Block knitting is a process by which a knitter can stretch and shape a piece of knitting to fit the dimensions required by a pattern. Block knitting can help to make two pieces of knitting conform to the same size and shape, and it can help even up the size and shape of the stitches. Some types of knitting, such as lace, require blocking to bring out the pattern. There are at least three common types of block knitting, including spray blocking, wet blocking, and steam blocking. Each type of blocking is appropriate for different patterns and different knitting wool fiber contents.
Wet blocking is the best method for knitting wool made of man-made fibers, novelty wools, and many wool or wool-blend fibers. To perform wet blocking, a knitter must typically wet the piece of knitting to be blocked. The piece of knitting should not generally be so wet that it drips.
Knitters will generally spread the wet piece of knitting out onto a sheet or towel. They then stretch the piece of knitting to meet their needs of shape and dimension. They use straight pins to hold the piece of knitting in the stretched position until it dries. Some knitters may use a wire frame. It's generally considered vital to use pins or wire that won't rust, as rust often stains the fibers of the knitted piece.
Steam blocking is a block knitting process used for fibers, such as cotton, that may lose their shape if they're allowed to get wet. Steam blocking isn't generally considered appropriate for man-made fibers, or wool, as the heat may damage them.
To perform steam blocking, knitters place a damp piece of fabric over the knitted piece. Knitters may either stretch and pin the piece of knitting before applying steam, or they may apply steam before stretching and pinning the knitting. Which method a knitter chooses relies largely on the natural elasticity of the knitting wool fibers. If the knitting can be stretched before steaming, this is generally considered preferable. Knitters then steam the piece of knitting by pressing a hot steam iron gently over the damp fabric, allowing the steam to penetrate through and relax the fibers of the wool.
Spray blocking is considered the most delicate block knitting process. It's used for delicate fibers such as silk. The piece of knitting is stretched and pinned in place. It's then sprayed with water from a bottle until it's damp, but not dripping. Once a piece of block knitting has dried, the pins and wires can be removed and the piece of knitting will usually hold the new shape.