Projects

The Origins and Remediation of Pad Marks on Milled Limestone

by John R. Hill and Margaret V. Ennis

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For many years, the dimension limestone industry in Indiana has been plagued by discolorations that appear unpredictably under pads used to separate palleted pieces of milled limestone. Such discolorations are of equal concern to architects and builders, some of whom have trepidations about the permanence of the markings. Certain types of padding materials, such as homosote, which is a dense fibrous material similar to Celotex, are particularly prone to forming pad marks. Exposure to moisture for prolonged periods has been another factor in the formation of these troublesome discolorations. Pad marks are known to disappear without remediation after exposure to rain and sun for periods that vary from less than one month to about one year.

Although the Geological Survey has collaborated with the Indiana Limestone Institute of America on a number of experimental efforts to determine the cause(s) of pad marks, the problem remains. This study used a variety of padding materials to separate rectangular prisms of Indiana Limestone milled to dimensions of two feet by four feet and stacked on wooden pallets just as they would be arranged on a job site. Our research goals include: (1) documentation of which padding materials produced pad marks, (2) time interval for pad marks to form, (3) photographically document pad mark formation, (4) determine the underlying causes for pad- mark formation, and (5) determine the time period necessary for exposure to sun and rain to erase the discolorations. Precipitation, temperature, and exposure to sunlight were documented over a period of one year.

The Bybee Stone Company provided 82 milled slabs of Indiana Limestone that were machined from standard gray, buff, and variegated stone quarried by several dimension-stone producers in the dimension-limestone belt of Indiana. Each of the rectangular-prism specimens was 2 feet by 4 feet by 2 inches in thickness and weighed approximately 75 pounds. The test samples were given sample numbers and stacked in sets of four on wooden pallets in the upper yard of the Bybee Stone Company property in Ellettsville. Padding material between the limestone slabs consisted of the following materials:

• industry standard homosote pads
• homosote pads separated from the limestone by a layer of plastic that covered the entire face of the stone
• homosote pads separated from the limestone by a layer of thin (5mm) Styrofoam ribbon
• 3/4-inch thick Styrofoam pads cut from construction grade Styrofoam sheeting
• Nylon spacers
• 3/4-inch thick Styrofoam pads separated from the stone face by a layer of plastic that covered the entire face
• pads of homosote and of Styrofoam set on top of a layer of limestone dust

The 82 limestone slabs were exposed to weathering, including sun, rain, snow, and dew, for a period of one year. Each of the stones was inspected for pad marks on an approximately three- month interval and photographs were taken of pad marks as they developed. A rain gauge was also installed, which was used to keep track of the total precipitation that fell at the test site.

Pad marks developed within two weeks of the project beginning (May 1999), some of which persisted through the end of the project (May 2000). The following summarizes the formation of pad marks:

• Pad marks formed in every case where homosote was in direct contact with the limestone prism surface.
• No pad marks formed under the homosote that was insulated from the stone face by a plastic sheet that covered the entire stone face. The same may be said for other pad materials that were placed atop a full plastic cover sheet.
• Negative pad marks (e.g., discoloration of the stone occurs around the pads, but not under them) formed under the nylon spacers, the plastic insulated homosote, the Styrofoam pads, and various combinations of Styrofoam ribbon and other padding material.
• Staining formed most aggressively on gray stone, irrespective of its origin and the date on which it was quarried.
• On variegated stone, pad marking was at least 50 percent more intense on the gray colored parts of the stone than on the buff regions.
• Limestone dust appears to have mitigated the initial effects of pad marking. Even standard homosote pads produced only faint pad marks when placed on a part of the limestone that had been thoroughly dusted with limestone powder.

While it is premature to state with certainty, it appears that pad marking is caused on stone enriched with interstitial organic matter by a shift from neutral pH (e.g., either alkaline or acid conditions seem to mobilize the organic compounds to form pad marks). Moisture is an essential component in the formation of pad marks, and humid conditions exacerbate the problem.

In April of 2000, the test samples were inspected a final time for pad marks and those specimens that were most notably discolored were segregated into two sets. One set of samples remained at the Bybee Mill, and individual pieces of the set were placed upright against a solid surface that faced either to the east or to the south. After approximately 60 days of exposure to bright sunlight, all traces of the pad marks disappeared on each of the individual test specimens.

The second set of pad-marked stone was moved to a fenced space on the Indiana University Bloomington Campus and individual samples were stood upright, but facing north. After 60 days of exposure to weathering without direct insolation, most of the pad marks had disappeared, but a mottled discoloration had formed on the north-facing part of four of the test specimens. Unusually wet conditions during the last month of the exposure period, coupled with basal contact of the samples with the ground surface, are the most probable causes for the mottled discoloration.

Exposure to weathering of pad-marked limestone that has been installed at a project site normally removes the pad marks within a few months. Stone exposed to direct sun light clears the most rapidly while stone on north-facing applications clears the most slowly.

On the basis of many years of observation by Geological Survey geologists who have studied the performance of Indiana Limestone as a building material, it is safe to say that normal discoloration of limestone facing stone, coping, and so forth that occurs on occasion during transport or during the period of site storage, disappears with time. The application of oxidizing reagents such as household bleach and hydrogen peroxide can, under some circumstances, accelerate the disappearance of stone discoloration.