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Historic Preservation - Technical Procedures

Spectitle:
Sandstone: Characteristics, Uses And Problems

Procedure code:
0447001S

Source:
Developed For Hspg (Nps - Sero)

Division:
Masonry

Section:
Sandstone

Last Modified:
02/24/2012

Details:
Sandstone: Characteristics, Uses And Problems



SANDSTONE:  CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of sandstone and identifies typical problems
associated with the material along with common causes of its
deterioration.


INTRODUCTION

Characteristics of Sandstone:

-    A sedimentary rock consisting of sheets of stone deposited one
    atop another under water.  

-    Very porous and water will penetrate it easily.

-    Brown, red, purple, and pink sandstones are commonly called
    brownstone.

-    Available in a variety of surface textures and earth-toned
    colors.

-    Weathers best when its end-grain faces the weather (naturally
    bedded).  (Face-bedded) stone is subject to greater
    deterioration.  Water damages a face-bedded stone by spalling
    or flaking off entire sheets of sandstone.  Also, freeze/thaw
    cycles allow water to get into the stone and then freeze and
    expand causing some of the top layer to split off.

    NOTE:  In many 19th century applications, the grain was placed
    parallel to the weather side (face-bedded) for aesthetic
    reasons.  This was especially common around doorways.


TYPICAL USES

Typical historical uses for sandstone included:

-    Urban row houses, commercial buildings and churches built from
    the 1840s through the early 20th century (this was usually
    brownstone).

-    Lighter-colored sandstones were used more frequently by the
    end of the 19th century.

Typical current uses for sandstone include:

-    New sandstone is not widely available for use today.

-    The popularity of sandstone today is focused primarily on its
    decay and on how it can be repaired and preserved.


PROBLEMS AND DETERIORATION

Problems may be classified into two broad categories:  1) Natural
or inherent problems based on the characteristics of the material
and the conditions of the exposure, and 2) Vandalism and human-
induced problems.    

Although there is some overlap between the two categories, the
inherent material deterioration problems generally occur gradually
over long periods of time, at predictable rates and require
appropriate routine or preventive  maintenance to control.
Conversely, many human induced problems, (especially vandalism),
are random in occurrence; can produce catastrophic results; are
difficult to prevent, and require emergency action to mitigate.
Some human induced problems, however, are predictable and occur
routinely.  


NATURAL AND INHERENT PROBLEMS

1.   Moisture-related problems:  May be evident in sandstone as
    spalling, erosion, cracking, flaking and deteriorated mortar
    joints.

2.   Weathering:  Disintegration of the stone's surface usually
    caused by erosion, chemical action, and moisture freezing in
    the stone.

3.   Exfoliation:  Separation and loss of large areas of stone
    along the bedding planes usually caused by the stone having
    been face-bedded.

4.   Blind Exfoliation:  Separation of stone along bedding planes,
    but where layers are still loosely attached behind the
    surface.  It is often caused by having laid the stone with the
    bedding planes running parallel with surface of the wall
    (face-bedding).  Blind exfoliated stone will sound hollow when
    lightly tapped with a rubber mallet.

5.   Blistering:  Swelling and rupturing of a thin uniform skin
    caused by air-borne chemicals reacting with the stone's
    surface, forming a hard, brittle skin.  The blisters will
    often pop when touched.

6.   Cracking:  Narrow fractures in the stone from 1/16 to 1/2 inch
    wide.

7.   Detachment:  A clean break in the stone often resulting from
    a sharp impact, or from stresses concentrated in a small area
    of stone due to structural settlement.


HUMAN-INDUCED PROBLEMS:

1.   Stone laid with its layers parallel to the wall plane (face-
     bedded) rather than perpendicular to the wall plane
    (naturally-bedded):  Face-bedded stone is more prone to
    deterioration by weathering as entire sheets of stone tend to
    flake off.

2.   Painting over a deteriorated stone surface:  This only leads
    to more serious moisture-related problems.

3.   Applying a cement patch over deteriorated surface areas:  If
    an inappropriate patching mix is used, it may be necessary to
    paint the entire stone surface to lessen the visual disparity
    between the two materials.

                         END OF SECTION