With respect to all, graphite coating is a separate thing from glazing. The quickest reference I have is from William Knight (the "Mad Monk"), in the collected articles on the Laflin & Rand website (
http://www.laflinandrand.com/page3.htm ):
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Before launching into the subject of glazing it would be best to explain that glazing has nothing to do with the use of graphite applied to the powder grains.
After pressing, the grained powder will contain varying amounts of water. The amount may vary from 1 to 2%. If the powder grains are dried on trays they will form loose deposits of potassium nitrate crystals on the surfaces of the grains. Any water migrating to the surfaces of the grains will do so as a saturated solution of potassium nitrate.
Press densification imparts mechanical strength to the mass of powder. Keep in mind that 75%, by weight, of the mass is potassium nitrate. The mass being pressed is about 2% water. Wherever potassium nitrate crystals contact each other they will begin to fuse together. This fusing of contacting surfaces promotes hardness and mechanical strength.
By tumbling the grains, during drying, the crystals of potassium nitrate are compacted and fused into a thin shell, or skin, covering the surfaces of the powder grains. Under high magnification this thin skin, or shell, will give the appearance that the powder grain had been coated with glass. The appearance of the glass-like skin is where the term "glazed" powder originated. Large grains of powder will have a thicker glaze since the large grain sizes have a greater amount of mass relative to their surface area.
The thickness of the glaze formed on the grains will, in part, determine ease of ignition of individual grains of powder and govern flame spreading rates within a mass of powder grains. Heavy glazing slows the process of ignition and combustion of the powder. Heavy, or thick, glazing was used to slow powder charges behind heavy projectiles.
When high-purity potassium nitrate is used to fabricate the black powder the glaze imparts a degree of moisture resistance to the powder grains. Below 90% relative humidity the powder will be little effected by water vapor in the air. Above 90% R.H. the powder grains will pick up only trace amounts of moisture which will be quickly passed back to the air when the R.H. falls below 90%.
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and
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Graphite coatings on black powder.
There are two misconceptions regarding the function of graphite coatings on grains of black powder.
One common myth is that the small letter g behind the grain size designation indicates that the powder has been graphite coated. The small letter g stands for glazed powder. Glazing and graphite coating are two entirely different things.
When glazed grains of black powder, lacking a graphite coating, are stored for any length of time the mass of grains will begin to clump together. The surfaces of the grains are covered in a thin skin of potassium nitrate and charcoal minerals. These are crystalline materials and will behave as all crystalline materials do. Surfaces of the same salt in contact with even slight amounts of pressure will begin to fuse, or bond, together.
Old military sources describe how kegs of black powder were to be removed from their storage magazine periodically and rolled around on the ground. A little mechanical agitation would break up any clumping without causing damage to the grains.
Graphite coatings on the grains act as an “anti-blocking” agent, preventing the grain surfaces from fusing, or bonding, together. An old duPont Blasters’ Handbook states that graphite is used where a free flowing powder is desired.
Graphite coatings on grains of black powder do not provide any form of moisture protection when the powder is subjected to humid air. The graphite will only hide the effect of additional moisture pickup by the powder. Heavy graphite grain coatings are a way of hiding the effects of the use of an impure grade of potassium nitrate in a powder.
Some black powder plants add graphite to the powder after corning but prior to the screening of the powder. The graphite acting as a screening aid to increase the rate at which the grains will pass through the screens.
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There is, naturally, much more in these articles.
Regards,
Joel