How did Corwin recover his sword?

Question

In the first book of the Amber series, when Random and Corwin travel to Amber, Random "adds" Corwin's sword when travelling through Shadow. The same thing happens in the second book, Guns of Avalon, when Corwin finds his sword in a tree trunk:

It mattered not if it had been somewhere in Amber. It was here now, for the wood that I walked was in Shadow.

Later in the series, however, we learn that it is impossible to find and transfer particular thing through Shadow. So, was Corwin's sword a copy in these cases, or did he have the power after all?

Answer 1

Grayswandir is a specialized artifact linked and attuned to the Pattern and to Corwin. Though Corwin's magical abilities were limited mostly to walking through Shadow and using the Trumps, he had a special ability to be able to call his sword to him as he moved through Shadow.

His entry in Wikipedia expands further upon Grayswandir's nature and powers:

• Grayswandir is Corwin's sword, also known as the Night Blade; its twin is Brand's Werewindle, the Day Sword.

• Both Grayswandir and Werewindle are inscribed with portions of the Pattern of Amber; this makes them particularly effective against creatures of Chaos.

• In addition, Grayswandir has one magical property that makes it particularly useful: Corwin can call it by creating a Shadow where he wills it to exist. Unlike a normal blade, Grayswandir can also harm apparitions, and allows Corwin to speak to the shades in Tir-na Nog'th.

• In the Amber short story "Hall of Mirrors", Corwin revealed that Grayswandir and Werewindle were transformed spikards. Spikards are rings of power that existed before Amber, and possibly even Chaos, were created; they allow the bearer to tap into the raw power of the Amber universe. For reasons unknown neither Corwin nor Brand used their powers during the Patternfall War.

• The name Grayswandir echoes that of Graywand, the longsword of Fafhrd in Fritz Leiber's tales of Fafhrd and the Gray Mouser, a likeness that may or may not be coincidental. Unlike Grayswandir, Graywand has no magical properties, or even identity as a unique artifact; rather, Fafhrd bestows this name on any longsword he happens to possess and use.

Answer 2

An interesting take on this, similar to Thaddeus's answer but not identical, can be implied from the Amber Diceless Roleplaying Game by Erick Wujcik, which had some creative input by Zelazny, so can be seen as at least partially true to canon, or at least a reasonable interpretation of it.

In the game, characters are created by assigning points to specific traits or attributes. Some of these points can be used to create a unique artifact for your character - often a weapon - which has the same realtionship to your character as Greyswandir to Corwin. This item isn't a mere accidental construct, but somehow Real(tm), like the children of Amber are real, and thus cannot be held in place by something as flimsy as Shadow - it is a part of you, and if you are travelling in Shadow, it is also travelling with you.

Based on this, Greyswandir's appearance is easily explained. Of course Corwin will find it in a convenient Shadow, just like he won't walk through them only to leave his right hand behind.