I poked around a little bit and could not find the answer. On Mohs Hardness Scale, what is the hardness of ice?
Mineral Ice is highly variable in hardness, depending on a number of factors. Mohs value of 1.5 is often give.
Tris
Yeah, temperature would probably be an obvious factor. I’ve heard that ice down around -50 C or lower is very different in some of its properties from ice near the ‘melting point’
lets say you took ice down really cold, like maybe one degree from absolute zero. anyone know the hardness then. i just want to know how hard ice can get. thanks
I’m not sure you’re going to get a useful answer. Ice as we know it is quite variable, and below around 70K it changes to another crystallographic form, Ice 6. Good luck finding its properties! it gets even more complicated when you factor in pressure changes as well - I count 20 types of ice on the ice polymorph table from this link:
http://www.lsbu.ac.uk/water/phase.html
Mohs hardness probably wouldn’t be used for ice anyway, it’s a simple comparative scale. A proper indentation hardness such as Vickers, Brinell or Rockwell is more meaningful. Googling turned up:
http://www.tpub.com/content/ArmyCRREL/AOS_SR96_23/AOS_SR96_230095.htm
The maximum measured ice hardness was 60MPa, measured by Vickers indenter. This corresponds to a Vickers Hardness of about 6Hv, which isn’t very hard at all. For comparison, a mild steel has a hardness of around 120-150 Hv, whereas a hardened blade steel is up around 800-900 Hv.
Actually, Tris’s answer of 1.5 Mohs looks right on the money! Halfway between talc and gypsum (plaster). Tris, how do you know that?
This paper has some data on the mechanical properties of ice, although not the hardness. It is inferior to concrete in all respects.
Ice just needs a little sawdust mixed in to toughen it up:
Pykrete
Except for one; under pressure or friction-generated heat, ice is “self-repairing”, i.e. it will fuse back together. Try that with cured concrete. For this and other reasons, insulated fiber-reinforced ice would make an almost ideal material for a large space habitat or a low thrust spacecraft.
As for the hardness, the previous posters are all correct; there is a great deal of variability in hardness and other mechanical properties dependant upon the temperature, pressure, allotropic form, et cetera. There is no one ready answer to this question.
Stranger
The Google fu of the elder master is a strong ally.
I googled “Hardness of ice” as a phrase, and checked to see if what I remembered from Mrs. Banks’ Earth Science class from forty five years ago was right. It was. Thank you Mrs. Banks.
Tris
What about sublimation?
I’ll go on ahead and toss out the Vonnegut *Cat’s Cradle * reference now, just to get it out of the way.
Oh, that will occur, of course, but at a surprisingly low rate at the low ambient temperature of space, hence why the ice would have to be radiatively insulated from the Sun or solar reflections. A thin layer of foil would be adequate to limit losses from sublimation due to sunlight to an acceptable level. Internal fiber reinforcement (similar to fiberglass in resin) would give shear resistance and some tensile strength, and a fiber wrap on the outside gives it additional hoop strength. You could use slag from metal smelting operations as an additional aggregate to give additional bulk or thermal mass.
I envision a large O’Neill “Island Three”-type cylinder (sans the windows) and put a layer of water next to the inner wall surface, which would double both as a water reservoir and radiation shielding, then place “land masses” on top of this, either tied to a frame or just floating on top of the water. The thickness of the wall would be controlled by the temperature of the “ocean”, and any cracking or punctures of reasonable size would be filled by water from the reservoir, making such an object robust and more failsafe than just a metal can. The mass bulk of the ice and water would also prevent internal hazard from orbital debris or the occasional micrometeorite, absorbing the energy and ablating away. Since you don’t have large strips of windows like the O’Neill concept you’d have to have a solar collector on one end (I think of it as “The Sunflower”) which is designed to face the Sun, and a large radiator on the other end which exhausts waste heat out to space (which is critically important, as excess heat will fill the “ocean”, resulting in heat transfer to the outer walls and eventually structural failure of the wall). The radiator could also be designed as a low effeciency thermal thruster to help keep the habitat aligned against precessive forces or make slow adjustments in orbit.
The upside of this is that it uses readily available and easily worked materials (ice, silicates) with only a moderate amount of prefabricated reinforcement and essentially no finished or refined metals or glass (for the main structure), and it is more resistant to damage or radiation than more conventional steel “eggshell” habitats. It has the additional advantage of being able to be expanded; cut the hoop reinforcement in a few select places, give it more spin and let the hull “flow” outward in a carefully controlled fashion, chill the oceans a bit more, and voila, you have an expanding egg, to which you can then apply a new outer reinforcement wrap.
The downside is that there is a large minimum size this can be scaled to in order to maintain the necessary thermal equilibrium, and so it would require a substantial investment in terms of collecting all the necessary materials to get started, and would thus be unsuited for small scale space habitats. It would also be unsuited to high thrust applications, as the ice “hull” would tend to flow under continuous thrust. So the current utility is pretty much limited to speculation.
And you could make pretty colors by dying the water in bands. 
Just remember, now that it’s in the public domain, you can’t patent it.
Stranger
Shouldn’t it be true that the variations in hardness occur only near the melting point, and that ice kept well below freezing should have the same hardness? Or, if ice well under freezing can have different hardness characteristics because of crystal habit or otherwise, then don’t we have to conclude that ice is not one mineral but several? IIRC calcite and dolomite have the same chemical composition, but crystallize differently.
Yes, the biggest hardness variations with temperature will occur close to the melting point. But just about everything shows a hardness variation with temperature. Blacksmiths work the iron when it’s red-hot for a reason! (Well actually many reasons, but iron and steel do lose hardness at elevated temperature.) Microstructure is also very important. In metals, strength increases with decreasing grain size, and it’s quite possible that the same applies to ice.
Absolutely! Diamond and graphite. White phosphorus and red. Although with pure ice, you probably can’t have many of the same mineral forms together under the same conditions. I would guess that once formed, cubic ice (Ic) is metastable when you warm it up a bit, so you could probably have chunks of cubic ice and hexagonal ice side by side in your freezer. Alternatively, a fast cooling of cubic ice and hexagonal ice might get you into Ice 6 temperatures without them transforming, so you could sit three types of ice side by side on your spacesuit glove. The hexagonal and cubic ice samples would be metastable - thermodynamically they should transform to Ice6 but the kinetics of the change are too slow.
Diamond, incidentally, is also metastable - thermodynamically, it’s “trying” to turn into graphite. But the kinetics of that change are so slow that we don’t worry about it at temperatures we’re used to.
Stranger, that’s a very neat design. I was worried about the ice composite creeping, but you have a temperature gradient through your ice-water wall with much colder ice near the outside… beautiful! I wonder if you could laser-launch shaped chunks of ice composite into orbit with no other superstructure at all?
These are known as allotropes (phases of the solid state) and water ice has sixteen known crystalline allotropes plus the amorphous solid phase.
Well, it will creep over time, so it can’t be considered an equilibrium static structure, but the rate can be controlled by temperature and the amount of reinforcement, which makes the structure self-repairing. For a smaller structure you’d probably need some way to insulate the inner layers of the water reservoir from the outer layers, so that you can keep the latter sufficiently cold such that the wall is kept at a minimum thickness and the interior isn’t frigid. To start constucting the structure you would inflate a large football-shaped balloon, the interior of which has been coated by nucleating agents, give it a bit of spin or enough pressure to keep it rigid, and then start blowing in water vapor, which freezes against the wall. There are a lot of engineering details that would need to be refined, but I think the basic concept is workable, stemming from experiments with pykrete ships during WWII.
As for lifting chunks of ice by their own ablation, it might be possible; however, I think even if you can cope with the thermal blooming issues involved in shooting through the atmosphere you are still going to have problems keeping the ice from flowing, melting, or cracking due to aerodynamic and thermal stresses, and in order to get the high thrust and specific impulse necessary to lift a large mass efficiently (i.e. before you burn most of it up) you are going to need a high pressure chamber to contain the pressure of the superheated steam before it expands. The advantage of beam propulsion is that you don’t need to carry the power source with the craft, but it still has to carry propellant, and pressurized steam just isn’t that great of a propellant. Laser or maser beam propulsion works for low thrust, high specific impulse applications and so might be suited to graceful orbital or interplanetary maneuvers, but it doesn’t scale up well for the high thrust requirement for lifting from ground to orbit. Besides, there is plenty of water already in space in the form of cometary water ice; a single moderate sized comet would yield more than enough water for a very large habitat.
Stranger
I never thought of ice as a mineral until I saw it listed in a mineralogical field guide. I was surprised, but it does meet the definition of one.
So those stereotypical “igloos”, though made of ice blocks, are actually stone masonry! 
I just read that the peak hardness of ice is actually 5.5 on Moh’s Scale, which is stronger than steel
Where did you read that?
** zombie/ice/mineral joke **
http://academic.emporia.edu/aberjame/ice/lec02/lec2.htm says:
“At 0°C, ice has a hardness of 1.5 on the Mohs scale; at -70°C the hardness is 6 (Nesje and Dahl 2000).”
The “Fusing back together” wouldn’t remove the weakness caused by the crack totally
Nor the cause of the cracking in the first place.