Use cotton thread with cotton fabric. Do not use polyester or cotton-covered polyester thread in either your applique or your piecing. Polyester thread is very strong. There is a danger that over time, as the fabric moves in the seams, the polyester thread can cut the fabric. You don't need thread that is stronger than the fabric you are sewing.
If you look on a spool of thread there is often a number. This number indicates the fineness of the thread. There is no universal standard that this number corresponds to. Each manufacturer assigns numbers based on the particular threads they offer.
Thicker thread is more visible in hand applique. Thicker thread takes up more space in a pieced seam than a finer thread does. We generally prefer to use finer threads for more invisible stitches and flatter seams. When using finer thread in machine piecing you will need to use a slightly shorter than normal stitch length.
We prefer 2-ply thread for both hand applique and machine piecing. Our favorites are Aurifil (50 wt.), DMC (50 wt.), YLI (60 wt.), Masterpiece made by Superior (50 wt.), and Mettler (60 wt.). Notice that we listed Mettler's 60 wt. cotton thread. It is finer than Mettler's 50 wt. silk-finish cotton that many of you are familiar with.
We have sometimes confused the weight of the thread with a number that would indicate the tensile strength of the thread. (OKâBecky admits that she has done this more than Linda.) In fact, the tensile strength of thread is measured in "centinewtons". A centinewton is a unit of measure that is standard for all thread manufacturers (according to Guy at the Mettler factory in Germany). The tensile strength measured in centinewtons is not listed on the spool.
We recently visited the Mettler thread factory in Germany. We had an opportunity to watch the machine that is used to measure tensile strength. The machine is mechanized. A length of thread is wrapped once around a spindle and gently pulled. The machine is able to measure how many centinewtons of force the thread can withstand before it breaks. Multiple lengths of thread from each lot are tested.
To summarize, use the weight number printed on the spool as a general indicator of a thread's fineness. Rely on your eyes and the feel of the thread to help you decide which thread is best for your project.
