FYI, if you are selling in the US, you can not legally call 10/40 wire (1/40th 10K RGP) gold filled. You must refer to it as rolled gold plate.
See http://www.artisanplating.com/articles/goldfilled.html "commercial designations and markings" or http://www.ftc.gov/bcp/guides/jewel-gd.shtm 23.4 for more information.
To meet the definition for gold filled, the wire must be at least 1/20th gold => 5% by weight. This is normally abreviated as 14K/20. 14K/10 is at least 1/10th gold => 10% by weight.
There is also a 14K/40. Beware here - 14K/40 means 1/40th by weight = 2.5%. There are vendors that call this gold filled. BY FED REGS (US), it does not meet the standard for gold filled and can only legally be called rolled gold plate. All gold filled can be called rolled gold plate - but in the US only items with 5% or greater gold by weight can be called gold filled.