There are two methods we use to get an image onto the pieces of wood that we use for our printing, and in this blog entry we’re going to look at transfer printing on wood. The other method is dye sublimation printing which is covered in another article. However we’re quite careful about which images we use dye sublimation for on wood – because the ‘paper’ colour is whatever the wood colour is, certain colours don’t mix well with it. Blue, for example, doesn’t print very well using dye sublimation onto wood, and we would generally recommend that pictures with blue sky in for example are printed as transfer printing on wood surfaces.
Why is that? Well, in order to get more realistic colour matching in almost any form of printing, the ink has to be on a fundamentally white background. Without getting too technical, that’s because normally a colour printer (even a big commercial printing press) prints the four ‘process’ colours cyan, magenta, yellow and black which build up in layers to form your full colour image. This process is “subtractive” – in other words each layer of ink reduces the colour range that you see from white to a more limited gamut, until the final layer is on and you see the colour as it was intended. If you’re NOT printing onto plain white paper, the colours you get in the finished product are affected by the colour of whatever you ARE printing on. In the case of wood, this gives some lovely effects with the right image – old mono photographs are particularly effective – but can make a lot of colours default to a kind of murky brown!
What’s the solution for these colours then? We print the white background onto the wood UNDER the image. So when we’re transfer printing onto wood we first print the image in reverse (a mirror image) on a sheet of specially prepared paper, and the LAST layer to be printed is a layer of solid white ink on top of all the other image areas. Then we transfer this to the wood using heat and pressure – because we’re transferring it, the white top layer goes onto the metal FIRST and becomes a kind of ‘undercoat’ that the other colour layers can go on top of, to build up the colour as you would on a sheet of white paper. That way, the cyan, magenta and yellow inks (we don’t use black, because the first three added together give a perfectly good black for most images we print) can build up into an image exactly as they would in a colour laser printer.
Obviously using both heat and pressure when transfer printing on wood comes with its own hazards, and we have to take care not to let too much heat build up, or leave the print in for too long, or let the pressure get too high – otherwise we can end up with a print that is ‘overcooked’ or cracked and split – but years of experience and many hours of practice mean that we don’t often have that problem! Finally, having used a water mist spray to ‘fix’ the image, we let it dry thoroughly and then seal it with a clear lacquer before fitting any extra parts like hooks or clock movements.