Interfacing: Where Cloth Has to Stop Behaving Like Cloth

FUSIBLE interfacing has heat-activated glue on one side and is ironed on. It is quick, it is stable, and it is the right answer for most modern light and medium cloth. SEW-IN interfacing is held by stitching. It is the answer for anything that cannot take the heat of fusing, for very textured or loosely woven cloth where glue will not grip, and for tailoring where a softer, more moveable result is wanted. Fusible on the wrong fabric is a lasting mistake: the glue can show through thin cloth as a faint sheen, bubble after washing, or refuse to stick at all. Test on a scrap of the actual fabric, wash the test, and look at it again — before fusing a collar.

Woven interfacing has warp, weft and bias exactly as fabric does, and it must be cut on the same grain as the piece it is backing. Cut it crosswise onto a lengthwise piece and the two layers stretch differently and the collar twists. Non-woven interfacing has no grain in the woven sense, which makes it simpler and also stiffer in every direction. KNIT interfacing stretches in one direction and is what a stretch fabric needs — a non-stretch interfacing on a knit garment stops it stretching where it was meant to. Match the interfacing to what the fabric must still be able to do. That is the whole selection rule, and it is why there is no single best interfacing.

Fusing needs all four, and the one people cut short is time. Ten to fifteen seconds in one place, pressing down firmly, is typical — and the iron is lifted and placed, never slid, for the same reason as any other pressing. Use a pressing cloth and a little steam unless the instructions say dry. The cloth stops glue reaching the iron, which is a mess that then transfers onto the next thing pressed. Trim the interfacing out of the seam allowances on anything thick, or cut it to the stitching line in the first place. Four layers of fabric plus four of interfacing at a collar corner is a lump no amount of pressing will flatten.

BUTTONHOLES, without exception. A buttonhole in unsupported cloth stretches into an oval with use and the button falls through. WAISTBANDS, or they roll over. COLLAR and CUFF, or they collapse. Also anywhere a fastening pulls: the front band a button strains against, the edge a zip is set into, a pocket opening that a hand goes into repeatedly. The common factor is a concentrated force on a small area of cloth. And anywhere a bias edge must not stretch — a shoulder seam, a neckline, a curved hem. A narrow strip of interfacing along that edge is a stay tape, and it is what keeps a neckline the size it was cut.