Understanding Kraft Paper Grain
What is Kraft paper grain, and why is it important? One of the most important aspects of working with Kraft paper when pasting shells, rolling tubes, or making leaders is being able to determine the "grain" of the Kraft paper. Kraft paper grain has characteristics similar to wood grain, and in the same way that a furniture maker needs to understand the properties of the wood he is using, the firework maker must understand the properties of Kraft paper grain so that he can use his Kraft paper to his best advantage.
When Kraft paper is manufactured, the individual fibers of the Kraft paper pulp tend to orient themselves in lines or rows. A good way to visualize the structure is to look at or imagine the bamboo place mats or window blinds, in which thin strips of bamboo are laced together with fine string to form a material which can only be rolled up in one direction. It is easy to see that if one tried to roll this mat around a mandrel or tube that there would be only one way in which this could be accomplished without bending or breaking the bamboo strips. With Kraft paper, this would be referred to as rolling "with the grain" as opposed to rolling "against the grain". Kraft paper, especially heavier weight Kraft paper and thin cardboard, exhibits this same characteristic, and the direction of the fibers should be noted for several reasons before beginning to utilize the Kraft paper on a project.
First, as was noted with the bamboo mat, Kraft paper and cardboard are distinctly easier to roll around a mandrel when making tubes, cylinder shell liners, or quickmatch leaders for instance, if the material is oriented to roll with the grain. Secondly, Kraft paper, like wood, expands when wet and contracts when dry. Again, the fiber structure explains why this happens. Imagine the bamboo mat, with its strips of bamboo rather loosely held together. If something like sand were poured on the surface of the mat and forced or pressed into the spaces between the strips, the mat would get slightly wider but not longer. With Kraft paper, cardboard or wood, the same thing happens on a molecular scale when water is applied to the material. The Kraft paper will swell, but mainly in one direction. This property of Kraft paper grain is very useful when pasting shells, especially cylinder shells, where larger sheets of Kraft paper are used. The Kraft paper is first pasted overall, causing it to expand, after which it is rolled with the grain around the body of the shell. As it dries the Kraft paper shrinks down to fit very tightly around the shell, making a hard smooth surface. If the Kraft paper is rolled against the grain, the shrinkage will not be nearly as great and the final product will not have the same smooth, tight appearance.
Several methods are available to determine Kraft paper grain: folding, tearing or wetting. For heavy Kraft papers and cardboard, merely folding and creasing the material first in one direction parallel to an edge and then making a fold perpendicular to the first fold works well. The fold on the grain of the Kraft paper (which will be parallel to the direction of the fibers) will be easier to make and will have a sharper appearance. Tearing is another quick method that works on either thin or thick Kraft papers. Tearing a line down the length of the fibers produces a straighter separation and is somewhat easier to do than tearing across the fibers. The best method however is by taking a small square of the Kraft paper and quickly wetting one side only. As the water starts to expand the fibers on that side, the Kraft paper will begin to curl and hump up. The curl will clearly indicate the proper orientation the paper needs to be in for rolling tubes, leaders, shell casings, or anything else in which the grain direction is important.
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