While there are several ways of sizing art paper (and I've tried a few with success), here I am trying gelatin hardened with ammonium dichromate.
I'm basing this on two sources, both highly recommended
The Carbon Print: The History, Theory, and Practice of Carbon Transfer Printing. Sandy King & John Lockhart (2017)
The Book of Alternative Photographic Processes (3rd Ed.) Christopher James ISBN 978-285-09831-7 (2016)
First, let's look at
Gelatin is a protein - well, it's bits of protein that have been pulled apart in the manufacturing process - and it is used widely in the food industry. And it's fed to kids as jelly. So it is about as safe as it gets. So no Safety Data Sheet is needed. Though I wouldn't mix my photographic gelatin with the kitchen variety, just in case.
Ammonium dichromate, though is poisonous and needs to be treated with respect - here is the Safety Data Sheet that came with mine. You will see it is poisonous if swallowed and carcinogenic if you breathe it in (wearing a mask when handling the powder is advised). And it's recommended not to get it on your skin, so wear gloves. If you follow these you should be safe, I know many people who have used ammonium dichromate (and potassium dichromate) for many years without problem.
Ok, I'm going to cheat - the chemistry involved is quite both complex and variable - the gelatin polypeptide chains link covalently in a number of ways and at several different points on their chains. The result is it becomes less soluble and the reduction in solubility is dependent on the amount of UV light it experiences - the relationship is very nearly linear making it ideal for high-quality photographs
I can't resist some form of equation, so here is the chemical equation of ammonium dichromate
I use 240 Bloom gelatin, which I got off the internet, and from which I will make a 5% m/v solution. I prefer to make up just enough gelatin to size the paper I need. I like to use large sheets of art paper that comes in Imperial sizes; 760mm x 560mm is my favourite. I tend to size several sheets at once. I use my own 'ready reckoner' (an Excel spreadsheet I carry on my phone) and this paper needs 25ml for two coats on one sheet.
I will do the calculations for 1,000ml (1 litre), but the proportions can be scaled down. 5% m/v requires 50g of gelatin
For thinner paper I often size both sides because it greatly reduces curling.
It is recommended that before you add all the water you start by adding the gelatin to a smaller amount and allow it to bloom.
This is the technique I am going to follow:
- The Gelatin Bloom Add the full 50g of gelatin to 300ml of cold de-ionised water and leave to bloom for 20 minutes. You will see the gelatin expand and soak up all the water. Then add the remaining 700ml of water and stir in before heating.
- Heating the gelatin I use a simple double boiler technique, which involves a pan of water on the kitchen stove, with a suitable container that holds the gelatin solution without spilling. The gelatin will dissolve completely at 50°C and the trick is to measure the temperature using a kitchen thermometer. I heat the pan and never let the water boil and slowly watch the temperature rise to 50°C. The trick is to keep it at that temperature while you coat. (By the way, normally when making jelly I use the microwave to great effect, I can't see why it could not be used here with care).
- Remove micro-bubbles Add 10ml of 100% isopropyl alcohol to help remove bubbles
- Soak the paper I am going to try soaking the paper, Saunders Waterford 300gsm HP, for 2 minutes and blot dry.
- Adding dichromate 10ml of 5% m/v ammonium dichromate solution is added just before coating and stirred in thoroughly. Add a squirt of 100% isopropyl alcohol - I use a spray can
- First Coat I prefer not to pour out the gelatin for sizing, instead I sue a foam brush (40mm for a large sheet of paper), which I make sure is damp, but not dripping, before first use. I like to dip and paint quickly but not so fast as to cause bubbles. With the dichromate added the areas that have been sized are clear on the first coat. Immediately after coating I hang the sheet up and give a first coat to all the other sheets.
- Second Coat I don't wait for the first coat to dry - the gelatin will set quickly and you can do the second coat almost immediately. It can be a little difficult to see what has been coated and what has not on a second coat so I set up several lights around the coating table as well as the overhead light and look for reflections from the wet areas to guide me. It does not really matter if the light sources are not tungsten filaments (lacking in UV) because you want the gelatin to be fully hardened anyway
- Drying The papers then all get hung up to dry - I normally leave them for a day. (Time to clean up and wait, remember to use gloves now that the dichromate has been added)
- Hardening I then use my own UV light box (description to be added in another blog). I use 20 minutes for each sheet individually
I won't attempt to clear or wash the papers, the dichromate will wash out later - or, if necessary, can be cleared with sodium metabisulphite
5% Ammonium Dichromate Solution
I use a saturated ammonium dichromate solution for my gum printing, which is 28% m/v
I store my solutions in 500ml wide-mouthed bottles
Calculating the amount of 28% m/v dichromate solution is need for 500ml:
- Mass of dichromate 500ml of 5% m/v contains 25g of ammonium dichromate
- Volume of 28% m/v dichromate needed There is 25g of dichromate in 89ml
Into an empty 50ml bottle I add 89ml of ammonium dichromate to which I then fill to 500ml with deionised water
- Compared to gelatin sizing hardened with glyoxal this was a simpler process. However, I prefer the results of the gelatin / glyoxal size
- I have used the dichromate-hardened gelatin size with gum printing and it is a poorer more blotchy size