Theoretically, one could probably build a thermal printer (maybe with a laser) that simply burns the paper black where you want it black. Conceivably without embrittling the paper too deeply or setting it on fire, if the energy is applied quickly and intensely enough.
Was that ever experimented with/implemented, and what (apart from fire hazards) makes it impractical?
It's a bit unclear what you're asking:
further:
To start with, there are only two ways to transfer heat in a focused way and that's direct contact of some heating/heat transfer element and a beam focusing, which in turn makes most sense by using a laser. I would assume the question tries to avoid contact based printing (which would as well be possible), so laser it is.
Even for thermal paper, the necessary power to 'burn' it in a time short enough to work as a printer, is way above what lasers could do until very recently. Or more exact, could do within an acceptable budget and size. Before high power laser diodes, it would have taken a quite large setup as well.
But even with diode lasers a setup would be far larger than what existing thermo printer need.
To burn coated paper (aka thermo-paper) with a speed equal to a cheap thermal printer, a power close above .1 Watt is needed. After all, it does not make any sense to spend more money or a slower printer. According laser diodes have still today a price tag of 10 Euro or more.
Doing the same with 'plain' paper requires either a laser many times the power or to accept slow operation. For example a .5 W laser may 'engrave' under optimal setup up to 100 mm/s on conventional (laser printer) paper. Not really anywhere near acceptable speed. To even come close to a thermal printer it would need a power past 5 Watt.
Power over time is also a big hurdle to overcome is the paper catching fire vs print speed. This goes hand in hand with focusing. To make it viable the paper would need to be transported in a well defined way - maybe exactly as with a thermal printer.
In addition, there is no 'plain' paper. Even what we would consider similar may be different and designed to purpose. Some may remember that in the early days of laser and inkjet printers one had to take care what paper to use, as sheets made for typewriters did not work well wit non impact technologies. So, even if someone would build a laser based printer, 'burning' results may greatly vary over paper type used. It would only work good with paper made to certain specs.
Bottom Line: It did never make any sense, neither technological nor financial, to build such a printer.
Not exactly plain paper, and these were analogue devices, but the Gestetner Gestefax and Gakken Fax stencil copier for duplicators used a photocell on the read drum and a small spark on the write drum to burn through the thin paper duplicator stencil. These were indirectly used for reproduction, but not as a printer.
The utterly terrible Sinclair ZX printer used a spark to burn through metallic coating on the paper, but that's not the OP's plain paper.
Not plain paper, but how about cardboard?
The CO2 Laser’s marking and engraving capabilities include marking date codes, serial numbers, and other product identification on wood, glass, rubber, plastics, cardboard, and product packaging.
