Foamed pottery, as described in Cyclic Fabrication System, is a broadly useful product, easily improving the properties of fired-clay ceramics for a number of uses. However, in its usual form, not only does it make a terrible smell; it also requires organic materials as an input, actually in larger volume than the clay; it requires oxygen to react with the materials; and it requires a fairly high temperature, at a bare minimum 250° but typically more like 800°. These requirements are not always desirable or feasible.
There are other uses for solids that can be thus "burned out" without disturbing surrounding materials; they can be used as support material for initially unsupported things, for example during assembly or 3-D printing, and investment casting and other lost-wax casting relies on burning out the wax from the mold. Similarly, in lost-foam casting, the foam pattern is burned out of the mold by the hot metal being cast.
So what kinds of "support materials" would make good candidates for such processes?
If you need to do your burnout at low temperatures, carbon dioxide sublimes at -79° and is very cheap and pretty inert. A number of other common compounds are solid at accessible temperatures below room temperature and are then easily evaporated, such as ammonia (-78°, boiling at -33°), sulfur dioxide (-72°, boiling at -10°), the highly toxic cyanogen (-28°, boiling at -21°), and water (0°). However, I can't think of any such compounds that don't melt first at atmospheric pressure or are totally nonpolar like CO₂; nonpolar compounds mostly tend to have a pretty wide liquid range, which is annoying here. At even lower temperatures the toxic, inflammable CS₂, may be an option, melting at -112°, but it doesn't boil until 46°! It's famous for dissolving insoluble things like cellulose, phosphorus, rubber, sulfur, and asphalt.
But at higher temperatures there are a number of inorganic and mostly-inorganic compounds that are easy to vaporize or thermally decompose into gases, which can perhaps then escape (through pores in the mass you want to remove them from, if they're embedded within it, as in the case of investment casting). Sometimes these gases are reactive, toxic, or both, especially at high temperatures; for example, table salt (NaCl) melts at 801°, boils at 1465°, and starts evaporating rapidly at much lower temperatures of 1100°–1200°, a property used in preparing fired-clay pottery — but the resulting sodium gas is highly reactive! Catalyzed by steam, it reacts with the surface of both the pottery and the kiln (or saggar) to produce a sodium silicate glaze, the desired result.
Even before the burnout, it's possible for solid support materials to react, especially if the material they're supporting is fully or partly liquid, as is the case with pottery clay bodies, for example, which are colloids plasticized with water. For example, dinitrogen pentoxide is a crystalline solid which melts at 41° into a liquid which boils at 47°, so it might seem like a reasonable candidate; but it reacts violently with water to form nitric acid, which can oxidize a wide range of materials to nitrates, so it will not work in systems where water may be present.
So with this burnout process, questions of support-material compatibility arise, especially at higher temperatures, as well as human and environmental safety if the process is being done near humans or within Spaceship Earth or another spaceship.
Formamide is organic but only one-fourth carbon; it melts at 2° and at 180° decomposes to mostly carbon monoxide and water. If overheated or catalyzed by acids it produces HCN. Formamide is miscible with water and so nontoxic that it's used as a cryoprotectant for vitrification, but it is also teratogenic.
Hyponitrous acid is an inorganic solid, if a dangerously unstable one, which spontaneously decomposes to nitrous acid and water over weeks at room temperature. I'm not clear on whether this decomposition takes place in solid form or not, or what its boiling point is.
Ammonium nitrite also slowly decomposes to water and nitrogen even at room temperature; perhaps it is stable at some lower temperature, but usually it is stabilized instead by an alkaline aqueous solution. However, it, too, is dangerously unstable under many circumstances.
Hydroxylamine itself, used as a photographic developer, might be a reasonable candidate: it's an inorganic solid, melting at 33° and decomposing at 58°, but unstable in a poorly-understood way. Still, its flashpoint isn't until 129°, and its oral LD₅₀ is around 400 mg/kg, which sounds considerably more innocent than most of the amines mentioned below.
Pyrosulfuric acid melts at 36° but may not be a good idea, being strong enough to protonate sulfuric acid. Moreover, I'm pretty sure that when it decomposes from heating, it decomposes into SO₃ and sulfuric acid.
Dinitrogen pentoxide melts at 41° and boils at 47°. It's a strong oxidizer and a strong acid, of course, but much less horrifying than the more common nitrogen oxides, which it will produce in ultraviolet light. Eventually it decomposes to nitrogen dioxide and oxygen even at room temperature, though.
Ammonium bicarbonate decomposes into ammonia, water, and carbon dioxide at 42° and is widely thus as a leavening agent for cookies and crackers, as well as an acidity regulator, fertilizer, and fire extinguishing agent. Decomposition is already rapid at 36°. It's an irritant that can cause lung damage.
Caro's acid melts at 45° but is probably contraindicated, being dangerously unstable itself and "one of the strongest oxidizers known".
NH₃OHNO₃ is another inorganic solid; it melts at 48° and decomposes somewhere in the 200°–300° range, but it is also very toxic and, except in aqueous solution, dangerously unstable.
Cyanogen bromide, a common organic synthesis reagent, is an inorganic solid which cleaves peptide bonds and reacts with water to produce HCN and HOBr. It also has a tendency to produce cyanide while in storage. It melts around 50° and then boils around 61°. Unlike almost every other material on this list, it contains no oxygen, but it does contain carbon.
Ammonium carbonate decomposes into ammonia and carbon dioxide at 58° and is used like ammonia bicarbonate for leavening, often in a mixture, as well as an emetic and photographic lens cleaner.
Ammonium carbamate occurs with the carbonate and bicarbonate, with which it is used as leavening (and with which it spontaneously interconverts), and it decomposes at 60°, also to ammonia and carbon dioxide.
Marshall's acid is similar to Caro's acid but instead decomposes without melting at 65° and, I think, isn't itself dangerously unstable, as long as you keep it far away from any organics. I imagine that it decomposes into mostly SO₃, though.
Ammonium sulfite, used as a food additive, a photographic fixer, and a safer alternative to lye for straightening hair, also decomposes at 65°, into "sulfur dioxide and oxides of nitrogen". It's also used to make blast-furnace refractory-lining bricks; US patent 2,724,887 from 1955 explains that in aqueous solution it works as a source of a sulfite ion which oxidizes to sulfate and somehow prevents iron-oxide contamination in the bricks from causing them to disintegrate under blast-furnace conditions. (Mysteriously, though, he forgot to patent this, patenting only the use of lithium chloride for the same purpose.)
Ammonium oxalate is organic but only about 20% carbon; it melts or possibly decomposes at 70° and presumably decomposes at a higher temperature, below about 130°, I think. It's so nontoxic that it's found in kidney stones and is used as an anticoagulant for blood transfusions. However, the decomposition products include, at first, oxamide, and later hydrogen cyanide.
In Project Pluto a similar purpose in assembling a hot reactor was answered with naphthalene mothballs, which melts at 80° and boils at 216°, requiring no oxygen. They also sublime pretty rapidly at room temperature, typically millimeters per month (or, in SI units, hundreds of picometers per second.) But mothballs are still organic.
NH₄ClO3 decomposes at 102° to nitrogen, chlorine, and oxygen, but is dangerously unstable; Wikipedia says, "Even solutions are known to be unstable ... it should only be kept in solution when needed, and never be allowed to crystallize." Ammonium chlorite and hypochlorite are even worse.
Ammonium acetate is organic but only about one-third carbon; it melts at 113°, boils at 117°, and decomposes to liquid acetamide and water at 165°. Acetamide is acutely nontoxic but possibly carcinogenic, and doesn't decompose until 221°. Ammonium acetate is deliquescent and sufficiently nontoxic to be used as a diuretic, a biodegradable de-icer, and a food additive for buffering pH. Crystallizing it from a water solution is difficult.
Just plain crystalline sulfur melts at 115°. In air it will burn enthusiastically shortly thereafter (its flashpoint is 160°, its autoignition temperature 232°), but absent oxygen, it boils at 448°. It evaporates with surprising speed even at and below its melting point, though.
A unique property among the materials mentioned here is sulfur's metastable "solid" amorphous form, easily produced by quenching molten sulfur from above 170°, where it is blood-red; this amorphous polymeric form is red or brown and very plastic, like chewing gum ("a more or less sticky mass"), and can be remelted at 120°. At room temperature this material initially seems to show some surface tension, fingerprints in its surface disappearing over the course of several minutes, so it is really just a viscous liquid, but in a few hours to days it hardens and becomes glassy, though without changing color, a phenomenon attributed to semi-crystallization of this amorphous polymeric phase into "ω-sulfur" and the simultaneous partial crystallization of the non-polymeric impurity.
Some sulfur dioxide in the sulfur is apparently necessary for the formation of this amorphous phase, and is normally formed when solid sulfur is exposed to air.
This "quick-quenched" form of sulfur reportedly has S₈ rings dissolved in it, lowering its glass transition temperature to -30°; these can be removed by washing with CS₂. (Some twenty allotropes of solid and liquid sulfur are known, complicating this enormously; some of them are even metallic.)
You could recrystallize this amorphous form either by waiting long enough (apparently many years) or by annealing it, reconstituting the familiar brittle yellow α-sulfur. Reportedly above 90° the recrystallization becomes "rapid", which seems to mean "hours" or "minutes" rather than "seconds", and is associated with a volume loss of some 8%. This may be useful for, for example, using the sulfur as modeling clay, then recrystallizing it to its usual form. However, four hours at 100° does not seem to be enough time to have any noticeable effect, even when the dark "amorphous" sulfur is in contact with what seems to be yellow monoclinic α-sulfur. Even remelting to the low-viscosity liquid form doesn't seem to revert it fully from dark brown to bright yellow.
Ammonium formate is organic but only 20% carbon; it melts at 116°, and decomposes into water and the nontoxic formamide at 180°, at which point the formamide decomposes into carbon monoxide and ammonia. Ammonium formate is deliquescent and relatively nontoxic. The above-mentioned considerations for formamide apply.
Hydroxylammonium sulfate is a stabler and less toxic salt of the hydroxylamine mentioned above; it's used in color film emulsions, decomposing at 120° to SO₃, N₂O, NH₃, and water, in a reaction exothermic if heated past 138°. It irritates skin but won't even damage your eyes if you splash it in them; however, it's acidic and a strong reducing agent.
The inorganic solid ammonium persulfate, used in hair bleach and as a food additive, also decomposes at 120°. However, though its toxicity is relatively low, it's a strong enough oxidizing agent to oxidize copper and nickel, and it's very acidic. Worse, I imagine that when you do manage to decompose it, it decomposes into ammonia and Caro's acid.
The fertilizer, flame retardant, and herbicide ammonium sulfamate melts at 131° and decomposes at 160°, presumably to ammonia and sulfamic acid, an "intrinsically safe" household cleaning product which melts at 205° and then decomposes to nitrogen, water, and sulfur oxides.
The inorganic solid Hydroxylammonium chloride decomposes around 156°, but I don't know what it decomposes into. I'm guessing that nitrogen oxides, probably hydroxylamine, nitric oxide, and hydrogen chloride would be in the mix; maybe nitroxyl, hyponitrous acid, and/or hydrogen, too.
NH₄NO3 is another solid that can be entirely decomposed with moderate heating, sometimes in a dangerous chain reaction. It melts at 170° and decomposes exothermically at 210°. This decomposition can produce relatively innocent nitrous oxide and water; even more innocent oxygen, nitrogen, and water; or deadly and caustic acids, ammonia, and acidic nitrogen oxides; depending on the conditions of decomposition. It, too, is entirely inorganic. An additional disadvantage, or advantage, is that it deliquesces above 59% humidity at 30°, or even lower humidities at higher temperatures.
Ammonium thiosulfate is a photographic fixer, fertilizer, defoliant, and nontoxic cyanide alternative for heap-leach mining of gold and silver. It presumably decomposes if you heat it up enough, but I don't know at what temperature; different sources suggest 180° or 150°. It decomposes to sulfur oxides and ammonia, normally, though sometimes it can produce hydrogen sulfide, and even without heating, it can corrode even copper.
Sodium persulfate is an almost non-hygroscopic compound used as a hair bleach, a soil conditioner, an oxidizer for zinc, a pickling agent for copper, and a polymerization initiator which decomposes at 180°. Presumably this yields sulfur oxides, probably SO₃, and possibly oxygen, and leaves behind a sodium oxide residue, which doesn't boil until 1950°. In some situations, for example in fired-clay pottery, this residue may be tolerable; most of the other anions mentioned here can be used with sodium similarly, but I will mostly focus on burnout without residue.
NH₄ClO4 is an inorganic solid that decomposes around 200° into HCl, nitrogen, oxygen, and water, but it's exothermic enough that this can be dangerous; its autoignition temperature is only 240°. It's also a fairly widely used oxidizer. Its acute toxicity is low, but its chronic toxicity is high, and of course the decomposition products are caustic.
Ammonium sulfate is another inorganic solid that can be entirely decomposed by heat at 235°–280°, producing ammonia, nitrogen, sulfur dioxide, and water. It's pleasantly stable and nontoxic, being widely used as a fertilizer, a food additive, 30% of worldwide fine particulate pollution, and (refreshingly for this list) a flame retardant. It's almost alone among ammonium salts in emitting no significant ammonia at room temperature. Ammonium bisulfate is an intermediate product in the decomposition, melting at 147°, for better or worse.
Ammonium chloride, when heated to 338°, decomposes into ammonia and hydrogen chloride. These are presumably quite caustic in the gas phase, but are known to recrystallize innocently on delicate fossils to reform ammonium chloride. And ammonium chloride is entirely inorganic, and could even be used if oxygen were scarce.
Phosphorus pentoxide melts at 340° and then boils at either 360° or 423°? I don't understand this but I don't want it going on anywhere near me. Phosphorus oxides are very complicated and hard to predict, and they tend to be hygroscopic.
Oxamide, used as a fertilizer and flame retardant, loses water at 350° to form the highly toxic cyanogen. It's organic but only about one-third carbon. Different sources claim that it melts at 163°, at 300°, or not at all.
Nitramide? Probably too dangerous. Ammonium dinitramide? Probably too dangerous, and also how to make it is a secret.
acetamide? Urea? Melts at 134°, only 20% carbon.
Sulfamic acid
Iodine
Thiocyanates? Thiocyanides? Bifluoride? Cyanate? Fluoride? Hydrosulfide? Iodate? Iodide?
Thiourea?
If melting like naphthalene rather than vaporizing is acceptable, then under some circumstances there are a variety of solids that can be easily melted.
If water won't damage the thing you're trying to remove the support material from — not the case for clay bodies for pottery, of course, but plausibly the case under many other conditions —
Ammonium nitrate
Calcium chloride
Magnesium chloride, zinc chloride, ferric chloride, carnallite, potassium carbonate, potassium phosphate, ferric ammonium citrate, potassium hydroxide, sodium hydroxide...
phosphorus pentoxide?
Pretty much any solid can be removed by the appropriate reagent, but not always quickly, and the trick is to pick something that won't damage the thing you're trying to remove it from.