Ballast turning green

[threelink]

I recently ballasted a length of track with a commercial ballast and dilute PVA applied in the conventional way. The ballast pack was at least 20 years old. The PVA was fresh. The ballast was grey, apparently crushed stone of some sort (unfortunately the bag had lost its label so I do not know what stone was used). To my surprise, when the PVA set, the ballast was a green/blue colour - hopelessly unrealistic. I had to paint it grey. I have never come across this before. Is there a ballasting expert out there who can throw some light on this?

[Rog RJ]

I've read of this happening a few times. Don't know why it happens though.

[DarkRedCape]

Was it Woodland Scenics ballast by any chance? I’ve read that people have had issues with discolouration when using PVA with this brand due to some PVA being slightly acidic and reacting to the ballast and minerals.

It can also happen with real ballast, as in crushed rock or coal.

You can get neutral not acidic PVA glue which may prevent ballast from changing colours.

However I will add that the actual reason for this happening is something that is debated. Some say it’s the glue, others say it’s the ballast you use.

[Bulleidboy]

I have also read about this. I have used Woodland Scenics ballast throughout my layout, and not had that problem with the ballast. I also recall reading that some makes of PVA could effect the final outcome - Evo Stik was mentioned. When laying my track and ballasting, there was a green tinge on a point, where the rails sat in the chairs. This was back in 2013/14 and it certainly is not there now.

[96RAF]

Could be the ballast has some organic residue that is going mouldy, or that the water in the PVA is releasing a hidden dye in the ballast, or as stated it’s a UV effect on the PVA, or it’s a chemical reaction twixt ballast and PVA. Either way it’s a pest and needs some fettling.

[ModelerXYZ]

Its the grass growing through it. Tell your cheif Pway engineer to get some weedkiller.

Seriously I don't know much about it. I hate balasting track because getting the ballast off the sleepers is so fiddly.

XYZ

[threelink]

Thanks all for your helpful comments. It looks as though it is some form of random chemical reaction between ballast and PVA. I do not know the make of ballast because the bag had lost its label. If it happens again, I'll just have to paint it.

[Rallymatt]

I have only heard of it with real stone ballast and it being a chemical reaction between the minerals in the stone and what’s in PVA and water. The non stone stuff is crushed nut (possibly walnut) shells and this apparently doesn’t react the same way although there is allergy warning on the packet!

[threelink]

@Rallymatt. Thanks for the info. I'll look for some non-stone ballast. The weird thing is that on the prototype crushed blast furnace [word deemed unacceptable but meaning one of the furnace by-products containing impurities from the iron ore mixed with molten limestone] was often used as ballast. When freshly broken it displays a green/blue glass like appearance, very closely resembling the nasty green of the ballast I used. The difference is that the [word deemed unacceptable] weathers to the overall grey that we associate with ballast, whereas the nasty green stuff on my layout stayed green.

[Potterton]

Back in the 90's I experienced this same problem using crushed stone and PVA which was perplexing as I was using the same method I had used previously albeit probably with a different brand of PVA to the one I used that time. When I switched to a different one, the problem was resolved so I assumed it was the PVA at fault. I can't be 100% of the brand, (it was in a red and white container), but I'm 90% sure it was Evostick as Bulleidboy said above. (Apologies to the manufacturer if I'm wrong.)

[Set in the West]

With how old the ballast is, something could've developed in it to make it turn that. The PVA could have contributed to that as well. I'm not a modeller like everyone else but that sounds like a good theory to me.

-Set In The West (LittleOldPug)

[What About The Bee]

On another project, I required casien glue. This isn't sold commercially anymore, but the Forest Products Laboratory (US Govt Agency) did preserve the recipe

Off I went, and purchased the requisite chemicals. As this was a marine application, a small dose of copper chromate was required. This is to be added last.

Imagine my surprise, upon adding the copper chromate to the pale grey glue, the entire mass turned bright purple!! Copper chromate is greenish. After drying, the glue eventually turned brown.

There is little doubt in my mind, ThreeLink, that your green ballast is the result of a chemical reaction betwixt your glue and stone.

Bee