Selecting The Right Platinumware for your Automatic Fluxer
In relation to automated fluxers, platinumware refers the crucibles and moulds. Selecting the best models for your specific application can be critical to your applications. With platinumware priced by weight, and platinum being relatively expensive, choosing the wrong model can be a costly mistake.
Platinum Alloy 95%Pt/5%Au
Platinum melts at 1768°C and does not oxidise under heat. It also does not react with lithium borate flux so will not contaminate your sample. Early glass makers found that the addition of gold to platinum improved its non-wetting characteristics and mechanical strength. Unfortunately, as gold content increased phase instability increased and the melting point decreased.
As a consequence, 5% addition of gold became the standard for glass makers as well as fusion machines for XRF sample preparation.
Dispersion Hardened Platinumware – DPH
DPH platinumware has minute amounts of zirconium oxide (ZrO2) or zirconia added. The zirconia grains are distributed in the Pt95%/5%Au alloy and reside in the grain boundaries pinning them, thus retarding grain growth and internal crystallisation. From an operators perspective, this slows down the onset of embrittlement, increasing the lifespan of your platinumware.
The Cost of Platinum
Platinum is expensive, but is also very durable. Over its life, platinumware can easily be worth less than 1 USD per fusion, and given its inertness and reliability, it is well worth it.
Platinumware is also fully recyclable, so old platinumware can be traded in against new, thus reducing outlay on new platinumware.
Crucibles are the tumbler-shaped receptacles to which you add the flux, sample and any other optional additives. The material they are made from needs to be inert so that it does not contaminate the material being analysed, especially given that the flux melts and dissolved the materials in the crucible at high temperatures.
Furthermore, the internal surface of the crucibles needs to be kept smooth to prevent the melt from sticking to it during pouring.
The mould is a saucer-shaped container into which the molten sample/glass is poured. Again, an inert composition is imperative to avoid contaminating the sample prior to XRF analysis.
It has a taper in it to facilitate release of the solidified bead and the upper mould surface needs to be kept polished to ensure the glass bead surface is as smooth and flat as possible to avoid any inaccuracies from an uneven sample surface.
The mould is preheated prior to pouring to avoid thermal shocking the sample and potential crystallisation.
Lighter vs. Heavier
While the mould diameter will be dictated by the sample size required by the XRF spectrometer you are using the wall thickness and hence the weight of the mould can often be chosen by the client.
While crucible selection is not critical, the general rule is, thinner-walled crucibles are suited to lower throughput applications and careful operators. Thicker-walled crucibles are better suited to high throughput operations and “less careful operators”.
Standard-shaped crucibles with near parallel walls are less expensive. However, crucibles with flared walls are structurally more rigid, our better, but do cost a bit more.
It has been the common belief that small diameter moulds (e.g. 30 and 32mm) are more rigid and less prone to warping. However, if you do not use a non-wetting agent (NWA) such as LiBr, LiI or NH4I, then larger, thicker-walled moulds should be used as they can sustain more re-polishing.
Deeper moulds allow you to melt more material which tends to produce a better bead shape and consistency and also helps to ensure “infinite thickness” for heavier elements. That said, if you use less sample and flux and avoid using NWA, then shallower moulds are recommended. Furthermore, avoiding the use of NWA can extend the life of platinum moulds by reducing the need for polishing.