Dear Forum:
I make 8x10 ambrotypes out in the field. I use Coffer's Old Workhorse, a Ferrous/glacial/grain alc developer, and fix with Sodium Thio.

I varnish with Soluvar Glossy Varnish mixed with Paint Thinner (1:1). This has been great to use for ease of mixing, and I have gotten good results generally, until recently.

I do not heat the plate, just pour, blot excess and leave in dish rack to drain/dry in a dust-protected space.

Question: what might be causing the very speckled surface after varnish dries? The speckling does not interfere significantly when viewing the plate, but it does not seem right to have this textured surface and, at odd angles to the plate, it is very distracting.

I suspect the following:
- some kind of dirt embedding onto the surface the collodion at some point when wet (though I cannot see a texture to the collodion when dry, before pouring varnish)
- some reaction of varnish to surface of collodion
- as it tends to be more pronounced in more humid conditions, possibly humidity on plate.

Any thoughts?

Thanks,
Mike K.

It's probably dust that's has either accumulated in the pour bottle from pouring it on and off plates, or dust in the air is settling on the surface as they dry. You could try setting the rack into a cardboard box and shut the lid on the box once you finish pouring...

Some people also have success eliminating airborne dust by misting the air with a water sprayer before they varnish. As well, anything you can do to speed up the drying process would probably help... Maybe putting them in a slightly warm oven at the very lowest setting for a little while to get things kick-started.

I use sandarac, so dry-time isn't an issue for me.

Cheers!

Andy

Thanks Andy.

Unfortunately it happens with newly mixed varnish in new pour bottles as much as when I pour re-used filtered varnish.

And I do isolate the drying rack in an enclosed space dedicated to drying plates.

Weird part about it is that the speckles do not vary much in terms of size and frequency, as I expect dust would.

There may be something on the plates to begin with before you varnish as well... Do you wipe you plates with anything before you varnish?

Could there be dust settling on to the plate before varnishing. I use a very soft brush to lightly brush them and it helps a lot. I do work outdoors in a tent a lot but it seems that no matter how short the dry time is dust will end up on the plate.

Im not sure about Solavar but do you know that Mark Osterman recently posted something on his FB page about archival protective sprays being harmful to the collodion layer, He posted images taken with a microscope to show this.

No offense intended but spray on stuff might make it easier but I dont think you can beat what they used back them. Why not try some Sandarac Varnish.

Thanks Joboo and Robert.

I don't wipe the plates before varnishing but I do hit them with compressed air. It is hard to tell if I am altering the collodion itself with the varnish or if it is dust. As I said earlier, I am less inclined to think it dust due to the way it looks the same across the plate.

Also, I am pouring the soluvar, not spraying, but of course archival quality is a concern. It seems that if it is used to protect paintings, as Soluvar is, it should also protect the plates, but I will look for Mark's FB posting.

Thanks,
Mike

Ive also used a rocket blower to blow the plates off. Is there any chance some liquid is coming out of the compressed air?

Turns out the thinner seems to the problem. I did a pour without the thinner and in a space without any thinner fumes from past pours and upon drying the plate is clear of speckles.
Thank you all for your input and attention.
M

Ah yes, that should be the problem. The root cause is the industrial formulation of the mineral spirits. They are defined by a lower and higher boiling point (distillation tower stuff). Thus mineral spirits is not pure and does vary not only within a brand but between brands.

Companies like Shell can give you more information about their products. In general the mineral spirits listed with the shortest range of boiling points and the lower boiling points are more "pure" and volatile which is what you want.

Stoddard's solvent is a well known mineral spirits and will always function better than cheaper blends.

The specks are caused by slower to evaporate mineral spirits which also have lower solvency for the Soluvar varnish. When they finally evaporate, they leave the bubble shell behind.

MatadorMac

Mark MacKenzie, M.A.C.
Art Conservator
Alcalde, New Mexico


a.ka.a. Matador Mac, SASS Duellist
But would like to be a wet-plate practioner.