A soap loaf is soap made in a block and sold either whole or pre-sliced. Buying by the loaf is cheaper per wash than buying bars, and it lets you cut the thickness you actually want.
Cold process and melt-and-pour
Cold process soap is made from scratch — oils and lye reacted together (saponified) — and it must then cure for four to six weeks. Curing is not drying out for the sake of it: water evaporates and the bar hardens, and a hard bar lasts far longer in a wet soap dish than a soft one. A properly cured cold process bar is a genuinely better object.
Melt-and-pour uses a pre-made base, remelted with colour and fragrance added. It is ready immediately, it is usually more transparent, and it is softer, so it goes faster. Neither is dishonest; they are different products at different prices.
Cutting a loaf
A long non-serrated blade and one continuous pull. Sawing tears the surface. If the loaf has been somewhere cold, let it come up to room temperature first — cold soap crumbles at the edges.
Around 2.5cm per bar is the usual thickness, which gives about thirteen bars from a standard 1.2kg loaf. Thicker bars last longer per bar but are awkward to hold.
Storage is the whole game
Cut soap wants air. Wrapped in plastic it sweats, and glycerine soaps in particular will bead moisture on the surface — that is glycerine pulling water out of the air, not a fault. Keep slices loose in a box or on a shelf with air around them, and only the bar in use goes near the bathroom.
A draining soap dish is worth more than any other soap accessory. Soap sitting in its own puddle dissolves several times faster than soap that dries between uses.
Scent, and how long it holds
Essential-oil scents fade faster than synthetic fragrance, especially citrus, which can be noticeably weaker within a few months. Storing slices in a closed box slows it considerably. If you have bought a loaf for a year's worth of washing, cut it as you go rather than all at once.