The top row was made with preshrunk Premier Home, 85% cotton yarn, and the bottom row was made with the same yarn, but unshrunk. Hook sizes for the columns were G, H, and I, in order of increasing size. The coasters were washed in hot water and dried in a hot dryer. Before and after this washing, the coasters in each row were weighed in ounces on my kitchen scale, and the average diameter was taken.
- The preshunk row before washing weighed 0.60 ounces, and had an average diameter of 3.62 inches. After washing it weighed 0.55 ounces, and had an average diameter of 3.27 inches.
- The unshrunk row before washing weighed 0.55 ounces, and had an average diameter of 3.31 inches. After washing it weighed 0.55 ounces, and had an average diameter of 3.31 inches.
After washing the preshrunk coasters felt a bit softer and thicker.
Thus the weight of the pre-shrunk coasters may have gone down a little with washing; this was not noticeable in the unwashed coasters. The average diameter of the unwashed coasters declined 0.35 inches, as opposed to the decline of 0.17 inches in the unshrunk group. So the preshrunk coasters had a 2 fold greater shrinkage compared to the unshrunk coasters, according to the diameter. This was 9.6 % by diameter as opposed to a 4.9% decline in diameter for the coasters made with the unshrunk yarn.
The area of the coasters is proportional to the radius squared, percentage is a proportion as well, and the radius is directly proportional to the diameter in all cases , to compare the change in area associated with washing and drying the different sets of coasters, I squared the respective diameters.
The results came out:
The preshrunk yarn coasters started out with an 8.2% larger area than the unshrunk yarn coasters before washing.
The preshrunk yarn coasters came out to have a 2.4 % smaller area than the unshrunk yarn coasters after washing.
The preshrunk yarn coasters came out to have an 18.4% smaller area after washing than before washing
The unshrunk yarn coasters had a 9.5 % smaller area after washing than before washing.
It surprised me that the preshrunk yarn shrank with respect to itself more than the unshrunk yarn shrank with respect to itself after washing and hot drying.
Did the preshrunk coasters actually get thicker after washing? To test this, I stacked the two rows.
The coasters on the left were unshrunk, and the coasters on the right were preshrunk. I positioned them on each side of a line in the table parallel to the camera, so I can use the image to measure them on the source photograph. On two different print outs of this photo, I determined the thickness of each coaster stack at the inner edge, close to the middle near the camera and at the out edge, and I added them up, took the difference and related it to the sum of the unshrunk height. The results were:
- The sum of the six preshrunk yarn coaster height determinations was 7.812 inches
- The sum of the six unshrunk yarn coaster height determinations was 7.562 inches
The difference of these sums is 0.25 inches, which means that the preshrunk yarn coasters are about 3.3% thicker than the unshrunk yarn coasters.
Thus, preshrinking the yarn made the washed coasters 2.4 % smaller area, and 3.3% thicker. These small measures are equal, considering the uncertainties of all these direct and indirect measurements. Thus the smaller area is probably due to an increase in thickness, caused by the prewashing. The volume of the yarn is conserved (area times height). This 85% cotton yarn is quite durable, with little of it washing out. It makes little difference if we prewash the yarn or not, and it is a lot of extra work.
With harsh washing and drying the unshrunk yarn resulted in coasters that shrank in :
- Diameter by about 5%
- Area by about 9.5%
These results are internally consistent.