Thursday, December 15, 2016

Density of Stone, Wood, Water and Ice

Stone wall, corner of Maple and Brooks Streets. Estimated
weight six tons. Click on photos to enlarge.
This website has density of >300 materials as kg/cubic meter and pounds per cubic foot:

Stone is heavy. Every stone mason who has ever blackened a fingernail knows this to be true. Granite weighs 165-170 pounds per cubic foot. One 2x3 foot piece of bluestone for a walkway, two inches thick, weighs 150 pounds. Filling a wheelbarrow with gravel will exceed the safe load capacity of the wheelbarrow. Tons of stone are needed for a not particularly long or tall stone wall. 

Steel is heavier. Steel weighs 490 pounds per cubic foot. Pieces of rail on old and abandoned railroads across New England are 13 yards long and weigh half a ton or more. Going price for scrap steel is roughly ten cents per pound. Silver, surprisingly, is not much less dense than lead. The two metals come in at 655 and 709 pounds per cubic foot, respectively.  In movies where silver is being cast into bullets (perhaps to shoot a werewolf?) the silver-colored molten metal is actually lead, which becomes liquid at 621 degrees Fahrenheit. Real silver melts at 1,763 degrees and glows red hot.

Twenty-four carat gold is 1,206 pounds per cubic foot. Standard-sized gold bars are a tad under 2x4x8 inches and weigh 27.4 pounds. A regulation baseball is 12.8 cubic inches – give or take a bit – and weighs 0.3 pounds. Granite carved to the same dimensions would be 1.3 pounds; steel 3.7 pounds; lead 5.3 pounds and gold 9.0 pounds. At a late November 2016 price of $38.80 per gram, that solid gold baseball would be worth about $160,000.

Wood floats, except when it does not. Water density is 62.4 pounds per cubic foot. Many dense-wood species of trees from around the world are called “ironwood.” In Spanish-speaking regions of South America, these trees are “quebracho,” derived from “quebrar hacha,” which translates as “axe-breaker.” Wood from quebracho, ebony, lignum vitae and other species can weigh as much as 80 pounds per cubic foot.

In Massachusetts, the tree species American Hornbeam is also called ironwood, but its density is approximately 48 pounds per cubic foot, so it floats. Density of wood in part determines how much heat is produced when burned. After sufficient air drying, softwood trees such as poplar, aspen and willow weigh 20 to 25 pounds per cubic foot. Ash, birch and elm are intermediate woods with weights of 35 to 40 pounds. The common hardwoods, which include apple, beech and oak, all exceed 40 pounds.

Cord for cord, the net heating value of oak or sugar maple will be nearly double that of poplar or aspen. An equal weight of coal will deliver nearly twice the heat of hardwoods. For all wood, the higher moisture content of green wood means less heat generated compared to wood properly air dried, the reason being that heat is lost vaporizing the water content. Splitting speeds the drying process.

Pure water weighs 1,000 kilograms per cubic meter. Converted to English units, that becomes 62.4 pounds per cubic foot. A gallon of water weighs 8.33 pounds; thus, the mnemonic "a pint weighs a pound," is close, but not spot-on. Sea water, because of the dissolved minerals, has a higher density of 1,026 kg per cubic meter. Ice from fresh water has a density of 919 kg per cubic meter. Dividing by 16.02 to get to pounds per cubic foot yields 57.3 pounds.  

Salt crystal melts surrounding snow.
Because of the salt content, ocean
water freezes at 28F. Water with a
higher salt content would have an
even lower freezing temperature.
As ice is only eight percent lighter than water, it would take an ice floe with the dimensions eight inches thick and 7x7 feet square to (barely) support a 160 pound person. Obviously, when walking on a frozen lake, it is the tensile strength of ice spreading the weight burden over a wide area rather than the floatation capacity of the ice directly underfoot that keeps you from plunging through. On fresh water, four inches of clear ice is considered the safe minimum for skating, walking or ice fishing.

Locally, winters average 40 to 50 inches of snow per year. The loose rule of thumb is ten inches of snow equates to one inch of rain. That puts snow at 100 kg per cubic meter, or 6.2 pounds per cubic foot. Wetter snow will be denser. Powdery snow will be much lighter. Roof melt that ends up on an unheated porch or garage roof can refreeze. In this way a flattish roof with an area of 20x20 feet could easily end up supporting two to three tons of dense snow and ice.

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