Steam blocking is a gentle way to shape knit garments, blankets and scarves. Instead of soaking the garment in water and pinning it to air dry—as you commonly do when blocking a knitted garment—you are steaming it, most commonly with a typical household iron. There are many benefits to steam blocking, including that you can shape your garment, fix or cover up small mistakes, and set stitches to appear more evenly.
In order to complete any blocking project, you will need a few tools in addition to your garment, such as pins, a blocking board, a tape measure, water (preferably distilled so that you don't have any extra hardening agents depending upon where you live) and an iron. There are a variety of blocking boards that can be used- any of which will get the job done. Many knitters use blocking boards made specifically for knitting, others use foam boards originally intended for science fair projects, and some find that using foam rubber baby floor mats is a great way to pin and block.
The good news is that regardless of your tools, steam blocking is easy! To steam block, simply pin your garment out to the desired dimensions making sure the wrong side is facing up. Wet an old pillowcase or thin towel and wring out the excess water so that it's damp and place it on top of the garment. Then use a household iron at its hottest setting, and press down lightly forcing the steam through the garment. Repeat this all over the garment until the pillowcase is dry. Then let the garment dry for approximately 30 minutes.
Another way to steam block is to take the iron, put it on the steam setting, and hover it about an inch to half inch away from the garment. The goal is for the steam to go through the garment without ever touching the iron to the piece. Keep the iron moving along the length of the garment and then let it dry for 30 minutes or so.
Steam blocking is a great way to get your wool or cotton knit items into the desired shape. For more delicate items like cashmere and acrylic, spritz blocking is recommended. For this, the item is pinned to the desired shape and lightly spritzed with water from a spray bottle and left to dry.
Acrylic yarn can also be "killed" to create a softer and more limp fabric. Killing it removes all of the structure from the garment so this should only be done when additional draping is desired. By applying heat to the garment though steam blocking or ironing the fabric directly, the plastic fibers in the acrylic yarn will essentially melt together. Though once you've "killed" a garment, it cannot go back to its original shape, so make sure you decide wisely which acrylic pieces to use this technique on.
In addition, you should never try to block silk, as the fibers are far too delicate to withstand the heat. If you need a certain size or shape with silk, always create a gauge swatch first instead of relying on water and steam to help.
No matter your method, steam blocking is the perfect way to make your garment behave and shape to the look and fit you desire. If you are looking to learn more about the more common process of knit blocking, this tutorial offers a step-by-step guide, while the class Blocking Handknits provides detailed, in-depth instruction for blocking.