Every one of us, crocheters and knitters alike, have found the perfect pattern, and then had the sinking feeling that comes with realising you can't get the yarn. It's discontinued. You're allergic to the fibre content. It's only available in New Zealand.
People come into the shop all the time with exactly this dilemma. So here's how to find a good substitute yarn.
The first thing to sort out is the weight of the yarn. In yarn-shop terminology, this refers to the thickness of the yarn (not what it weighs, as we'll see). In principle, DK weight yarns will happily sub for each other, so if your pattern specifies DK, that's the place to start. If it wants Aran, then sub with Aran.
If you're making something that needs to come out a particular size, then the size of the stitches matters just as much when you're using a different yarn, so you need to swatch. So buy a single ball of your intended alternate, and check that you're getting the pattern tension. But swatching tells you far more than just your tension. You can be reasonably sure that the pattern designer has chosen a yarn that suits the project, but if you're subbing, you need to consider the fabric a little more carefully.
Not all fibres behave the same - for example, wool is elastic and resilient, cotton is inelastic and tends to grow downwards with gravity. For that reason, it's usually not a good idea to use cotton for a pattern written for wool - the designer will have taken the elasticity of the fibre into account, and using one with little stretch but lots of sag won't work. If your heart's set on cotton, much better to search out a pattern that was intended for it. In the same way, if you want to make an slinky evening top with gorgeous drape, then a pure wool DK may not be the best choice.
Your swatch will also tell you whether that yarn will work with your intended pattern - if it's very soft and fluffy, it mightn't show off the intricate cables you've planned. If it's very smooth and tightly plied, perhaps you don't want to make a diaphanous evening shrug from it.
To give an idea of how much yarns of one designated weight can vary, have a look at the picture up above. They're all DK, but they're very different. As you can see from the shape of the balls, the fibres behave very differently - Louisa Harding Merletto
on the extreme left is smooth and drapy compared to the floofy Sublime Lustrous
at top right.
These two balls of yarn illustrate our final point very well. In patterns, yarn is typically described by weight (in the non-yarn-shop sense): you're told to get ten 50g balls for your jumper. So it's easy to assume that you need to get ten balls of your substitute yarn. That can go very badly wrong indeed.
Because different yarns, even of the same fibre content, can be spun very differently, and because different fibres are less bulky than others for the same weight, you cannot rely on the weight in grammes of the substitute yarn. Here's an example: the Merletto on the left in the picture above weighs 50g. Suppose you're subbing it into a pattern that calls for ten 50g balls of Debbie Bliss Cashmerino
. It's easy to think that ten balls of the Merletto will be just fine.
Easy to think, but quite wrong: Merletto's 50g only stretches for 90m. Cashmerino's 50g is 110m. So ten balls of Merletto is 900m, while ten balls of Cashmerino is 1100m - or over two balls of Merletto in difference. If you know this when you're buying the yarn, you can get enough to complete your project. More and more patterns tell you the total yardage required, but not all do and it's easy to check when you're aware of it.
Indeed, the yarn at the top right in the picture illustrates how weight and thickness can be deceptive: that's Sublime Lustrous, a very luxurious yarn that's new for Autumn/Winter. To look at it there, it's close to Cashmerino in thickness and floof. When you pick it up, you notice the difference. A ball is 95m, which makes it shorter in yardage, but that 95m only weighs 25g. In other words, it's half the weight of all the others. Light and warm for winter? Oh yes!
Of course, we're delighted to help with substitution issues in the shop as well, so if you come across your ideal project and want to change the yarn, it's possible and it's fun. And who knows - your version in your newly chosen yarn may even be lovelier than the original!