Is boiled wool just thick felt?

They are close, but not the same. True felt is matted from loose fibers and was never knitted or woven, so it is rigid. Boiled wool starts as a knit and is fulled until dense, so it keeps a little stretch and has an underlying looped structure.

How is felt different from fulled woven or knit wool?

Felt is non-woven: its fibers are matted directly into a sheet with no yarn. Fulled woven wools (like melton or loden) and fulled knits (boiled wool) start as cloth and are then matted, so they have an underlying weave or knit that felt lacks.

Why does neither fray?

Fulling mats the wool fibers together so tightly that there are no loose threads to unravel. True felt has no threads at all, and boiled wool's loops are locked by the felting, so both can be cut and left unhemmed.

Side by side

Felt vs Boiled Wool

Felt and boiled wool are both dense, matted wool fabrics with no visible weave, made by the same fulling that mats wool fibers together with heat, moisture, and agitation. The difference is the starting material. True felt is not made from yarn at all; its fibers are matted directly into a sheet. Boiled wool starts as a knitted fabric and is then fulled until it shrinks dense, so it keeps a little of the knit's stretch.

Felt

No. 054

nonwoven, matted fiber · first documented Antiquity

Boiled Wool

No. 140

knitted wool, fulled until dense · first documented Middle Ages

The differences

Aspect	Felt	Boiled Wool
Starting point	Loose fibers, never spun or woven or knitted.	A knitted fabric, then fulled until dense.
Structure	A homogeneous mat with no threads at all.	A felted knit; the loops are matted but still underlie it.
Stretch	None; a rigid sheet.	Retains some of the knit's give.
Edges	Never fray; can be cut anywhere.	Resist fraying; can be left unhemmed.
Typical use	Hats, crafts, gaskets, padding, banner letters.	Tyrolean and Austrian jackets, slippers, structured knitwear.

Which to choose

If the dense wool sheet was never spun into yarn, it is true felt; if it began as a knit and was then fulled, it is boiled wool. Felt is rigid and made from raw fiber; boiled wool is a felted knit that keeps a little stretch.

Common questions

Is boiled wool just thick felt?: They are close, but not the same. True felt is matted from loose fibers and was never knitted or woven, so it is rigid. Boiled wool starts as a knit and is fulled until dense, so it keeps a little stretch and has an underlying looped structure.
How is felt different from fulled woven or knit wool?: Felt is non-woven: its fibers are matted directly into a sheet with no yarn. Fulled woven wools (like melton or loden) and fulled knits (boiled wool) start as cloth and are then matted, so they have an underlying weave or knit that felt lacks.
Why does neither fray?: Fulling mats the wool fibers together so tightly that there are no loose threads to unravel. True felt has no threads at all, and boiled wool's loops are locked by the felting, so both can be cut and left unhemmed.

Full entry: Felt Full entry: Boiled Wool

Sources & References

1.Felt, Wikipedia
2.Felt, Encyclopaedia Britannica
3.Boiled wool, Wikipedia
4.Fulling, Wikipedia