Observations on the determination of gold in gold alloys cupellation method (Fire Assay) Dippal Manchanda MSc CSci CChem FRSC Technical Director & Chief Assayer WHAT WE WANT TO ACHIEVE TODAY 1. Understand the technicalities behind the fire assay process and how these affect the accuracy of your results. 2. Determine what you need to ask any laboratory before sending them your test samples so that you can be certain that the results you receive are fit for your purpose. 3. A brief insight into the expertise of Assay Office Birmingham in these two very important issues. 1
WHAT WE WANT TO ACHIEVE TODAY 1. Understanding the technicalities behind each and every step followed during the fire assay process and how these affect the accuracy of the results. HOW DO WE KNOW RESULTS ARE ACCURATE? We run minimum two reference samples of known purity usually of 99.99% and subject them to the same treatment as we give to the other test samples. The matching trend in the results of reference samples indicates if the fire assay operation has been performed satisfactorily and results are accurate. Reference samples will show matching results only if the correct process has been observed and if the factors affecting accuracy of the results are regularly controlled. So what are the factors which affect the accuracy of our results? I will now take you through the fire assay process and discuss these factors one by one. 2
WEIGHING - FACTORS AFFECTING ACCURACY BALANCE SENSITIVITY 1. Calibrate balance before use. It is advisable to use a balance of 0.001mg sensitivity for optimum accuracy. Why use a balance of 0.001mg sensitivity? 100.006 mg to 100.009 mg on micro balance = 100.01 mg on macro balance. SAMPLE SIZE 2. Sample size is another factor which affects the accuracy of results. Use a higher sample weight for the optimum accuracy of results. Why is a higher sample weight preferred? SAMPLE SIZE VERSUS ACCURACY Initial weight (mg) Fineness (ppt) Final weight. (mg) Final weight (mg) [systematic error of 0.1 mg] Fineness after error (ppt) Difference in fineness (True Actual) 30 375.0 11.25 11.15 371.7-3.3 ppt 55 375.0 20.62 20.52 373.1-1.9 ppt 120 375.0 45.00 44.90 374.1-0.9 ppt 230 375.0 86.25 86.15 374.6-0.4 ppt 500 375.0 187.5 187.4 374.8-0.2 ppt NOTE: Make sure your preferred laboratory is using higher weight of sample if optimum accuracy is important to you. Provide sample size accordingly. 3
WEIGHING - FACTORS AFFECTING ACCURACY 3. Add silver equivalent to 2 to 3 times the weight of fine gold present (Standard Industry Practice) What is the optimum gold/silver ratio? By experiment it has been found that the most accurate results are obtained by using a ratio of silver to gold of about 2.17 to 1. Try in your Laboratory and see the difference yourself. WEIGHING - FACTORS AFFECTING ACCURACY 4. Where the sample has a low copper content, a small amount of pure copper is also added. Why is copper added? It prevents spitting or spurting due to the escape of Oxygen from the Au-Ag button and also Increases the malleability of the button for rolling. 4
CUPELLATION FACTORS AFFECTING ACCURACY 5. Place the assay and the proof assay samples on cupels in the cupellation furnace. What are the main requirements of a cupellation furnace? PARTING FACTORS AFFECTING ACCURACY 6. Carry out parting in 1.2/1.3 specific gravity nitric acid respectively. Why do cornets sometimes break into fine powder during parting? The probable causes for the above phenomenon are i. The presence ofan excessive quantity ofsilver in the cornet; ii. Presence of lead i.e. Incomplete cupellation; iii. High concentration of acid; iv. Parting started in cold acid; v. Parting started in vigorously boiling acid; vi. If platinum is present in more than 5% of the weight of the bead; vii. Cornets that contains impurities to an extent greater than 1% usually. If above factors are present above the tolerable level but below the threshold level then it will cause uneven surcharge and in turn variable results. 5
CALCULATION - FACTORS AFFECTING ACCURACY 7. Calculate the results as below (formula proposed in NEW ISO 11426 standard): Formula: Mean weight of proof(s) weighed out initially Factor = -------------------------------------------------------------------------------X 1000 Mean of cornet weight of proof(s) final weight of sample Fineness (ppt) = ----------------------------------------------------X Factor Initial weight of sample (ppt) = parts per thousand. ATTENTION This formula will not provide accurate results if you have included additional gold from the out set, particularly when alloy `contains high Pt/Pd or is low in gold etc. YOU could be losing money. ASSAY OFFICE BIRMINGHAM FORMULA - WHEN ADDITIONAL GOLD IS INCLUDED 8. When additional gold is included, particularly when analysing gold with a high platinum/palladium content, dental alloys or low purity gold use the following Assay Office Birmingham formula : (M2 F) + (mean T / Pcm) * M2 W (Au) = -------------------------------------------------------------------X 1000. M1 W (Au) = gold content (parts per thousand), F = (fine gold mass added*fineness of fine gold mass)/1000, T = surcharge ( T = Psm Pcm), Psm = Initial weight of proof sample (Sample Mass), Pcm = Final weight of proof (Cornet Mass), M1 = Sample mass (mg), M2 = Cornet mass of the sample 6
WHAT WE WANT TO ACHIEVE TODAY 2. What you need to ask any laboratory before sending them your test samples so that you can be certain that the results you receive are fit for your purpose. 9 THINGS YOU SHOULD KNOW ABOUT YOUR LABORATORY Accuracy of assay results depends upon the quality control measures employed by your preferred Laboratory. If optimum accuracy is your concern then ask the following questions before sending your samples - I. Is your preferred Laboratory using the micro balances and are they calibrated and verified regularly by authorised personnel? II. III. In-homogeneity in your test samples can only be measured if replicate sample portions are analysed. Ensure that replicate sample portions are analysed and an acceptable precision must be obtained before reporting. Ask for their results reporting policy. If SRM are regularly analysed and control chart are plotted to verify compliance. Please note that Standard Reference Materials (SRM) are analysed to verify the accuracy of the analytical equipment/procedures that are used in the analytical laboratory. 7
9 THINGS YOU SHOULD KNOW ABOUT YOUR LABORATORY IV. If cupellation, annealing furnaces and exhaust systems are regularly serviced by qualified personnel and kept in good working order. V. If test results are periodically cross-checked for accuracy by an alternative method where possible, or by regular participation in Proficiency Testing Scheme/s. VI. VII. Submit previously analysed samples for analysis to check repeatability of the procedure and the effectiveness of the analyst. Always ask your laboratory to report measurement of uncertainty along with the results to verify if the results are fit for your requirements. 9 THINGS YOU SHOULD KNOW ABOUT YOUR LABORATORY VIII. Is their test procedure independently accredited and witnessed on a regular basis to ensure that analytical procedures and protocols continue to be followed as documented by the laboratory. IX. Finally, cost and turnaround time. 8
WHAT WE WANT TO ACHIEVE TODAY 3. A brief insight into the expertise of Assay Office Birmingham in these two very important issues. Dippal Manchanda MSc CSci CChem FRSC Technical Director Anchorcert Analytical Assay Office Birmingham P.O. Box 151 BIRMINGHAM B3 1SB ENGLAND Dir: +44 (0)121 262 1052 Dir: +44 (0)121 233 4209 Fax: +44 (0)121 236 3228 Mob: +44 (0)7595709714 www.anchorcertanalytical.com dippal.manchanda@theassayoffice.co.uk 9