Re: A few gum things
Oh, I would like to also add that I tried that sizing suggestion in
James' book (and I saw it mentioned somewhere else, too) where you
size with gum and dichromate-- I tried that twice and couldn't get it
to work. Has anybody ever actually tried that, and does it work?
Diana and others,
I found Ryuji's very informative post of a few years back and here it is below. It may answer some questions about using gesso etc., too, for you, in why it is not a good item due to hydrophilic/hydrophobic qualities, although some get it to work quite well. So believe me, when I finally settled on glut I weighed all this information very carefully. I do think formalin works pretty darn well but in MT I could only size in the summer. I have not yet tried Kerik Kouklis' handy method of adding 6 drops of formalin per 10ml hot gelatin at time of brushing it onto paper which would probably be the ticket.
I know I have an extremely sensitive nose, but the problem with formalin was not the smell but the stinging eyes. And the broken door knob. It is possible this problem occurs more greatly in MT because of our low humidity--essentially 20% in the winter. Well, maybe summer, too. But perhaps if I had left the paper completely dry in the garage overnight it might not have outgassed so badly when it came into the house and warmed up to 70 degrees.
BTW I always keep my sizing in a thermos, and pour out what is needed at time of use. I have used a tray hardening glut solution, brushing unhardened sizing on in the house, drying, and then hardening in a 6ml 2.5% per liter water outside in the garage, and that worked well but requires two steps and an open tray.
My bottom line is that as far as toxicity, glyoxal and formalin are just as toxic and that tends to be downplayed in comparative discussions about glut.
I have tried acrylics and unhardened gelatin and chrome alum and gum/dichromate and nothing provides me with a suitably smooth sizing layer. Glyoxal and chrome alum are a tad gritty/crystalizy. Acrylic provided very spotty, uneven layers. Chrome is a bit blue of a coat and gum/di is too brown for my tastes.
In my view, an ideal hardener meets the following criteria:
(1) crosslinks gelatin to reduce swelling factor to 5x or less
(2) crosslinks gelatin reasonably fast
(3) crosslinks gelatin in wide range of conditions (robust reaction)
(4) does not compromise archival properties
(5) has relatively low toxicity and manageable risk in case of accidents
(6) has low environmental load (when inactivated prior to disposal)
(7) has reasonably long shelf life
(8) is preferrably nonvolatile
(9) is preferrably inexpensive
(10) is preferrably commercially produced and available
No known hardener meets all of them.
In my view, glut solution prepared according to my recommendation meets all but 8. On the other hand, monofunctional aldehydes (such as formaldehyde) and shorter bifunctional aldehydes (such as glyoxal and succinylaldehyde) lacks 2, 3, 8, and more arguable on 5 than glut, due to larger quantity required in practice, and therefore larger quantity of excess agent. Glyoxal is also more susceptible to aerial oxidation and shelf life, 7, is inferior. Chrome alum fails 2, 3, 6.One problem I have with chrome alum is that, the viscosity of gelatin-alum mix becomes very viscous. This is not very easy to handle. Glut can increase the viscosity somewhat if the gelatin concentration is high, but bisepoxide doesn't do this. Chrome alum works best on alkaline side of isoelectric point (5 for lime treated cattle bone gelatin, but not very well defined for acid treated pigskin gelatin, though pigskin's pI can be as high as 9), but chrome alum will precipitate and become inactive at high pH. So the window of optimal pH is actually very narrow and tricky. Reaction of chrome hardening is pretty slow. So people tend to put a lot of it, and then they'll face afterhardening. Glut and other organic compounds can be readily destroyed to make inert form before they get dumped down the drain, but chrome alum is still chromium no matter what. Sure it's not as bad as hexavalent dichromates but I generally prefer to minimize unnecessary risk. I also have a few other preferred hardening agents. Bisepoxides are another class of excellent hardening agent. You can buy this in a bottle and you can mix them in gelatin solution as-is. This is inferior on 2, and the costs a bit more than others, but meets all other criteria. People who need nonvolatile hardening agent should consider this kind of hardeners. Indeed, those who size a lot of paper to supply for months of use, this is probably the most ideal agent, as you can work indoors with no special ventilation while coating and drying the paper. Glyoxal is a well studied compound and we have good knowledge of what kind of reactions it gets involved. But because it can react in many different ways, it's nontrivial to predict what is going on in yellowing of paper. It can get oxidized, it can polymerize, it can generate corrosive acids, etc., and combination thereof in various sequences. The reaction is heavily influenced by humidity of the air, pH, presence of trace impurities, etc. and all these factors only add up on the uncertainty in the usability of the sized paper. I've tried bisepoxide compounds to harden gelatin. As I anticipated from molecular structures and chemical properties of epoxides, theyreact much more slowly than glutaraldehyde (glut). My preferred compound for this purpose was diglycidyl ether of glycerol (epox). There are several other diglycidyl or polyglycidyl ethers of glycols and similar substrates, which can be useful. A 2 to 10% of dry gelatin weight is a good starting point for the hardener dose.
The basic strategy in multilayer coating is that, you want to make small stepwise change in hydrophilicity and swelling from one layer to the next. For example, in the case of sheet films, the base is polyester, which is hydrophobic and nonswelling. The first layer of subbing is weakly hydrophilic and weakly swelling, like latex
polymers. The next layer will be more hydrophillic and more swelling, and so on. You don't want an abrupt change between adjacent layers; damage due to mechanical distortion is a lot more likely when the material is wet and swollen. This can be emulsion flaking off, reticulation, or other forms of mechanical damage. Gum is very hydrophilic. So the layer under gum shouldn't be very hydrophobic. Straight acrylic medium is a poor choice for this reason. BTW, IIRC Getty Museum is running acrylic longevity tests... I guess > we'll have much more info about this issue soon. Archival property of acrylic medium with pigments is a different story from using it as a part of photographic material, though. I'm less worried about acrylic medium itself, but I am very leery of accepting titanium white as a part of sizing layer without doing extensive tests.
Maybe until 1970 formaldehyde, sometimes mixed with chrome alum, was used as a hardener but generally it was replaced with glut and other hardeners. So I don't know of good modern examples to begin with... But if you look at old literature, formaldehyde is generally used at a rate of 1 to 1.5g per 100g of dry gelatin. Formaldehyde hardened material is generally allowed to sit for two full weeks before use. (Chrome alum is also used at 1 to 2g per 100g dry gelatin, and similarly, two to three weeks are allowed before use.) If more hardener is added, gelatin will harden sooner, but then gelatin will overharden afterward, and the quality won't be stable
over time. Plus, excess aldehyde agent may evaporate, or stay in paper and cause problems. Loris's prototype formula uses 20g formaldehyde per 100g of dry gelatin, and this is a lot of hardener. More than 90% of this amount of hardener will be wasted in some form. I'd cut this to 2g or less and allow plenty of time for hardening. In order to buffer the pH to alkaline region to optimize hardening rate, borax can be used as suggested. Also, besides borax, disodium phosphate or triethanolamine may be used. TEA also works as a mild plasticizer. (Ammonia, primary amines, etc. should be avoided because they'll react with the hardener, by the same mechanism as gelatin crosslinking!) Gelatin solution's pH varies depending on the gelatin. So the amount of borax can't be determined without experimenting with the particular gelatin. However, 0.2 to 1% solution of borax is probably a good starting point.