The Good Delivery List (GDL) Refiners have most certainly kept the GDL team busy this month. There are nine Good Delivery applications in process, Proactive Monitoring (PAM) is in full swing, and several refiners are upgrading their production equipment, which is always good to hear. There have also been a couple of relocations and ownership changes.
The Proficiency Testing Scheme 2022 was launched with a registration deadline of 23 September 2021.
Proactive Monitoring (PAM)
In the previous communication, the results of gold data from the PAM and Good Delivery List (GDL) application assay results from April 2005 to May 2019 were discussed. In this article, the results of the silver data will be examined.
A key indicator is the difference or divergence between the assay laboratory reported fineness values and LBMA reference fineness values for the same test samples. LBMA values come from the Referee Laboratories (RL). Each RL produced a set of test samples and verified the homogeneity by assaying many sub-samples. LBMA conducted blind cross checks of their assays in comparison to two or more other RLs to arrive at a reference assay value.
Analysing the Assay Divergences
The divergence is calculated by:
∆ = LabAssay – LBMA
For silver samples < 999.5‰, there were 253 assays compiled. Overall, 97% of assay divergences were within ± 0.15‰ which is the limit for this material according to the GDL Rules. A frequency distribution histogram is shown in Figure 1. The distribution peak is at 0.00‰ divergence and the divergences are normally distributed. There is very slight tailing towards the negative divergence side which indicates some laboratory assays are low biased.
Figure 1. Divergence frequency distribution histogram for Ag < 999.5‰ for data collected from April 2005 to May 2019.
For higher purity silver > 999.5‰, 277 assays were reviewed. Overall, 93% of assay divergences were within ± 0.05‰ which is the limit for this material according to the GDL Rules. A frequency distribution histogram is shown in Figure 2. The distribution peak is centered at 0.00‰ divergence and the divergence distribution is slightly skewed towards the negative divergence side. This indicates that there is a slight tendency for low bias reporting for some laboratories.
Figure 2. Divergence frequency distribution histogram for Ag > 999.5‰ for data collected from April 2005 to May 2019.