Main Difference
The main difference between Pirn and Firn is that the Pirn is a Rod for weft thread or yarn and Firn is a granular snow, especially on the upper part of a glacier, where it has not yet been compressed into ice.
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Pirn
A Pirn is a rod onto which weft thread is wound for use in weaving. Unlike a bobbin, it is fixed in place, and the thread is delivered off the end of the pirn rather than from the centre. A typical pirn is made of wood or plastic and is slightly tapered for most of its length, flaring out more sharply at the base, which fits over a pin in the shuttle. Pirns are wound from the base forward in order to ensure snag-free delivery of the thread, unlike bobbins, which are wound evenly from end to end.
Pirns became important with the development of the flying shuttle, though they are also used with other end delivery shuttles. Power looms which use pirns generally have automatic changing mechanisms which remove the spent pirn from the shuttle and replaces it with a fresh one, thus allowing for uninterrupted weaving.
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Firn
Firn (; from Swiss German firn “last year’s”, cognate with before) is partially compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé. It is ice that is at an intermediate stage between snow and glacial ice. Firn has the appearance of wet sugar, but has a hardness that makes it extremely resistant to shovelling. Its density generally ranges from 0.4 g/cm³ to 0.83 g/cm³, and it can often be found underneath the snow that accumulates at the head of a glacier.
Snowflakes are compressed under the weight of the overlying snowpack. Individual crystals near the melting point are semiliquid and slick, allowing them to glide along other crystal planes and to fill in the spaces between them, increasing the ice’s density. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles.
In the summer months, the crystal metamorphosis can occur more rapidly because of water percolation between the crystals. By summer’s end, the result is firn.
The minimum altitude that firn accumulates on a glacier is called the firn limit, firn line or snowline.
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Pirn (noun)
A cone-shaped spool or bobbin on which thread or yarn is wound, used most often for weaving.
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Pirn (noun)
The amount of thread wound on a shuttle or reel.
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Pirn (verb)
To wind yarn onto a pirn, usually by means of a pirn winder.
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Firn (noun)
A type of old snow which has gone through multiple thaw and refreeze cycles and thus is made of numerous small icy grains, though it is not nearly as saturated with water as snow-cone slush is; can be hard or somewhat soft depending on recent and current weather conditions.