In the botanical printing groups online, you will read the term 'iron blanket' often. And I would like to explain to you what it is and how you can use it.
In short: an iron blanket is a piece of fabric soaked in a ferrous sulfate solution.
The ferrous sulfate reacts to tannins in the leaves, and sometimes tannins in the target piece. The target piece is the fabric we want to have printed as our final result.
Ferrous sulfate and tannins always create different shades of gray to black, depending very much on the type of tannins used.
The classic way of creating blacks and other dark backgrounds in botanical printing is the use of tannins in combination with ferrous sulfate (often called; iron). It's quick, easy, and does not contain too many steps which makes it a popular choice, but it must be said that the ferrous sulfate will inevitably weaken your fibers over time. One extra reason to use it with good measuring and not just in the 'rusty nails bucket', where we have no real control over the amounts of ferrous sulfate.
How to make an iron blanket
Take a piece of cotton or linen that is the same size as your target fabric and weigh it, this will be your iron blanket.
After you have weighed your blanket material dry, and you have noted the weight on a piece of paper, you now have to soak it in cold water. This is called 'wetting out'. Wetting out makes sure your uptake of ferrous sulfate will be even and that will avoid blotches and stains.
Weigh 1% WOF Ferrous sulfate and dilute it with plenty of warm water in a bucket.
1% Weight of Fiber means that for every 100 grams of dry fabric, you will need one gram of ferrous sulfate.
Put your pre-wetted fabric in the new bucket of ferrous sulfate solution and soak the 'blanket' for about 10 minutes, working the fabric well in the water so to create an even distribution. Failing to do this will create dark spots in your final project!
After soaking, rinse the fabric and wring out most of the water, leaving the fabric.
Now your iron blanket is ready for further use.
There are different ways of using this iron blanket.
The first:
Spread your target piece (mordanted with whatever you want.
Add leaves
Put the iron blanket on top of this
Add a barrier
Roll the bundle.
This method creates black outlines on your tannin-rich leaves. We love this method for simple and quick botanical prints on silk.
The second way to utilize an iron blanket: is to create a different colored background on your botanical print.
Mordant your project piece (silk or wool) with the tannin of your choice at 10%WOF.
Take a piece of cotton or linen that is the same size as your target fabric and weigh it, this will be your iron blanket.
Measure 1% WOF Ferrous sulfate and dilute it with plenty of warm water in a bucket. Soak the 'blanket' for about 10 minutes, working the fabric well in the water so to create an even distribution. Failing to do this will create dark spots in your final project!
Take the silk or wool out of your tannin bath and rinse it so there will be no powder residue left. Wring it and spread it out on your work surface.
Take your leaves and lay them down side facing the target fabric.
Take the blanket out of the ferrous sulfate bucket and wring well.
Lay the iron blanket on top of your leaves. You now have a sandwich of target fabric, leaves, and an iron blanket. It is a good idea to now add a barrier of brown paper, or recycled plastic.
Roll up the entire sandwich using a dowel and tie the 'sausage' well using rope or strips of cotton.
Steam or boil for 90 minutes at least.
The iron will create a dark background where there are no leaves.
