement of missing architectural features should be based on accurate duplications of features, substantiated by historic, physical or pictorial evidence rather than on conjectural designs or the availability of different architectural elements from other buildings or structures.
7. The surface cleaning of structures shall be undertaken with the gentlest means possible. Sandblasting and other cleaning methods that will damage the historic building materials shall not be undertaken.
8. Every reasonable effort shall be made to protect and preserve archaeological resources affected by or adjacent to any project.
9. Contemporary design for alterations and additions to existing properties shall not be discouraged when such alterations and additions do not destroy significant historical, architectural or cultural material, and such design is compatible with the size, scale, color, material and character of the property, neighborhood or environment.
10. Wherever possible, new additions or alterations to structures shall be done in such a manner that if such additions were to be removed in the future, the essential form and integrity of the structure would be unimpaired.
Resurfacing stone surfaces: Is it historically appropriate?
Is refinishing and/or aggressive grinding the stone floor of a classic building historically appropriate? If we abide strictly by the standards, the first responses would be “no,†but the standard concerning resurfacing is somewhat vague. Standard Number 7 listed above states, “Sandblasting and other cleaning methods that will damage the historic building materials shall not be undertaken.†That being said, we need to question whether or not resurfacing should be considered damaging. And in order to understand this matter, we need to take a look at the history of stone surfaces.
The resurfacing question proposed here applies to stone flooring only. All other stone surfaces, such as walls, columns, decorative features, etc., shall not be resurfaced. Other tile surfaces, such as ceramic, glass or porcelain, are not candidates for resurfacing. Any cleaning required for these surfaces can be accomplished by gentle cleaning methods, and under no condition shall these surfaces be refinished.
The history of stone floor installation
In the 1800s, when many of our historic commercial buildings were constructed, marble was the preferred flooring choice. The marble was typically installed in large slab pieces and set in a thick cement-based setting material. The thickness of these slabs varied considerably, and for this reason it was necessary to grind the floor flat to give the floor an even appearance. This was common practice and, in fact, was necessary. Today, stone flooring materials are calibrated so that all tiles are uniform in appearance, and hence do not require grinding.During installation, the marble flooring was treated with some very aggressive abrasives, which resulted in removal of a large amount of the stone surface. Many conservators and stone contractors believe that because the floor was originally ground, it is appropriate to grind it again for restoration. This is not the case. Once a marble or stone floor is ground, it should never be ground again. However, it may be appropriate to lightly resurface or hone the surface to remove any surface imperfections. The terminology has become confusing, though, and this is where many architects and designers get into trouble. “Grinding†is the removal of large amounts of stone material, whereas “honing†is a light resurfacing. To help understand the difference, it is necessary to know how the abrasives used during this process are sized.
All abrasives are graded based on how aggressive they are. Typically, the larger the particle size, the more aggressive the abrasive. To size abrasive particles, the abrasive powder is sifted through a screen with a predetermined number of holes in the mesh. The mesh is nothing more than a screen with a given number of holes. If the screen or mesh has 60 holes per square inch, then the grit or particle size is labeled a 60 grit. The more holes present, the finer the abrasive will be. The standard grit sizes are 16, 24, 36, 60, 120, 220, 400, 600, 800, 1800, 3500 and 8500. Typically, any grit up to a 60 is considered a grinding grit size. Grits from a 120 and up are considered honing grit sizes.
How much material is removed during honing?
Abrasive Technology, a manufacturer of tooling and accessories for stoneworking, conducted a study to determine how much material is removed during any grit size. Figure 1 shows a summary of this study:
As you can see, if a marble floor is lightly honed with a 400 grit, you are only removing .0008 inches of material. Most historic stone flooring is in excess of 1 inch thick. Therefore, it is unlikely that a light honing will remove any significant amount of material.
So, is it appropriate to resurface a marble or stone floor? The general rule, as outlined in The Secretary
of the Interior's Standards for Rehabilitation, is to use the gentlest means possible. For stone flooring, first try a mild cleaning with a good neutral cleaner first. If those results are not acceptable, the next step in the process should be to try an alkaline cleaner. Honing should be the last attempt for resurfacing an historic stone floor, and should be attempted only if other means do not achieve the desired results. If in doubt, it is always best to consult with a stone conservator.
A note on polishing historic stone
For many historic stone flooring surfaces, the material was actually installed with a honed finish. Any sheen or polish was achieved by waxing the floor with a natural wax such as beeswax. Today, the use of these waxes is not appropriate. However, it is acceptable to use a polymer type coating to emulate the wax finish if desired.End box
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Fred Hueston is founder and president of The National Training Center for Stone and Masonry Trades, a successful marble and stone consulting and training company. He is recognized as a nationally known consultant and has written over 28 books on the subject. If you have any questions for The Technical Forum, please e-mail Fred Hueston at fhueston@aol.com.
