Twelve spear throwers from the tomb of the "Lady of Cao"were analyzed with a portable equipment which uses energy-dispersive X-ray fluorescence (EDXRF). The pre-Columbian tomb of the Lady of Cao is located about 50 km north of Trujillo, in the north of Peru, and is dated around 350 A.D. The spear throwers are on gilded copper over a wood cylindrical structure. All analyzed spears have a similar gilding, and four of them have also additional much smaller gilding rings covering the wood structure. These rings are clearer and contain more silver, in three cases about the double, in one case about five times the silver of "normal"gilding. There were two problems to solve: the first one is related to the composition of the gildings, the second one is related to the determination of their thickness. The last problem, which is much critical, was solved using a new method based on the different attenuation of fluorescent X-rays by the gildings.