by Chuck Safris
It is always a thrill to find a cracked concretion that separates cleanly in the field with a light tap of a rock hammer to reveal a beautiful fossil. More often, however, a promising looking concretion is not already cracked and resists being broken. When if finally breaks under the blows of a rock hammer, there is some shattering or uneven breaking which can damage the enclosed fossil. An even then, there is a chance that the enclosed fossil is not exposed. With a little patience, there is another way to expose the enclosed fossil. Artificial weathering is a simple alternative that may lead to the collection of fine, undamaged, fossils if the matrix material is suitable.
Freezing and thawing causes accelerated mechanical weathering of a rock. If water can seep into pores or micro cracks in the rock, the rock will become saturated. When the water expands as ice forms during a freezing cycle, pressure is exerted on the rock, leading to cracking or exfoliation. If the fossil is a carbon film, then the fossil is a natural weak spot in the concretion and with luck the subtle pressure of freezing will open the concretion so that the fossil is perfectly exposed and undamaged.
The process is very simple for any rock that will take up water. A container (other than glass, which might break during the freezing process) suitable for the specimen's size is selected and the concretion is covered with water and allowed to soak for several days. Then a series of freezing and thawing cycles are achieved by using the freezer in the summer or the back porch in the winter. By achieving a freeze thaw cycle every day, the process is accelerated. It is important that loosened residue from each cycle be removed and examined because if there are any fossils reveled, the next freeze cycle could destroy them.
If you live in a northern climate and there is no hurry, the suitable rocks could be placed in container full of water and simply left outdoors all winter where the daily temperature swings would do all the work. It has been reported that thousands of Mazon Creek (IL) fossils have been exposed using accelerated artificial weathering this way.
In Iowa, fossil collectors are on the lookout for blade shaped nodules of limey shale in Pennsylvanian exposures and stream beds. With any luck and some artificial weathering, a well preserved, beautiful fern frond is likely to join your fossil collection.
Reference: "Freezing and Thawing of Fossils"; J. Pojeta and M. Balanc, U. S. Geological Survey, Reston VA, Undated.
The reference material is printed in the book "Paleotechniques". The book is subtitled "The Paleontological Society Special Publication No. 4", 1989 and is edited by Rodney M. Feldmann, Ralph E. Chapman and Joseph T. Hannibal. It is published by the Department of Geological Sciences, The University of Tennessee, Knoxville, TN. Its ISBN number is 0-931377-04-2. The article appears on pages 223-226 and is not dated.
The article goes deeply into the use of various freezing agents, of which tap water is the one I wrote about.
Chuck Safris, Editor
Submitted: January 1999
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Last Revised on
October 17, 2011