Saturday, June 13, 2015

Oh That Nasty F Word: Firescale!! and metal clay findings too!!


(***NOTE:  I wrote this article before the advent of PMC Sterling and the 960 alloy, so please read this with Fine Silver Clay in mind.  Now that we have the above alloys, strong clasps can be constructed entirely of these two new metal clays.  In addition, you can implant all the silver wire into them as well.)

FINDINGS, the mechanical bits that differentiate a bracelet from a tie tack, from an earring, have a number of things in common.  To function correctly, clasps and findings must have strong working parts to hold together and to support weight.   The finding must be fashioned in such a way that the wearer feels secure that the piece will stay on the body.  Ease of use is an advantage.  Integration of the finding into the overall design is always desired.  Commercial findings have their place, but when a designer can meld function with form, it is the best solution and tends to make a more successful and pleasing piece of jewelry.

Wire is the main form of metal that makes up a variety of clasps and findings.   Since PMC is fine silver, it is obvious that the implantation of any piece of fine silver wire will survive the highest firing schedule of 1650° F for 2 hours and anything below 1650° F.  Jump rings or eyelets of a thin gauge of fine silver wire are suitable for any small, light element that might dangle from an earring, for instance.  Even larger, heavier gauge jump rings in fine silver will function well if they are embedded to cover the joint.  But relying on fine silver of any gauge would be a mistake when the finding needs to have tensile strength (tension or spring) in order to function properly.  Without the presence of the small amount of copper as in sterling silver, fine silver wire cannot be work hardened for strength.  Therefore, sterling is the material that is preferred for creating findings in PMC.

A red flag should wave when considering implanting sterling into PMC.  At temperatures above 1200° F, sterling’s molecular structure will begin to change.  Initially, heat will soften the sterling and anneal it (about 1100°F).  Beyond about 1500° F sterling will become brittle and begin to reticulate or wrinkle on the surface.  Sterling silver melts at 1640° F which would be an obvious problem when using the 1650° F firing temperature for any duration of time.  Therefore, it is preferable to use only PMC 3 at the 1110° F temperature.  The duration of time at 1110° F doesn’t seem to be an issue.

Another change that Sterling undergoes when heated in the presence of oxygen is the formation of copper oxide, or firescale.    During heating, the copper at the surface is converted to Cu2O, cuprous oxide, which has a reddish color and then to cupric oxide, CuO, which is black.  We are used to seeing the black CuO on the surface after removing a finding of PMC 3 and sterling wire from the kiln, but what you don’t see easily is the Cu2O which is absorbed by the silver and the copper and then resides in the interior of the silver.  Prolonged heating and in the case of soldering, insufficient flux or wearing out of flux, causes firescale to go deep into the surface making abrasives necessary to remove it.

Firescale can be dealt with in a number of ways.  After firing, abrasives such as emery can be used to scrape into the surface of the sterling and remove the firescale.  As a total preventative measure, one can paint the entire surface of the sterling with PMC 3 paste. The paste will fuse to the sterling and create a layer of fine silver over the sterling and negate any oxidizing action in the kiln.  Depletion gilding is also useful in removing copper oxide. Pickle can be used to remove some of the surface firescale.  If the firescale is deep, abrasives must be used.  If using pickle make sure to neutralize the acidic action by soaking the metal clay in a saturated solution of baking soda and water.  Leaving pickle in the porous structure of metal clay is not a good practice.  ( see a comprehensive article on firescale:

If a mirror finish or without patina finish is desired, firescale must be removed because of the off-color of the oxidized metal.  Polished firescale has a grayish, purplish tinge that is immediately noticeable if not removed or disguised by patina chemicals.  (Tumbling will not remove firescale—only acids, acidic chemicals and abrasives will work.)  It is not good craftsmanship to see polished firescale on a finished piece.

(If I know that I am going to be using patina chemicals to blacken my piece, I just leave the firescale surface intact and use it to my advantage by integrating the color of the oxidized sterling with the patina treated areas.)

To create various clasps and findings one can insert pieces of sterling wire into the soft clay.  Even though the sterling is fused to the PMC and captured by the shrinkage, a little insurance in the form of a wiggle in the imbedded wire is not a bad idea.   If one has a dry, unfired piece of PMC or one that has already been fired, sterling wire can be buried onto the surface with unfired  PMC 3.  Fire those types of PMC and then add your sterling and fire at the 1110 F temperature.

Once you have fired your piece with the implanted sterling, and removed the firescale, it may be necessary to work harden the sterling so that it will function properly.  For example, if you are making a hook and eye clasp, the hook must be strong and springy.  One can carefully hammer that hook form with a steel hammer on a steel block and create that spring.  Another finding that needs this step would be French or shepherd-type earring hooks.  Jump rings and eyelets whose joints are buried don’t usually need to be hammered.  Tumbling does not work harden annealed sterling sufficiently for it to function as a finding.

The configurations, forms and varieties of findings that one can create with PMC and sterling are limitless.  Being able to create a customized clasp that integrates aesthetically with a piece of jewelry is very advantageous to the overall look of the piece.  Not having to haul out the soldering paraphernalia is also a pleasure!