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Hartz Stone Blog - Winter And Frost Action On Stone And The Evaluation Of The Soundness Of Stone
Winter And Frost Action On Stone And The Evaluation Of The Soundness Of Stone 
The decay of rock and stone has been well known to conservators and architects of historical buildings and monuments since the days of the Roman Empire. We can understand most phases of physical and chemical weathering as a result of minerals and rocks attempting to reach equilibrium under conditions at the earth's surface. And today there is not much known of the time required for these processes to take place in the complex environment of the earth's surfaces.

The frost action on stone in those moderate humid climates has long been known as a disruptive factor which deserves our attention. The action of frost results from a combination of factors, such as volumetric expansion from the water to the ice phase, the degree of water saturation of the pores, the critical pore size distribution, and the continuity of the pore system.

The danger to stone by frost action depends on the pores size distribution, the relative humidity (RH), the water saturation, and the possible presence of salts, especially Na2SO4. Ice increases its hardness with decreasing temperature from MH = 1.5 (Mohs Hardness) at 0 degree C to MH = 6 at -60 degree C, which is the hardness of granite. The volume increase of water from 4 degree C, the densest point, upward and down toward freezing appears to have some influence in confined capillaries.

Specimens of stone soaked continuously before freezing are more susceptible to decay than specimens which are soaked and subsequently dried at 75 degrees C and 50% RH. The sensitivity of quarry-moist stone blocks is therefore not surprising. The curing of such blocks has been practiced since Roman times. It was well known to the architect, Christopher Wren, who cured blocks of Portland stone on the beaches of England before use.

The following is a partial list of observations of Physical damage during winter exposure or after cold exposure of stone to the elements of weather, usually cold;

1) Cracks, in sandstones, marbles, and granites, caused by stress relief and uneven loading of a building.

2) Scaling and Flaking, in all rock types, by hygric action, frost or salts.

3) Surface crumbling, in sandstones, some granites, and marbles. Detection of traces of efflorescent salts.

4) Porosity, changes due to weathering, transport of grain cement, effectiveness of consolidants or sealer.

5) Ultrasound testing for quality of stone; data of weathered stone should be compared with quarryfresh material, dry and water soaked; ultrasound tests may replace unsightly test drilling in many instances.

6) Moisture testing: approximate moisture content in masonry and stone can be tested with several types of instruments in the field. The method is limited; more precision is required to determine minor quantities absorbed from high relative humidity.

With the experience of most stone restorers, conservators, and preservationists; especially in these economic times, it is often more attractive to restore rather than replace. Our experience is that if the stone only had more care and maintenance or proper restoration, these great stones can be cared for properly and preserved. Since our company serves the New York Metro area, we especially see many old stone buildings with stone that is in advanced degenerative phases, inside and outside of the buildings. Working mechanically with these stones can often cure the problems, refine the finish, and preserve the stones.

All of the cities in this country has as its' lifeblood, its landmarks - its historic neighborhoods, its incomparable buildings, its distinctive streets. They hold the history of this nation built on democracy and capitalism. Histories so important to embrace, protect, and learn from. Like our Constitution, this is something to preserve. Like stone, a reminder of what was, what is, and what can be preserved and not destroyed. It is times like these that we restore. Given the chance, "yes, we can." The old does have an enduring significance. Until next time, "hold onto your stones" - build or maintain and restore, and do not destroy what is good.

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