Some areas of the evil meant lair are not for the eyes of the average minion. There is only one way in and out of this area, there is no door blocking the way, but a lava waterfall pouring from the ceiling, completely covering the section of the corridor. The lava is actively recycled and a mechanism "opens" the waterfall from the center when the villain approaches.
Is this setup possible? Are there materials that can consistently contain molten rock without melting?
Consider using molten metal instead!
Most of the problems facing your lava waterfall can be avoided by using a more easily workable material. Relative to lava, almost all molten metals emit more heat and light than lava at a given temperature as you can see from the chart below. (stainless steel, tungsten, copper, aluminium, etc. almost all metals will follow this pattern)
In addition to aesthetics, metals can be heated using induction which is much more efficient and compact than whatever mechanism would heat your lava. This process also makes no direct contact with the metal in question, so it will not wear out as easily.
By using electromagnetic induction from a high-energy coil, the molten metal's temperature can be very accurately controlled and will also cool more quickly than stone (meaning the transport system needed to recycle it will be much easier to create and maintain).
Let's suppose you were to use a molten metal like tin or pure iron (avoid steel due to the sparks of super heated carbon that jump out). It can be made to flow much more easily (and better resemble a waterfall) than lava. It is highly receptive to inductive heating. It releases none of the toxic fumes that true lava would, and unlike lava, it can even be moved around using a ceramic pump. Furthermore, molten metal can be more easily contained, guided, split, or pushed out of the way to reveal your door due to its lower viscosity at higher temperatures.
Suppose the following for your villainous lair!
your passageway to the uber-secret room is disguised as the idealized lava wall you desire, radiating heat and light to whatever specifications you wish. The flow of metal falls cleanly from a slot in the ceiling into a tungsten grate in the floor that drains it away into a reservoir by a trough. the metal cools during this process, becoming easier to transport, but not solidifying. It is pumped through a series of tubes that have inductive coils around them as shown which sustain the metal at this temperature:
From there, the molten metal is taken back up into the ceiling where a series of final high-energy coils return it to the yellow glowing radiance you desire. It is then flowed back over the lip of the ceiling, completing the loop.
In order to reveal the secret massage, one need only turn off the pump, or one could divert the flow to stop only the middle of the lava wall. One need only cross the glowing tungsten grating (perfectly safe if done quickly enough) and behind the wall is a hallway or catwalk or door to your lair.
Edit: also consider this design used by foundries to improve laminar (smooth) flow and prevent strain on the pump
Go ahead and make lava.
https://www.gamma-meccanica.it/mineral-wool-production-lines/rock-wool/melting-furnace/?lang=en
Rock is melted and whipped into insulation. Depicted - a factory doing just that. More here on this related question - Can stone be "recycled" by melting and cooling it?
For purposes of making a glowing molten wall, you could use other industrial processes - a metal refinery has all sorts of glowing hot molten stuff pouring here and there (hopefully mostly there). Or a glass recycling plant. Bonus - your Evil Lair could have a plausible raison d'être as a factory of some sort.
Ceramics would work nicely. They would hold up well in the heat and wouldn't deform under load like softer metals could. However they would be subject to wear. Modular units should be used so they can be replaced as they fail.
A simple ceramic Archimedes screw could be used to pump the lava. The screw should be long enough to put the drive mechanism a safe distance from the heat. To avoid the replacement of the screw in the event of a failure there should a sufficient gap between the screw and the pipe for lava to escape back down the pump if the screw stops or seizes.
Locks should be implemented to segregate top, pump, and bottom sections for maintenance.
Withstanding the heat is the easy part.
Continuously flowing lava is by far harder than the heat, even the air is too cold and will cause lava to solidify. lava falls are temporary occurrences. the better you want it to flow the hotter it has to be. Lava is also extremely abrasive and will scour material very quickly, like faster than most industrial abrasive processes quickly. flowing lava can carve away inches of concrete in a single event.
As for containing it, yeah that's the easy part. We even have steel alloys that will handle it just fine, tungsten steels can handle even hte hottest lavas. even the hottest lavas are only 2000 degrees celsius, to get it to flow easily you only need to get it to around 1200 degrees. You will need to replace your container often however as no matter what it is it will wear away/out fairly quickly.
Sapphire.
I'm serious! Here's an interesting thing: Randall Munroe was once asked what could be used instead of glass for a hypothetical lava lamp that containing actual lava. His response includes some interesting tidbits:
You have a few choices for transparent materials that could hold the lava without rupturing and splattering half the classroom with red-hot droplets. Fused quartz glass would be a great choice. It's the same stuff they use in high-intensity lamp bulbs, the surface of which can easily get up to mid-range lava temperatures. Another possibility is sapphire, which stays solid up to 2,000°C, and is commonly used as a window into high-temperature chambers.
If the sapphire wall/floor/ceiling is thick enough, it will not be too transparent.
But if you are not into gems and want to save some money, you could, you know, just build your room out of the stuff that lava usually flows over.
<img height="300" alt="She's one of my favorite gems too" src="https://i.sstatic.net/QELHN.png">.
$\endgroup$
Evil super-vilian security theater!
Instead of making a true lava waterfall, make a liquid that has the same appearance, but is significantly cooler. I find a vinegar/baking soda with some red dye added to usually be sufficient, but with the resources to construct an evil lair in the first place, you're going to be able to produce a higher quality psuedo-lava. Once you have that, any sort of typical indoor waterfall setup will work.
Also install industrial heaters in the walls radiating heat into the corridor, so that the heat from even approaching the lava waterfall is unpleasant or even painful, and everyone will be convinced that it's a genuine lava waterfall.
Are there materials that can consistently contain molten rock without melting?
The problem is that most ceramic furnace materials will slowly react with your lava. You also need a very high temperature for your lava (higher than 1200 °C) for it to actually flow (or add sodium, but it will evaporate eventually requiring you to refill it occasionally).
Lavas are reactive with most of the stuff they get into contact with. Pretty much the only materials with a high-enough melting point that will not react with the lava (thus changing the composition of the lava and the container) are platinum and iridium, and they are expensive. Really expensive. If you want to build a system of pumps and tubes to move the lava around, you will need so much platinum or iridium that I'm not certain there is enough of it out there being mined, or you might actually temporarily increase the prices so much to cause a global financial crisis.
Silver has several properties that you want:
You could just use something that only looks like lava, like water or oil, and is still hot enough to burn anyone that touches it. And it's much easier & cheaper to heat & pump, and any metal or glass could contain & direct it easily, building a moving waterfall should be no problem.
Nearly boiling water is plenty dangerous (and would add a cool "steamy" effect), or hot oil could be downright lethal (185C/365F).
Yosemite National Park's Horsetail waterfall sometimes looks like Glowing Lava and that's just natural sunlight:
With oil it could look similar to a big "lava lamp" if you wanted a clear glass area somewhere too:
So I would suggest using real lava is virtually impossible, and this lethally scalding hot oil is a better solution.
