Spiral-wound gasket utilizes a mix of metallic material and “filler material” generally the gasket has a chosen metal, normally a carbon rich or stainless steel, wound (hence the name) outwards in a circle (although other shapes are possible this is the primary) with the filler material, generally a flexible graphite, starting at the opposite side of the circle and winding in the same direction. This leads to a growing circle of alternating layers of filler and metal.
These gaskets have proven to be reliable in most applications and although more expensive than solid material they do not require as high of bolt forces to be effective. This is possible mainly because the graphite makes the primary seal with the flange and the metal only acts to keep the gasket structurally sound.