Drawing Board

A plain flat board used by applying pressure to the surface. They are usually about twenty inches square, but there have been much smaller and much bigger.
It can be changed, but the tricks to making these things is a bitch in the first place, requiring saliva from something I haven't invented yet--but standard colors are black with white impressions. The impressions tend to swim in opalescent white colors.
You can use your finger to make impressions, but it feels kind of like petroleum jelly (no residue, just slimy) so most tend to not, and will instead use something else narrow and small. It feels like those weird gooey drawing things with the liquid over the rainbow stuff.
The surface of the writing pad is kept upright, usually on a wall. When lain flat on either front or back, the whole thing is 'pressed' upon and it is effectively blank, or scribbled out, depending on how you want to see this. 
Standard creation is that impressions last three days. It's a pain in the ass for messages, but good for warnings or invitations for tea, making notes, or drawing charts. Secret languages or adaptable communications are not unheard of. (It's actually like that to prevent secret messages.)
The linked devices themselves have a set time for all impressions upon it, and this is set at creation. You can have five linked boards that all keep a message for three seconds each or for ten days each. You cannot have five linked boards, one that lasts for three days and the other lasts for two while the other three last for four. 
Impressions keep shape of whatever is impressed. If you want to translate what you wrote, you have to do it magically or manually, it won't do it for you. (That is to say, no 'automatic translation spells' to reshape impressions. Most attempts work like... attempting to take a photo of Chinese writing and turning that visual into similar-looking Romanized lettering and trying to translate the results. These boards have no magical AI of any kind, so it'll do what you ask it with what it has to work with, which is nothing.)
The thing will not break. Short of deliberate magical destruction, these things, once made properly, are nearly impossible to destroy. Making one properly, however, is difficult, and most settle for something less durable but similarly functional (Gonna call it a notepad, heeee)
The construction involves the hide of a sort of cave frog, blood and bones from a sort of forest-dwelling goat, and Nymphwood (This is the step everyone fucks up. Stolen nymphwood is no good. It'll heal, but you can absolutely break your thing if you don't have nymphwood with her blessing still on it, and it may reshape at random.)
The technology originated with the The Scribes Post Scattering.