This specimen is from the geological collection of the City Museum of Berlin, and the image is used under a Creative Commons license. Click to enlarge. The keys to identifying chalk are its hardness , its fossil content, and its acid reaction. At a glance, diatomite and gypsum rock have a similar appearance. An examination with a hand lens will often reveal the fossil content, separating it from gypsum.
The acid reaction will surprise you if you are used to testing other types of limestone and have never tested chalk. When you apply a drop of acid, capillary action pulls it deep into pore spaces of the specimen.
There, the enormous surface area of calcium carbonate that contacts the drop of acid usually produces a spectacular effervescence. Instead of holding the specimen in your hand during the test, place it on surface that will not be damaged by the acid, with a couple paper towels beneath it. Fields are shown in yellow, well locations are shown in green and red. Image by the United States Geological Survey. At a microscopic level, there can be a lot of space between the fossil particles that make up chalk.
Land underlain by chalk directly below the soil is often well drained. In these areas, water that infiltrates into the soil encounters the top of the chalk and easily flows into the chalk's pore spaces.
It then flows downward to the water table and then follows the direction of groundwater flow to a stream or another body of surface water.
In some areas, people drill water wells into subsurface chalk layers for residential, commercial, and community water supplies. In areas where oil and natural gas form in the subsurface, the pore spaces of chalk can serve as a reservoir. Many oil and gas fields are located where subsurface chalk units serve as reservoirs.
It yields oil and natural gas from both conventional and continuous reservoirs. Small pieces of chalk have been used by students for over years for writing on small slates and large classroom panels known as "blackboards".
It is an inexpensive and erasable writing material and the most widely known use of chalk. Much of the early blackboard writing was done with pieces of natural chalk or natural gypsum.
Today pieces of natural chalk and natural gypsum have been replaced by sticks manufactured from natural chalk; sticks manufactured using other sources of calcium carbonate; or sticks manufactured using natural gypsum. Gypsum chalk is the softest and writes smoothest; however, it produces more dust than calcium carbonate chalk.
Yellow became the preferred color for chalk. Almost all chalk produced today is dustless. Earlier, softer chalk tended to produce a cloud of dust that some feared might contribute to respiratory problems. Dustless chalk still produces dust; it's just that the dust settles faster. Manufacturers accomplish this by baking their chalk longer to harden it more. Another method, used by a French company, is to dip eighty percent of each dustless chalk stick in shellac to prevent the chalk from rubbing off onto the hands.
The main component of chalk is calcium carbonate CaCO 3 , a form of limestone. Limestone deposits develop as coccoliths minute calcareous plates created by the decomposition of plankton skeletons accumulate, forming sedimentary layers. Plankton, a tiny marine organism, concentrates the calcium found naturally in seawater from. To make chalk, limestone is first quarried, generally by an open pit quarry method.
Next, the limestone must be crushed. Primary crushing, such as in a jaw crusher, breaks down large boulders; secondary crushing pulverizes smaller chunks into pebbles.
The limestone is then wet-milled with water in a ball mill—a rotating steel drum with steel balls inside to further pulverize the chalk. This step washes away impurities and leaves a fine powder.
The base of pastel chalks is calcium sulfate CaSO 4 , which is derived from gypsum CaSO 4 -2H 2 O , an evaporite mineral formed by the deposition of ocean brine; it also occurs disseminated in limestone.
Chalk and dehydrated gypsum thus have similar origins and properties. Pastels also contain clays and oils for binding, and strong pigments. This mixture produces sticks that write smoothly without smearing and draw better on paper than on chalkboards. Although great care is taken to eliminate contaminants when chalk is manufactured, some impurities inherent to the mineral remain.
Chief among these are silica, alumina, iron, phosphorus, and sulfur. In less significant, amounts, manganese, copper, titanium, sodium oxide, potassium oxide, fluorine, arsenic, and strontium may also occur.
After grinding, the chalk particles are sifted over vibrating screens to separate the finer particles. The particles are then mixed with water, extruded through a die of the proper size, and cut to the proper length.
Finally, the chalk is cured in an oven for four days. Chalk that is intended for the classroom must undergo stringent tests in order to perform well and be labeled nontoxic. All incoming materials are tested for purity before being used.
After the chalk has been made into sticks, one stick from each batch is selected for tests. The density and break strength of the sample stick are determined.
The sample is then used to write with, and the quality of the mark is studied. Erasability is also studied. First, the chalk mark is erased using a dry eraser, and the quality of erasure is examined. Then, the chalkboard is washed, and again the amount of chalk left on the board is examined. Furthermore, a sample from each batch is kept for five years so that it can be inspected if in the future its quality is questioned.
Chalk for classroom use adheres to the American National Standards Institute performance standards. Written specifications state the proper length of the chalk stick, as well as how many sticks should go in a box. Moist chalk sucks because it's lost the ability to absorb moisture. Once chalk picks up the moisture in the air, it becomes less effective.
Now that you know what to look for in great chalk let's address how Spider Chalk approaches making superior grip products. Good chalk costs more than bad chalk, of course.
But if you use chalk often then you have to consider if the value makes sense. For instance, if you find yourself using less chalk because you don't want to decide between buying food or chalk this month, then you're paying too much and sacrificing your performance in the process.
Particle size determines the feel of the chalk. Small particle size rubs into the hand easier but has a slick feeling. Large particle size feels rougher but doesn't stick as well. Our particle size is unique in that Spider Chalk powder and therefore our blocks and liquid has a combination of large, medium and small particle size.
That unique combination ensures rough friction with the ability to rub into the grooves of the hand. We use industrial dehydrators to keep our chalk from absorbing moisture. Spider Chalk produces almost completely anhydrous magnesium carbonate with ultra-low moisture content. There's a tendency for chalk companies to cut corners by adding drying agents like Mag Sulfate or Silica to absorb the moisture and make the chalk feel dry even in high-humidity environments.
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