White gold was developed in the 1920's as a platinum substitute. Adding white metals to gold will lighten its color, and some metals have a stronger bleaching effect than others. Nickel, iron, chromium, palladium and platinum are the most effective whiteners. Zinc and silver work almost as well. There is no set standard in the industry to define white gold's color, and the only regulation about white gold at all came January 20, 2000 in Europe pertaining to the use of nickel. Nickel can bleed from white gold and cause an allergic reaction in some people. European Union countries are trying to eventually eliminate the use of it in all jewelry. In the US there is legislation to label nickel alloys with a health advisory. Alternative options like manganese, iron and chromium have been experimented with but unfortunately these metals make white gold alloys that are difficult to cast and are much more likely to crack or tarnish. Palladium and platinum make the whitest color alloys and are easy to work with, but are cost prohibitive - they can cost more than gold itself per troy ounce depending on the current market. Another drawback is they have a higher melting point which makes casting a bit more difficult and less energy efficient and therefore more expensive. There are many formulas for white gold; here are just a few:

18K Nickel white gold
75% gold + 2.2% copper + 17.3% nickel + 5.5% zinc

14K Nickel white gold
58.5% gold + 24% silver + 10% nickel + 7.5% zinc

18K Palladium white gold
75% gold + 15% palladium + 10% silver

14K Palladium white gold
58.5% gold + 5% palladium + 32.5% silver + 3% copper + 1% zinc

There are even alloys that use both nickel and palladium. No matter what elements are used in white gold all but the best alloys have a dull finish. To overcome this, rhodium plating is used. Once again there is no standard on the thickness of the plating nor is it mandatory that the consumer even be told that electroplating is used. Here's an earring with some of the plating removed to expose the mediocre white gold underneath:





The World Gold Council in London has proposed grading white gold into categories. 3 options are Premium, Standard, and Off-white. Premium has the best white color and would not require rhodium plating, but would be very expensive due to the amount of palladium used. Standard would be a slightly lesser grade and rhodium plating would be optional. Off-white would necessitate rhodium plating.

 

Gold is a color in addition to being a metal. Making it a different color (white) and then plating it with rhodium---this makes no sense to me.

 

White gold is generally not as “white” as say silver, platinum, palladium, rhodium etc. Plus, white gold alloy tends to tarnish in areas not exposed to surface wear which self clean, in a way. That tarnish is usually an unattractive brownish colour. Rhodium plating helps keep white gold alloy, white, in areas where tarnish is prone to occur. I have heard that chromium has been used to plate white gold items, as well.

 

Thank you, this is fantastic information. Would a white gold watch case, (edit follows for @ulackfocus) "manufactured by producers of high quality brands and not limited to a certain Swiss brand and model some argue are overly represented on this forum, notwithstanding that unsaid brand's name is included in the name of this forum", be rhodium plated to overcome inherent dullness in the alloy or do manufacturers ensure the alloy is of a quality that it isn't necessary? I've never heard that watch cases are rhodium plated and presumably that would be problematic when there's a need for light polishing or major repairs. If the alloy is platinum / palladium to avoid the need for plating, and it makes for easier working, how does hardness compare to 18k yellow gold?

.... and, SORRY, I mentioned the name starting with S.

 

To answer your questions, a comparison of the metals is in order.

Platinum is denser than gold in their pure states (100% pure unalloyed element) and also a bit harder, although both are relatively malleable compared to other metals. However, once gold is mixed with other materials it can become stronger and more scratch resistant depending on the other metals, particularly when you know that platinum alloys are 95% pure compared to gold alloys being 75% pure or less. Platinum alloys are more brittle and stiffer than gold alloys, so it's much more difficult to work with. Platinum loses less metal than gold when polished.

18K white gold is generally more grey than platinum, even when the gold is alloyed with platinum or palladium, so rhodium plating is often applied. Rhodium is part of the same metal group as platinum and palladium, and is a little more resistant to scratching and tarnish than the other metals in that group. The layer of flashing (thin electroplating) over white gold is usually less than 2 microns because any thicker coating could increase brittleness. The thicker the coating, the longer it lasts. If serious damage is done to a white gold watch or jewelry with rhodium plate, replating will be necessary after the repair. Fortunately, that isn't too expensive because the coating isn't as thick as the 10 - 40 micron layer for gold plating.

EDIT: just posted the H101 article on Platinum - https://omegaforums.net/threads/horology-101-platinum.76910/