This is not a question about purity; rather it is a question about colour.
How do you assess the degree of polish or lustre on a silver item? What is the different in colour between an item which has been silver or rhodium plated and the polished silver alloy? Is there a difference in colour between different silver alloys?
All of these questions go through our minds when we examine a finished piece for quality or consider the merits of a different silver alloy for our work. Many of us ‘shade’ a finished item with a piece of paper to assess its surface finish and so reduce the effects of reflected light which might mask surface defects. However, it is the use of reflected light which forms the basis of colour measurement systems.
In its simplest form you take a light of known intensity, shine it on a surface and measure how much light is reflected back to you. This measurement of ‘reflectivity’ gives a simple measure of the brightness and whiteness of the object being tested. To standardise the test you can specify the power of the light source and the angle at which it is shone at the object under test, but how can this principle be used for test for colour?
White light can be split into different coloured components. (This gives me an excuse to include the iconic Pink Floyd Dark Side of the Moon album cover which shows white light being split into separate coloured components by a prism!)
So to measure colour we need only to do the same thing in reverse; shine a white light at an object and measure what coloured components are reflected back, rather than the overall amount of white light.
This is the principal behind the Yellowness Index, the colour measurement system used to determine for white golds which are white enough not to require rhodium plating. (For those interested the Yellowness Index is based on ASTM standard D1925, but I do not intend to go into its details in this post.)
In fact white golds are split into three categories dependent on their colour; premium white grade, these alloys have a good white colour and do not need rhodium plating; standard grade, rhodium plating is optional for these alloys and off-white, these alloys need to be rhodium plated. Any alloys falling outside of these three grades are classed as non-white.
To take this principal of colour measurement further, if we can quantify what colour silver is then we can also use colour measurement as a way of measuring tarnish (or to put it another way, colour change with time). There is an established technique called the CIELab system which is widely used for measuring colour using a colour photo-spectrometer (simply a device which can detect the different wavelengths of light reflected into it and measure the intensity of each wavelength).
The diagram opposite shows how this system can use three values, or co-ordinates, to describe in a quantitative way any colour.
The L* co-ordinate measures the degree of brightness or lightness from 0 which is black to 100 which is white. This is also a measure of the reflectivity of the item. The a* co-ordinate measures the red-green component of colour and the b* co-ordinate measures the yellow-blue colour component.
So in this system a perfect pure white would have L* =100 and a and b =0 and also any colour can be described by these three values, L*, a* and b*.
In another blog posting I will describe how this colour measurement system can be used to quantify the degree of tarnish present on a silver alloy after testing but I hope this explains how we can assess the different colours of white metals and explains how we can measure what colour silver is.
