Ptc Chalk
Floriana Grundy
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk is common throughout Western Europe, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as the Dover cliffs on the Kent coast of the English Channel.
Chalk is mined for use in industry, such as for quicklime, bricks and builder's putty, and in agriculture, for raising pH in soils with high acidity. It is also used for "blackboard chalk" for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals, or gypsum.
Chalk is a fine-textured, earthy type of limestone distinguished by its light colour, softness, and high porosity.[1][2] It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton, such as foraminifera or coccolithophores.[1] These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as molluscs, echinoderms, or bryozoans.[3][4][5]
Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent.[3] While it is similar in appearance to both gypsum and diatomite, chalk is identifiable by its hardness, fossil content, and its reaction to acid (it produces effervescence on contact).[5]
In Western Europe, chalk was formed in the Late Cretaceous Epoch and the early Palaeocene Epoch (between 100 and 61 million years ago).[6][7] It was deposited on extensive continental shelves at depths between 100 and 600 metres (330 and 1,970 ft), during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. The lack of nearby erosion explains the high purity of chalk. The coccolithophores, foraminifera, and other microscopic organisms from which the chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition, resulting in the early cementation of such limestones. In chalk, absence of this calcium carbonate conversion process prevented early cementation, which partially accounts for chalk's high porosity.[3] Chalk is also the only form of limestone that commonly shows signs of compaction.[8]
Flint (a type of chert) is very common as bands parallel to the bedding or as nodules in seams, or linings to fractures, embedded in chalk. It is probably derived from sponge spicules[4] or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).[9]
Chalk is so common in Cretaceous marine beds that the Cretaceous Period was named for these deposits. The name Cretaceous was derived from Latin creta, meaning chalk.[10] Some deposits of chalk were formed after the Cretaceous.[11]
The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage.[3] Some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Mns Klint in Denmark.[12]
Chalk deposits are also found in Cretaceous beds on other continents, such as the Austin Chalk,[13] Selma Group,[14] and Niobrara Formations of the North American interior.[15] Chalk is also found in western Egypt (Khoman Formation)[16] and western Australia (Miria Formation).[17]
Most people first encounter chalk in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacturers now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing world, use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as "dustless chalk".[20][5]
Coloured chalks, pastel chalks, and sidewalk chalk (shaped into larger sticks and often coloured), used to draw on sidewalks, streets, and driveways, are primarily made of gypsum rather than calcium carbonate chalk.[21]
Chalk and other forms of limestone may be used for their properties as a base.[23] Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water.[24] In agriculture, chalk is used for raising pH in soils with high acidity.[25] Small doses of chalk can also be used as an antacid.[26] Additionally, the small particles of chalk make it a substance ideal for cleaning and polishing. For example, toothpaste commonly contains small amounts of chalk, which serves as a mild abrasive.[27] Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.[28]
In southeast England, deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but perhaps the most famous is the extensive complex at Grimes Graves in Norfolk.[31]
Chalk may also be used as a house construction material instead of brick or wattle and daub: quarried chalk was cut into blocks and used as ashlar, or loose chalk was rammed into blocks and laid in mortar.[34][35] There are still houses standing which have been constructed using chalk as the main building material. Most are pre-Victorian though a few are more recent.[36]
The CHALKSPRAY is a pigmented chalk paint that is ideal for outdoor marking (temporary only on non porous surfaces) as well as for indoor studio artwork. Available in 10 powerful color shades that are deep matte and high covering. Montana CHALKSPRAY can be applied on various surfaces. Depending on weather conditions and climate outdoor marking lasts between several weeks to several months. It fades faster during extended periods of rain or in high traffic areas. On non-porous surfaces like glass or metal the pigments can be removed easily. Removal from porous surfaces might be difficult. Trial application and removal on a non-visible section of substrate is recommend when working on a non-porous surface. Montana CHALKSPRAY impresses by its feel and look - the typical rough chalk effect.
Depending on weather conditions and climate outdoor marking lasts between several weeks to several months. Montana CHALKSPRAY works also well on canvas, cardboard or other primed grounds due to its great coverage. The CHALKSPRAY will become permanent when sealed with Varnish (when used on a canvas i.e.). It also allows further application by brush, scraper or other tools. For clean and precise lines we recommend to use Montana LeveL caps 3 to 6 (3 and 4 for fine and medium lines, 5 and 6 for wide lines and filling large areas).
After my last chalk paint project, I have to admit that I took a little break from chalk painting, despite the fact that at any given time there are probably about a hundred things in my house I'd like to chalk paint. Yesterday, I might have even been eyeing the guest bath vanity. Keep in mind that our house is only 1.5 years old so nothing is really in need of refinishing...yet. Someone please slap me. Anyway, I finally picked up my brush again a few days ago and refinished a piece that I have contemplated several times selling or sending to the curb with a sign that says "Take Me." It's one of those floater pieces that gets moved to nearly every room in the house but never quite seems to work anywhere. We had a teeny breakfast room in our last house, and it was a perfect fit there, but it just wasn't landing so well in our new house...that is until I tried it out in our guest room, and it totally worked! Yay! The only problem is that our guest room is light and airy, and the piece I'm talking about was black and heavy looking. Color-wise, it just wasn't a good fit for the space. My solution, chalk paint. It's one of the best ways I know to quickly and relatively easily give a whole new look to a piece of furniture. I'm by no means an expert on chalk painting, but I have learned a few things by doing it, so I thought I would share some of my tips with you today to help get you started, especially if this is something you've never done before. There are so many brands of chalk paint to choose from, but Annie Sloan is my favorite, so these tips have her paint in mind, but they should work for most brands of chalk paint. Okay, grab that cup of coffee. Here we go!
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