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Color difference
Hello colleagues. I am developing a balm type whitening cream that uses iron oxides as colorant. After multiple trials of the same formula, I sometimes obtain a little variation in color. What can be the cause of this?
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7 Replies
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Iron oxide pigments are so strong in colouring power that I suspect it’s simply a measuring problem.
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Unlike dyes, which produce the same color every time the same amount of dye from the same lot is added to a batch, iron oxides are pigments, which are not soluble, so the color comes from suspended oxide particles, rather than from solutions of individual dye molecules.
The intensity of the color of an iron oxide suspension depends largely on the number of iron oxide particles suspended in the product. For any given weight of iron oxide pigment, the color will intensify as the iron oxide particles are ground finer and made more numerous.The issue is complicated by the fact that iron oxide particles agglomerate (stick to each other) readily, so not only do you have to grind/de-agglomerate the particles, you also have to make sure that they do not re-agglomerate in your product.Pigment wetting can also be an issue - a pigment particle that is completely wetted will appear darker/more intense than one that still has air bubbles trapped on its surface.In order to get consistent batch-to-batch color using iron oxide pigments, you have to do four things:1) weigh your pigments very precisely2) grind/mill/homogenize them into your batch exactly the same way every time3) Use the right amount of wetting agents, suspending agents and anti-agglomeration ingredients4) adjust your pigment amounts every time you get a new lot of pigments -
@Belassi maybe this is one factor, since our weighing scales have a precision of 0.005 g. I’ll have to check on that.
@Bobzchemist is it acceptable to establish a Light-Standard-Dark shade? -
I’m sorry, but I don’t quite understand. Are you talking about QC standards?
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Yes @Bobzchemist. Like in labels and packaging, they establish a LSD standard. Can this also be applied to bulk products?
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Yes, absolutely. This is typical in situations where products are not being matched with a colorimeter, and also where the QC department does not really have trained color evaluators.
In this kind of situation, you have to allow for the fact that the untrained eye can’t tell when a match is close enough. The procedure I’ve used in the past uses microscope slides. You put 1 drop of each standard on its own slide. Then, you put a drop of the batch on both sides of each of the standards (this eliminates the confusion of “is the standard on the right or the left” - the standard is always in the middle) Then, you cover each of the slides with another slide, and evaluate the color - if the batch is darker than the dark standard, or lighter than the light standard, it fails.With enough experience, color evaluators won’t need the dark and light standards anymore - but in some cases, acquiring this level of experience takes years. -
@Bobzchemist Absolutely agree with you as I have seen in our lab where it takes years to develop that special ‘color eye’.
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