Hello Andrea,

as indicated by Simpson in the publication you mention, the definition of the redox number is derived from empirical data based on the measured effect of a given raw material on the batch (reducing or oxidizing effect). There is no general "rule of thumb" which allows for calculating the redox value for each raw materials. It has to be determined experimentally for each one of them. Of course assumptions or trends can be derived (you can know that a given raw material will be rather oxidizing or rather reducing), but to derive the exact value of the contribution of a given raw material on the batch redox number, it requires experiments.

The contribution on the Simpson redox number has been empirically determined for the main raw materials employed in the glass industry and some types of cullet, but there is still quite a large list of raw materials for which no redox number have not been derived.

Note also that these values are empirical, and that some glass producers sometimes adjust the values used for a given raw material, based on their observation (their own empirical observations). Also, last but not least, the organic contamination content in the batch has a very strong (reducing) effect on the batch redox state, but it is very hard to quantify, as the exact amount of contamination is not always easy to determine, and different types of contamination can have different influences on the redox (i.e. contamination of traces of sugar will not have the same effect as a similar amount of contamination by traces of paper, if you take an example of addition of cullet still contaminated by a bit of the product it contained or by a label which was on top of the glass).

I hope this helps. If you have further questions please let me know. Best regards,

Mathieu

Thank you Mathieu for you long reply. You gave me a very good explanation. Basically I should calculate the chemical oxygen demand of the raw materials. Right?

But if I make and iron doped soda lime silicate glass from only high purity oxides and carbonates, then the redox number should be zero and not influence the redox. If I do so, I should expect only an influence of the furnace atmosphere or of iron oxide chosen. Please correct me if I am wrong.

thanks

Hi Andrea,

Redox in glass in one very broad (and one of my favorite) topic in glass production. There are a lot of parameters to take into account, and as you point out indeed the melting atmosphere also has a strong influence. There are also many ways to characterize the redox, such as the Simpson number and the COD of the batch, the pO2 in the melt... The more you know about these, the more you will be able to actually quantify the effect of different parameters (raw materials, contaminations, atmosphere...). For a SLS glass melted from pure SiO2, limestone and soda ash only, technically the simpson redox number should be 0. A measure of the COD could inform you on the presence or not of organic contamination (often the case if mineral raw materials used), which would then affect your redox. Also, if you add a fining agent, this will influence the simpson redox number. For an iron-doped SLS glass, the source of iron will definitely have an influence on redox (i.e. more oxidizing if you introduce it as Fe2O3, more oxidizing if you were to introduce it via pyrite FeS2 for instance). Also here consider that iron may be present as impurity in your raw materials (especially in the sand!).

If you want to continue this discussion and/or have more questions, please do not hesitate to send me an email at: [email protected]

Best regards,

Mathieu

Very helpful discussion, Many thanks, and indeed I do hope you can continue this discussion on the website.