Tuesday, September 12, 2017

Grist and Using Ashenhurst’s Rule

During a recent spinning gathering, I showed my grist weighing setup and several of us discussed grist and how to relate it to diameter. One lady, an excellent and very experienced spinner, knitter and weaver, mentioned Ashenhurst’s Rule.  Peggy Osterkamp has a nice explanation of it on her blog.  I also read a few scholarly articles about it.

Ashenhurst’s Rule computes a number (A) that is the maximum sett possible from a yarn of certain grist.  It is supposedly a diameter, but no weaver could ever weave something this dense. The best analogy I can think of is absolute zero temperature – when all motion, even atomic motion, stops.  It cannot be achieved, but is it the theoretical maximum cold.

The basic Ashenhurst formula is:

For firm yarns, A = .9*√(YPP)
For woolen yarns, use .86 instead of .9

This is basically the same formula Alden Amos provided in his Big Book of Handspinning, when discussing grist but he doesn't call it Ashenhurst's Rule.  However, later in the book, page 254, he does discuss it.  Since I am not a weaver, I sort of skipped this section and thus failed to connect this rule to spinning.  The original paper by Ashenhurst is listed in his bibliography.

Since this is a theoretical minimum diameter, you still have to adjust this number to get a reasonable diameter for that amount of grist.  In other words, you still have to multiply A by another factor.  This is a sticking point, because few agree on what that factor should be.

Yarn diameter is commonly measured 2 ways: through wrapping or compared against a control card with bars of known thickness.  Both are fairly unreliable measures and very subject to how you measure and perceive things.  While that is true for all measuring devices, measuring WPI is particularly flaky.  Some measure WPI by wrapping the yarn fairly loosely and then shoving the strands together until they just touch.  Alden Amos recommends wrapping the yarn fairly snugly (but not to the point that the yarn flattens) and then shoving the strands together tightly (but not overlapping), known as “pack to refusal”.  The difference between these 2 methods is big.  Measuring against a control card has the problem of laying something round on top of something flat and trying to determine when the 2 diameters match.  Any variation in the angle you are viewing it can change the measurement.  The WPI I get using a card is often one size larger than when using wraps.  Being off one size results in about a 246 yard per pound error - so you need to take some care here.

The real goal is to come up with something that is consistent for you.  In other words, you have to calibrate yourself so that the results are consistent enough for you to achieve the diameter you want.
I got out 10 commercial yarns and 10 of my handspun yarns.  I tried to get a range of diameter sizes of both.  I then measured the grist of each using my mg scale and a centimeter tape.  Since elasticity of yarns varies, I decided to average the lengths gotten when the yarns were stretched and when they were just barely relaxed (straight with no kinking).  I entered all the data on a spreadsheet and did some calculations.

1)  0.cm/0.xmg = m/g * 496.5 = YPP
2)  A was calculated using the 0.9 factor throughout.
3)  I measured the WPI both by wrapping and using the control card.
4) I calculated the ratio of measured WPI to A.
5) I averaged the ratios for the commercial, for the handspun and for all samples.
6) I ran the stats to determine the variability of my samples, which was low enough to be dependable – in other words, I had measured enough yarns to get a good answer. The only yarn I threw out was the acrylic.

The commercial yarns average ratio was 0.55 and the handspun .59.  The higher the ratio, the closer the actual WPI is to A.  This means my handspun is thinner for the same grist as the commercial.  Something I already knew, but now I had a measure of how much thinner.

I am not trying to recreate commercial yarn, because often it is underspun and has its issues, like pilling and other stuff.  But I am trying to better manage both grist and diameter to get a yarn to meet a specific need.  The problem is that I, like most spinners, use twist to get the diameter I want rather using grist and spinning a diameter appropriate to it.  The analogy here might be using a corset to get a small waist, rather than diet and exercise.

Using 0.55 as a reasonable WPI/A ratio, I came up with a guiding table – mind you, this is for me.  Your ratio might be different.

I am now trying to use all this info and see if I can create a yarn with some specific characteristics.  I am aiming for 4 m/g (1986 YPP) and I want the WPI to be around 20 to 21 WPI (my WPI is a bit higher than what the Yarn Council deems as fingering weight, because of how I measure.) This is to be a 2 ply and therefore my singles need to be 1.5 times as fine as the 2 ply, or 30 to 32 WPI.
I took a few grams of the fiber (a dyed, very neppy Targhee) and spun what looked like the right diameter.  I then took a plyback sample.  

 From the get go, I realized I needed a bit more twist to keep this stuff from falling apart.  Then I measured the grist.  First one was too light (5.2m/g), the second one was a bit too heavy, 3.65m/g. On my third try it was just right:  4m/g.  I pulled off some of the singles immediately prior to this sample and wound them stretched onto a card.  This is what the right size should look like while drafting. I measured them to be about 35 WPI.  I also attached the plyback to this card and made some notes.  I will now spin using the card as my guide, but measuring the WPI along the way as well as doing a grist measurement every now and then.  I don’t want to do too many grist samples, as the samples will not become part of the yarn (aka, destructive sample).  

I created a little WPI tool for singles that seems to work well.  Rather than winding to a known width and counting wraps, I created bars that 5 wraps (10 for really thin yarns) should cover at specific WPI.  It is quick and easy to use on the fly and doesn’t require one to pull out a long length of singles.  I have compared the results I get with this to regular wrapping and it is usually right on.
The amount of twist I am putting into the singles is enough to create a pleasing looking plyback that is close to the WPI I want. I will adjust the final diameter using the ply twist, bearing in mind that this yarn will likely bloom a bit.

Tool to determine WPI of singles, based on 5 wraps, except 14WPI, based on 3.
 The backside has even finer WPI.

Will I always try to achieve that .55 ratio?  No.  I can see wanting a higher ratio for something like sock yarn - higher grist and tighter twist to create firm, dense, wear resistant yarns.  I might shoot for a lower ratio for something airy that has a long enough staple to support less twist.  But now I have the measurement tools and knowledge in place to start understanding and learning how to make specific yarns.  (I hope!)

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