HP-97 Case Yellowing


Has anyone been successful in reversing the case yellowing of
HP-97 Calculators?
I suspect the yellowing is due to a chemical change in the plastic so a possible thought would be to put it in a bag with some (yet to be identified) odoriferous agent that would penetrate and reverse the action.
Has anyone explored this or some other approach? All thoughts and ideas will be appreciated -- Mike


You mean that yellow color is not original? ;-)


Usually, the process of yellowing (UV rays destroying macro molecules, aka plastic) is not reversible. You could try H2O2 (should be easily obtainable), which has a bleaching effect. Remember that the 97 case was not white though - so a white case is probably not what you want. In short: just leave it untouched :)


I have considerable experience with many PVC slide rules (e.g. Aristo and Faber-Castell). Some of them had yellowed. As an experiment, I placed them exposed to UV (sunlight). Voila, the yellowing disappeared! As an architect (retired) I have also watched white PVC facade elements and window frames. After years, no yellowing observed except, in some cases, on the inside! All of which leads me to believe that UV isn't necessarily the cause, possibly only the effect of certain indoor climates or conditions(?)


PVC slide rules

I am not sure whether the HPs were made of PVC. Cases (of HP21 and HP29C) look more like polypropylene or something alike to me.

Well, perhaps not quite PP, but PVC neither, I think.


Isn't the HP calculator cases built on ABS plastic?

Best regards to all,



the Voyagers (11c) seem to be made of polystyrene...


I remember reading somewhere (the Hp97 manual ?) that the case was made from a high grade ABS plastic of the same type as used in crash helmets. I think the same was true of the SPICE series, and my HP10C and 11C look like they were made of the same stuff..

It is certainly tough enough, I was once careless enough to allow my HP32E to slide off the desk I was working and out of a first (US second) floor window. I survived a 14' fall onto a concrete surface, suffering only a reasonably minor chip to the bottom left hand corner of the case where it hit the ground - in all other respects it didn't seem to be affected at all (I don't think I even lost the contents of the X-Reg!). I think I aged more than it did!

I still use it today, 24 years later.

Mike T.


Hi Mike,

I succesfully revert severely yellowed HP97 cases by leaving them for 1 to 2 days in a chlorine solution. Here in Holland we have something for cleaning your toilet based on chlorine (called Glorix Bleek, dikste ooit). The stuff is quit a thick fluid substance (high viscosity) if you know what I mean.

I put 1 1/2 liter (3 bottles) in a stainless steel pan and let it soak.

After 1 day it lookes likes original and has a very "clean smell".

As far as I have seen no impact on structure etc. of the plastic itself.

I do not heat the solution to speed up the proces, for I am afraid the plastic will deform otherwise.
With this there is no need anymore to use abbrasive polish etc.

Succes and let me know how it worked for you.




Could you kindly check the label of "Glorix Bleek" and list the ingredients? I will investigate what products are available in the USA that most closely match up. And, thank you very much for your post! --

Warmest regards,

Mike DiGirolamo






Sodium Hypochlorite

Sodium Chloride

Cocamine Oxide

Sodium Hydroxide

Sodium Laurate


Sodium Silicate

(source Unilever Nl)

Personal comment: Do NOT swallow!

Have a nice day!


Edited: 4 Oct 2005, 1:51 a.m.




Yes, that is the correct label and ingredients,

Is is basic chlorine stuff used to clean the toilet. Only with some trick to make it a bit more syrup like so it will stream away directly.

The chlorine is the bleaching part.

However be carefull do not overdo do it for it will in that case belach some parts of the plastic almost white and some and the desired colour. I am not sure why that is, maybe that the plastic not fully homogenious ?

Also make sure you submerge it completely otherwise you get some parts more bleached that the others if you do match timng very carefully. (I have to confess I found this out the first time).



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