Tuesday, 13 May 2008

Yarn calculations

Yesterday I wanted to plan a yarn order. There are two particular projects I have in mind. The first is to weave a twill sample blanket, following instructions in Janet Philip's new book, Designing Woven Fabrics.

This requires 6/2 and 12/2 cotton yarns. Most helpfully she gives weights of yarn required in her book. I am going to alter the colours slightly. The two main colours she uses are white and navy. I have done a lot of samples in white and navy recently, so I have chosen to work in primrose yellow and royal blue instead. This still gives good strong contrast to show the weave patterns.

Project two is a lace weave cloth in mercerised yellow cotton for my sister's dressing table. She has a beautiful Victorian chest, french polished, and is concerned that it might be scratched by the bits and pieces she keeps on top of it. Yellow is far and away her favourite colour, and will go well with the yellow-orange-red colour range of the patchwork quilt I made for her wedding.

For project two, I needed to do calculations to find out how much yarn I need. This is something new for me. I went to Janet Philip's first book, The Weaver's Book of Fabric Design, pub. Batsford in 1983. She gives equations for warp and weft calculations.

The amount of yarn needed for warp is the ends per centimetre x width in centimetres x length in metres x Tex number of the yarn, divided by 1000.

What is the Tex of my yarn? This information is not given by my supplier. I know it is 12/2, what exactly does 12/2 mean, does it help for working out Tex? I found a definition of Tex: the weight in grammes of 1 kilometre of yarn.

This looked like a very difficult task. It took me some time, and plenty of digging around my books and files for information. Along the way I found various bits of information I thought needed keeping together. I started a new folder for yarn information, including things like sett ranges for different yarns, reed sley charts (I found I had 4 different ones!), notes on yarn counts and project planning sheets. It's a bright red folder so hopefully easy to find when I need it.

I discovered that my 12/2 cotton yarn means that the yarn is 2 ply, and the 2 single yarns are both of cotton count 12. This seems to be an imperial measurement. I found in notes from another weaver that the cotton count is the number of hanks of yarn of 840 yards which weigh 1lb.

The yarn count didn't seem to help with Tex. However, then I found notes I made from the SI/Metric website. The purpose of this website is to assist with Standard Imperial to metric conversions. There isn't much on yarns, but I found in my notes I had put information on yards per pound / tex together with an article by Rosemary Brock from her website called "Amount of Twist". The yards per pound of 12/2 cotton - 5040 from SImetric gives a tex of 98 in Rosemary's chart (for which she gives credit to Peter and Jacquie Teal).

I decided to check this against my own yarn. I measured out 10 metres of yarn and weighed it on my scales (which are accurate to 0.01g). The weight was 1.1g. This would be 110g of yarn per kilometre - Tex 110. That's reasonably close to 98, my yarn must be slightly fine for it's count.

So, now I could make the calculations. I just had to convert ends and picks per inch to ends and picks per centimetre. The sample I wove that I want to base the cloth on was at 24 epi. I recalled that an inch is about 2.4cm, so decided to work with 10 ends per centimetre and picks per centimetre.

This was taking a fairly long time to work through, but next time I will know what I'm doing.

I multiplied: 10 ends per centimetre x 46 cm wide x 1.2 c.m. long x Tex 110, divide by 1000. I got 54g. Plus 54g for the weft calculation. Only 128g? I was surprised. Then I started to think about loom waste. I realised that my loom needs 1.5m for loom waste (that's slightly generous) and I want some extra for sample length in case I need to make changes or adjustments before weaving the piece. Say, 2m extra. Well, that about doubles the yarn needed. I got my boyfriend to check all my calculations for me. He did, and then pointed out that weaving on a loom like mine is a production method - it's for producing large amounts of cloth. I'd better think again and weave more than one small piece of cloth! After all, she might need at least two so she can wash one from time to time.

Now I can order some yarn.

5 comments:

Peg in South Carolina said...

aaaak! Is the English system really that difficult? Guess I'm just not used to it. I use Asenhurst's formula, found in chapter 13 of Osterkamp's first volume, but that is all based on pounds and yards. And of course sett in the U.S. is not metric either.

Dorothy said...

What makes it difficult is that in spite of government directives most people use both metric and imperial. When I started school everything was in imperial measurements. I'd just got the hang of measuring things when all of a sudden we went metric... or did we? I just ordered yarn, the spools weigh 225g, that's the same as one pound.

Dorothy said...

Oops, got that wrong (again!) 225g is a half pound, it's the 450g cones that are one pound. Of course prices are given per kilo.

Leigh said...

I think Peter and Jacqui put some good information on this on the Online Guild Yahoo files. I'm not exactly sure where it is, but I remember something about it awhile back.

Anyway, I hope you got it all figured out correctly!

Amanda said...

Hi Dorothy,
I am studying weaving in Melb, Australia and we just went through the whole thing of calculating the TEX today in class!!!
BTW, one inch is 2.54cm