Part of a series that begins with Blocking Notes (Intro).
In general reading the ball band is a good idea, both for general care instructions, and to know what the yarn is made of. If in doubt, test your blocking methods on a swatch to see what the likely effects will be on the finished article. There are some general comments that can be made based on the type of yarn.
Wool and most wool-like fibres can be wet-blocked or steam pressed warm. Angora and mohair are less elastic than other wool-like fibres, while cashmere felts easily, so you need to be careful with these fibres. Some people say that spritz blocking is safest with delicate fibres such as cashmere and alpaca (see [THOM] for example), but since you will want to wash any garment you wear eventually, I see no harm in gentle wet-blocking. Try it out on a swatch first to be safe.
Silk tends to stretch and so after blocking you may want to pull a little on the thoroughly dry swatch before measuring. It is also fragile when wet, so block gently using wet-blocking or spritz-blocking. Cotton, linen, hemp (and other plant-based) yarns tend to stretch with wear so after blocking the gauge swatches you may want to hang a light weight on the dry swatch for a few hours to mimic wearing the article before measuring the final gauge. These yarns can be wet-blocked or steam pressed warm. Linen is strong when wet, so can take a lot of wet-blocking without damage.
Most synthetics don't respond to blocking the way woolen fibres do; they tend to bounce back to the pre-blocked state rather than keep the blocked form. There is a method used for acrylic yarns called “killing” [a href = "http://www.runedesigns.com/2008/12/02/blocking-notes-intro/#DRUC">DRUC] which involves pinning the wet article to shape, then steam pressing through a wet cloth. This is said to make the article keep its shape after blocking, and to drape beautifully. I haven't tried it myself so would recommend testing the procedure on a swatch first.
Stitch Pattern Effects
Wet blocking can be used for any stitch pattern, although care must be taken when pinning the article out to not stretch ribbing or cables too much. If stretched too much, these won't bounce back when dry.
When steam blocking, don't press ribbing, cables, or any embossed patterns, as the pressing will flatten and lessen the three-dimensional effect of the patterning.
I soaked a few swatches at a time in lukewarm water to which a small amount of a gentle hair shampoo had been added for about 20 minutes. I swished the swatches around gently a couple of times. I pressed out the water, and rinsed the swatches using lukewarm water. Then I rolled them in a towel. I pinned the swatches out on a foam board covered with a clean towel. I blew up a balloon inside the hat to block that, measuring to make sure I didn't blow it up too far. Then I let each swatch and the hat dry before unpinning the swatches and deflating the balloon.
I wove in the yarn tails before blocking for the swatches. For the hat, I wove in the yarn tails before blocking and then checked they hadn't puckered after blocking. I sewed on the i-cord loops after blocking.