As a designer, my most frequently asked questions is "why is your gauge so weird?" (A: Everyone is different, and I generally prefer densely knitted fabric?), but the second most frequently asked question is do you think X pattern will work in Y yarn?
The answer? Probably.
The best way to figure out if a yarn substitution will work is to swatch it out and give it a try.
But that's not always practical.
No Need to Fear Substituting Yarn
There's a good chance that the sample itself was done in a substitute yarn. About 50%+ of the time I do editorial designs (for magazines, books, etc.) the yarn I submitted my design swatches in is not the yarn that's used in the final design.
Sometimes I'll go out and buy something if I'm looking for something specific or need a different color, but I usually do my design swatches from my stash, which mostly contains a lot of leftovers from previous designs. Sometimes, I'm attached to the yarn I send the design swatch in on, sometimes I'm not, but if it's for editorial purposes, it's best not to get attached because it's likely that its going to change. I'd also advise that if you're designing for magazines that your design isn't yarn-dependent for the same reason.
I'm not an editor, so I don't know how the final yarns are chosen, but I imagine it's a combination of trying to have variety, relationships with yarn companies, desired color stories, etc.
Cormac is a bit of an oddity in that the sample in the magazine was knit in the exact yarn I submitted with in the exact color I used. But some of that was because the submission call specifically called for using chainette-style yarn, and there's not a huge number of those on the market, and I specifically did buy yarn for swatching.
How to Choose a Substitute Yarn (if you don't have something in your stash) - Option #1
Maybe you're allergic to alpaca or the suggested yarn is too expensive, or you want something more attuned to your climate, or it doesn't come in a color you like - there are dozens of reasons to choose a substitute yarn and hundreds of choices to sub. How do you narrow it down?
For now, I'm going to assume you generally want your finished sweater to look more or less like the sample (we'll talk more drastic changes later)
Here's where the Ravelry database is going to be a huge help.
Step #1 - Look at the suggested yarn page on Ravelry (or on the manufacturers site). Even if you know you don't want to use it. What are it's basic qualities?
For Shibui Knit's Maai the basics are this - DK weight, 70% alpaca/30% merino. So it's mostly alpaca, which is fairly drapey and has a bit of a halo/fuzz factor. The chainette construction makes it light and "springy" per the description. Looking at the design, the need for drapey is high - otherwise it would be pretty stiff and boxy, the fuzz factor is less evident. The pattern calls for size 8 needles, which is fairly big for DK, and the pattern is open, so that'll provide some drape there too.
So to replace it, we're looking for a drapey DK weight yarn.
Alpaca is drapey, so is silk, bamboo, tencel, and linen. But there's the springy factor too. So you might want a firmer fiber like a bit of wool or hemp or cotton in there to help the sweater holds it's shape a little. Something like the 70/30 blend of the original or up to a 50/50 mix of drapey/firm.
Step #2 - Back to Ravelry, this time to the advanced yarn search page. To start off I'm going to looks for the following criteria - Not discontinued, dk weight, contains alpaca and wool. This is going to be the closest to the Maai, without being Maai.
We've got 203 matches there alone. Let's look at some of our top-provided options.
Drops Lima - 65% wool/35% alpaca - this could work, but it's a little heavy on the wool side.
Classic Elite's Soft Linen - 35% Linen, 35% Wool, 30% Alpaca. That's 65% drapey (linen/alpaca). Looking pretty good here. The linen could help make it an even more transitional piece.
Queensland Collection Rustic Tweed - 63% Wool, 27% Alpaca, 7% Acrylic, 3% Other. Again heavy on the wool. It's also rustic and tweedy, which is a bit style shift.
The Alpaca Yarn Co's Astral - 50% Tencel, 30% Alpaca, 20% Wool. That's 80% drape, so super drapey. It also looks hand-dyed, which could mean alternating skeins in lace.
Berroco's Fiora - 40% Cotton, 30% Rayon, 15% Alpaca, 5% Wool. 45% firm/ 45% drape (not sure what the missing 10% is). Could work, be a solid choice for a more summery version, but keep in mind that that is a high percentage of cotton, which is heavier than most other fibers.
What if you look at Cormac and think - forget fall, that'd be better as an awesome beach pullover for the summer? Then you could search for a linen/cotton or cotton/bamboo blend (drapey/firm) or something similar.
How to Choose a Substitute Yarn (if you don't have something in your stash) - Option #2
Alternatively, if scrolling through pictures of yarn isn't your thing, you can do a project based-search. In this case we'll search for similar patterns (DK weight, lace, pullover) and see what other people knit them out of. There's a lot of options, but most of them aren't as open as Cormac, but Amy Miller's Stonecutter Sweater has a good number of similarities (there's nothing new under the sun, right?)
The original Stonecutter was knit in Lion Brand Cotton Bamboo, so that could be a good choice too. But wait, there's more! Stonecutter has been around for a while, and over 100 people have knit it and if you click the little "yarn ideas" tab, it'll show the most popularly used yarn subs and you have another 2 pages of yarn to choose from (back to the scrolling through little photos of yarn, sorry).
Already Have a Yarn in Mind?
Maybe you've got something in your stash your'e itching to use up. I've had a few message in my inbox regarding more summer-specific yarn subs for this project, specifically: Berroco Weekend DK, and Shibui Linen.
My first stop when considering a specific yarn substitution is to look at projects on ravelry that have been knit up in the yarn that have some similarities to the project I’m trying to do. In this case, a lace garment of some kind. For the left we have an example for the Weekend DK and one for the Shibui Linen on the right.
This can give you a good hint of what the fabric would look like. Both of them seem to work in lace pretty well, though I will note the following two things - the yarn is held double in the lace project and that cotton and acrylic don’t block open as well as animal fibers (though linen blocks fairly well). However there appear to be some well-blocked lace projects in Weekend DK, (Also the sample wasn’t aggressively blocked).
While I specifically designed the Cormac Sweater with Shibui Maai in mind, and tried to take advantage of the specific qualities of that yarn, it doesn't mean you have to use it! The great thing about knitting is you can make it your own and you have hundreds of options to choose from that will work.