I'm designing the type of ships an amphibious race could use for long distance travel, like a floating house, following oceanic currents. I imagine it shaped like an iceberg, with more structure below the water than above.

Essentially a big ship made of sculpted pumice. But I saw in various places that it cannot be done.

Why? What could make a ship made of pumice seaworthy?

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    $\begingroup$ Have you heard of pumice rafts? $\endgroup$
    – Gimelist
    Feb 4, 2019 at 7:42
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    $\begingroup$ Yes but pumice raft are dozens (or hundred) of mile long. I'm asking about a "real" ship. $\endgroup$
    – Juanito
    Feb 4, 2019 at 7:48
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    $\begingroup$ Could you link to or cite a reason given for why it supposedly cannot work? Pumice floats, and i see no reason why something that naturally floats should not be usable as a boat. Or is your question about the singular use of pumice, wihtout any other structural material, thus actually being a question about the mechanical strength of pumice? $\endgroup$
    – bukwyrm
    Feb 4, 2019 at 7:54
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    $\begingroup$ You could make a pumice boat about the same as an expanded Styrofoam boat. Use the Pumice as a core and skin it with a glass fibre resin composite. Remember if you want to make a boat that displaces water (has a hold) then you will need to have a keel weight (free with steel and concrete boats) needed with pontoon and wood boats. You are proposing to use raft material to make a boat so you will have to make allowances. $\endgroup$
    – KalleMP
    Feb 4, 2019 at 18:08
  • $\begingroup$ @Gimelist, a pumice raft has zero structural integrity. It exists as a coherent object only so long as winds and currents are favorable for holding it together. $\endgroup$
    – Mark
    Feb 4, 2019 at 23:00

4 Answers 4


But I saw in various places that it cannot be done.


There are two main issues for carving a boat out of pumice:

  1. Mechanical properties: a boat hull has to withstand loads without breaking, pumice alone is a glassy material, rather brittle. You don't want your boat to crack in two after taking a wave.
  2. Water permeability: pumice is not water-tight, meaning that some water will reach into the hull, leading to the sinking of the boat.

What could make a ship made of pumice seaworthy?

  1. Use some other material to add structural robustness: wood, steel, concrete, as long as the pumice provides just the shell.
  2. Make the pumice water-tight. Apply a glassy enamel on the outer surface, for example.
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    $\begingroup$ Enamel? Wouldn't the tried and true solution of "slather it in pitch" work just as with other boats? $\endgroup$ Feb 4, 2019 at 12:07
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    $\begingroup$ @rackandboneman, I don't know how pitch interact with glassy materials. On the other hand, flaming the surface could quickly seal it with an enamel like material. $\endgroup$
    – L.Dutch
    Feb 4, 2019 at 12:10

Pumice is composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock...It forms when volcanic gases exsolving from viscous magma form bubbles that remain within the viscous magma as it cools to glass...Pumice has an average porosity of 90%, and initially floats on water.

...When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite. Pumice is considered a glass because it has no crystal structure. Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water.

After the explosion of Krakatoa, rafts of pumice drifted through the Indian Ocean for up to 20 years, with tree trunks floating among them. In fact, pumice rafts disperse and support several marine species. In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts, some as large as 30 kilometers (19 mi) that floated hundreds of kilometres to Fiji. (ref)

It seems you can make rafts of pumice.

enter image description here(ref)

Featherock pumice is made of volcanic glass. It has been sitting at the original site of its eruption for thousands of years. It will not deteriorate over time outside...Pumice is a very soft rock that will carve very easily with regular metal tools. (ref)

You can also sculpt pumice easily and it won't fall part when exposed to the elements.

There’s nothing new about pumice concrete. The Romans were using it 2000 years ago. Roman engineers and builders have been celebrated for their enduring wonders—especially for the most well-preserved of all Roman architecture, the Pantheon of Rome. The dome itself is a world-class wonder. It was expertly cast using a concrete consisting of hydrated lime cement, fine-grained pumice pozzolan (critical!) and lightweight pumice aggregate.

Modern scientists have determined that the manageable weight of the Pantheon’s huge, unreinforced concrete dome is attributed to lightweight pumice aggregate, while the secret to Pantheon’s (and other Roman structures) longevity is pozzolan: fine-grained pumice (known as pozzolana to the Romans). (ref)

Pumice is also fairly durable, at least when mixed with some other materials.

But can you make a boat out of pumice?

Well, you can make a small boat out of concrete.

You can even make large boats out of concrete.

enter image description here

So, sure, it sounds like a pumice boat would float.

You might want certain parts to be poured concrete made in part from pumice stone, rather than just carved pumice. Not only will be be easier to get the shapes you want, but it will hold different pieces of pumice together and it will make a smoother surface, something your amphibians will appreciate.

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    $\begingroup$ in contrast to concrete, pumice is not water tight. $\endgroup$
    – Trish
    Feb 4, 2019 at 17:33
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    $\begingroup$ @Trish A lot of materials used for boats are not waterproof as is. Wood, for example. But they float. Waterproofing can come later. Also: concrete is not waterproof either. concretenetwork.com/concrete/waterproofing_concrete_foundations $\endgroup$
    – Cyn
    Feb 4, 2019 at 17:54
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    $\begingroup$ Well the easy answer would be using pumice concrete. $\endgroup$
    – KalleMP
    Feb 4, 2019 at 18:05
  • $\begingroup$ @KalleMP which is why I included it in my answer :-D $\endgroup$
    – Cyn
    Feb 4, 2019 at 18:06

Perhaps take the idea from Christopher's answer and use the pumice as a component in concrete. A trick people use to make concrete canoes is to use hollow glass micro-beads as one of the components. So in your world perhaps you can crunch up your pumice into very small pieces, coat the little pieces with with a resin, or use some sort of surfactant to keep the density light while the mixture hardens and then end up with a very light concrete to make your boat. That would be a little different than the Roman concrete that used volcanic materials, and probably lighter.


You need something to bind the pumice together and seal its pores

Pumice doesn't exist in large boat-sized chunks that you can carve into a boat. It's made of smaller pieces of unreliable composition. You need a resin, or a binder like used for concrete, to stick it all together. Then you build a mold to shape the thing, fill the mold, and let it set. You could instead try making planks out of the bound pumice, but that would result in a weaker boat.

Also, pumice is swiss-cheesed with holes. Water must be kept out, which is accomplished by the resin. Careful - fill too much of the internal pores with heavier-than-water resin and abruptly you have something which doesn't float anymore.

At this point, you're using a tremendous volume of not-very-strong material as the outer walls of your boat. Pumice is fragile, and anyone could step a foot through or drop a hammer through it.

The fundamental problem with the idea of a pumice boat is that the floating ability of the raw material is not as important as the airspace in the middle of the boat.


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