Kraft paper
From PyroGuide
Kraft paper is the name for common brown paper of the type many shopping bags are made. This thick but inexpensive material is the basis for most fireworks articles. It comes in large rolls and various thickness and can also be either virgin or recycled.
Kraft paper weights and thicknesses
| Paper weight #(lbs): | approx. Paper weight (gsm): | Thickness (inches) | approx. Thickness (mm) |
| 30 | 51 | 0.003 | 0.08 |
| 40 | 68 | 0.004 | 0.10 |
| 50 | 85 | 0.005 | 0.13 |
| 60 | 102 | 0.006 | 0.15 |
| 70 | 119 | 0.007 | 0.18 |
| 80 | 136 | 0.008 | 0.20 |
| 90 | 153 | 0.009 | 0.23 |
| Conversion factor -->gsm: | Conversion factor -->lbs: | ||
| 1.695 | 0.59 |
Explanation:
Generally paper thickness is described by giving the weight (in pounds) of a ream of paper. Unfortunately the basis sizes for a ream vary with the paper type, which means that two different kinds of paper can differ from each other in thickness, even if the same lb value is given on the packaging. A ream of kraft paper consists of 480 sheets of 24 inch by 36 inch (basis size). Accidentally there is a relationship between the ream weight and the thickness (in inch) of a given kraft paper: The latter is the former divided by 10000. This is only a rule of thumb though, because two krafts can show differently dense material.
Most pyrotechnic sources refer to kraft paper thickness giving the pound values. Shell makers and other fireworks people frequently talk about e.g. "a number seventy kraft paper" (#70 kraft). Anything heavier than the #70 seems to be both rarely used and hard to find. (Some types of protective paper for use when painting are actually about a #60, and some other extra tough ones employed for the same use can even be #110.)
However, the lb-inch system is popular in the english-speaking part of the world only. Given the basis size of a ream of kraft and the lb-value, one can easily calculate gsm (grams per square meter) and vice versa. The paper weight in gsm turns out to be approx. 1,695 times the lb-value. Thus, when knowing gsm the paper weight in lbs is about 0,59 times the gsm-value. If we do not know lbs nor gsm when buying a paper, we can roughly determine its thickness by measuring with a digital caliper, only slightly touching the paper´s edge.
Theory:
For everyone interested, here are the steps for calculating gsm when knowing lbs (hopefully I did this properly):
An easy calculation shows that a ream of kraft consists of 267,5607552 sqm of paper (480*24*36*2,54²/10000); this is a constant. If we multiply the given lb-value with (0,453592*1000) we get the total ream weight in grams, which we divide by the ream constant to get gsm. As the factors for the conversion lbs-grams are constants too, gsm turn out to be 1,695286 times the lb-value of our kraft.