Jewelry Concepts & Technology by Oppi Untracht

Author:Oppi Untracht [Untracht, Oppi]
Language: eng
Format: epub
ISBN: 978-0-307-78411-7
Publisher: Knopf Doubleday Publishing Group
Published: 2011-01-26T05:00:00+00:00

FIRESTAIN: A subsurface scale

Silver alloys containing copper repeatedly subjected to high heat treatments without being properly protected with scale-inhibiting flux or other inhibitors will develop a subsurface scale, commonly called firestain or fire by silversmiths and jewelers. This appears as a shadowlike, dark gray patch or patches below the surface. Firestain is more likely to occur as the alloying copper content increases. Eventually, this suboxide penetrates the grain structure, which becomes harder than the original silver alloy. Where it exists near a soldered joint, the joint is weakened since it hinders solder flow. Ordinary pickling may temporarily obscure its presence by leaving the surface thinly coated with pure silver from which the copper has been leached, but firestain appears again when the surface is polished. When allowed to remain, firestain may spoil the work’s appearance, and its presence hastens further oxidation or tarnishing.

PREVENTING FIRESTAIN FORMATION To avoid firestain formation, take advance preventive measures. The simplest measure is to precoat the entire article: with a paste mixture of flux applied by a brush: by dipping the heated object into a flux solution then draining and allowing it to dry; or by spraying it with flux. A fixative sprayer, insecticide hand pump, or other sprayer can be used. Preheat the article beforehand somewhat so that the spray will dry on contact. Reheat the work and spray it with a second coat of flux. During soldering, at an elevated temperature the flux fuses and forms a glassy coating on the metal that seals the surface against oxygen contact so firescale and firestain do not form. After soldering, fused flux is removed with pickle or an ultrasonic cleaner.

CURING TREATMENTS TO REMOVE OR TREAT FIRESTAIN

Acid: Paint joints front and back with protective molten wax. Immerse the work for short intervals in a strong 50% solution of cold nitric acid, remove, and rinse. Observe the result by scratch brushing. Repeat this procedure until the subsurface firestain level is dissolved.

Abrasives: If metal thickness allows, remove firestain with abrasives. Polishing with tripoli usually is not enough unless firestain is shallow, but it can be tried first. Otherwise, use a wet waterproof abrasive paper, dry emery paper, or pumice paste and a felt buff. Alternately, rub the surface with a fine Arkansas stone, or a file.

Electrostripping: Electrolytically strip the silver by deputing with the article as anode. Remove the work at intervals to observe its condition as excessive stripping roughens the surface.

Electroplating: After polishing and thoroughly cleaning, electroplate the entire article using a pure silver or rhodium anode to completely cover the firestain.

Creating a uniform firestain: Make the entire surface appear a uniform bluish gray firestain color by annealing the work until it just shows redness. Allow it to cool somewhat, then pickle it. Repeat the process about six times, then polish with rouge.

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