Boiled book
general journal junk
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idealog
In the simplest terms, boiled book (or eco printing), is where (generally on) plant materials are bundled between the papers you want to print on, and are them boiled to release the dyes found inside the plants. This process creates one-of-a-kind prints that can then be bound into a journal, used as backgrounds for other art, backgrounds for framing photographs, or framed on their own as decor. You can use other than natural materials, but you want to make sure that whatever you use isn't going to out gas and cause illness.
There's a lot of information on YouTube and elsewhere online.
Here's a list of materials that could conceivably be used for this method:
Bark, Charcoal, Coffee grounds, Fabric scraps (those that bleed colour), Feathers, Flower petals, Juice, Leaves, Lichen, Moss, Mushrooms, Natural fibres, Onion skins, Paper scraps (coloured), Peppercorns, Rusted metals, Sand (regular and coloured), Seaweed, Seeds, Soil, Spices, Tea, Threads (that bleed colour), Tissue paper (coloured), Twigs and branches, Wine