RESEARCH ARTICL E
Towards the colonization of Mars by in-situ
resource utilization: Slip cast ceramics from
Martian soil simulant
David Karl ID
1 ☯ ‡
* , Franz Kamutzki
1 ☯ ‡
, Andrea Zocca
2
, Oliver Goerke
1
, Jens Guenster
2
,
Aleksander Gurlo
1
1 Fachgebiet Keramisch e Werkstoffe / Chair of Advanced Ceramic Materials, Technisc he Universita
¨ t Berlin,
Berlin, Germany, 2 Bundesans talt fu ¨ r Materialforschung und –pru ¨ fung (BAM), Berlin, Germany
☯ These authors contributed equally to this work.
‡ These authors are co-first authors on this work.
* david.karl@ceramic s.tu-berlin.de
Abstract
Here we demonstrate that by applying exclusively Martian resources a processing route
involving suspensions of mineral particles called slurries or slips can be established for
manufacturing ceramics on Mars. We developed water-based slurries without the use of
additives that had a 51 wt. % solid load resembling commercial porcelain slurries in respect
to the particle size distribution and rheological properties. These slurries were used to slip
cast discs, rings and vases that were sintered at temperature s between 1000 and 1130 ˚C
using different sintering schedules, the latter were set-up according the results of hot-stage
microscopic characterization. The microstructure, porosity and the mechanical properties
were characterized by SEM, X-ray computer tomography and Weibull analysis. Our wet pro-
cessing of minerals yields ceramics with complex shapes that show similar mechanical
properties to porcelain and could serve as a technology for future Mars colonization. The
best quality parts with completely vitrificated matrix supporting a few idiomorphic crystals
are obtained at 1130 ˚C with 10 h dwell time with volume and linear shrinkage as much as
~62% and ~17% and a characteristic compressive strength of 51 MPa.
Introduction
A promising concept to explore and subsequently colonize the Moon and Mars is in-situ
resource utilization (ISRU), the practice of on-site collection, processing, storing and use of
native materials encountered in the course of human or robotic space exploration. Early colo-
nization scenarios propose the direct use of the rock covering, loose granular surface media
(including dust, soil and broken rock) composed of various oxide minerals and referred to as
Lunar and Martian regoliths. The chemical composition of Lunar and Martian regolith
( Table 1 ) makes conceivable the extraction of metals and ceramics. For the smelting of
regolith in blast furnaces and bloomeries to produce base metals, the availability of ceramic
tools is an important prerequisite. The ISRU approaches towards ceramics include (i) dry con-
solidation [ 1 – 3 ], (ii) melting [ 4 , 5 ], (iii) self-propagating high temperature synthesis and
PLOS ONE | https://doi.org/10.1371/journal.po ne.0204025 October 11, 2018 1 / 11
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OPEN ACCESS
Citation: Karl D, Kamutzki F, Zocca A, Goerke O,
Guenster J, Gurlo A (2018) Towards the
colonization of Mars by in-situ resource utilization:
Slip cast ceramics from Martian soil simulant.
PLoS ONE 13(10): e0204025. https://doi.org/
10.1371/journal.pone. 0204025
Editor: Ross James Friel, Hogskolan i Halmstad,
SWEDEN
Received: February 22, 2018
Accepted: August 14, 2018
Published: October 11, 2018
Copyright: © 2018 Karl et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License , which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper.
Funding: This work was supported by Technische
Universita ¨ t Berlin through an internal funding grant.
We acknowledge support by the German Research
Foundation and the Open Access Publication Funds
of TU Berlin. The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.

geopolymerisation [ 6 , 7 ]. For the first of these approaches, which represents the most realistic
initial colonisation scenario, loose regolith powders are pressed into bricks and fused by direct
compression or sintering [ 1 – 3 ], such bricks could be used for masonry construction [ 8 ]. How-
ever, dry consolidation routes are often not suitable for ceramic parts with complex shapes. In
respect to Mars, despite research efforts, it remains challenging to assess the feasibility of the
ISRU approaches discussed above, especially as many of the proposed routes are not used to a
great extent on Earth. It is surprising that to this day wet processing of minerals—the oldest
and most universal processing route towards earthenware pottery, established around 30,000
years ago, has not yet been discussed for ISRU. In traditional pottery hydrous aluminum phyl-
losilicates are mixed with water, then molded into a shape, dried, and fired [ 9 ]. Here we pres-
ent the water-based slip casting technology for fabricating various ceramic parts of different
complexity (discs, rings and vases) using solely theoretically available Martian resources, i.e.
regolith, gypsum and water. We followed a common approach in simulating extraterrestrial
regolith and its properties by using regolith simulants [ 10 ]; here we apply the Martian regolith
simulant JSC-Mars-1A, which is a natural glassy volcanic ash composed of finely crystallized
and glassy particles of Ca-rich plagioclase, Mg-rich olivine, Mg-rich pyroxene, Ti-magnetite
and nanoparticulate iron oxides and oxyhydroxides known as npOx which are also responsible
for Mars’ reddish appearance [ 11 – 16 ]. We chose this simulant as it is the best-established Mars
regolith simulant, allowing for good comparability. The second resource needed is gypsum,
which was used for the plaster molds in the slip casting presented in this paper, it can be found
in gypsum-rich veins which have been detected at various locations in sedimentary rock on
Mars [ 17 , 18 ]. The third resource for wet processing of minerals is water, which is found in the
Table 1. Chemical composition of the Martian regoliths and the JSC-Mar s-1A regolith simulant.
Compound Regolith JSC-Mars-1A
Martian regolith stimulant
Utopia Planitia  Ares Vallis Mermaid Dune   Columbia Hills of Gusev crater    Orbitec data sheet     Authors’ anlaysis 
SiO
2
43 50.2 36.1 43.5 37.27
Al
2
O
3
7 8.4 2.56 23.3 20.74
FeO n.a. 17.1 15.4 n.a. n.a.
Fe
2
O
3
17.8 n.a. 4.84 15.6 14.71
MnO n.a. n.a. 0.37 0.3 0.24
MgO 6 7.3 21.6 3.4 3.2
CaO 5.7 6.0 1.69 6.2 5.46
Na
2
O n.a. 1.3 1.0 2.4 2.07
K
2
O < 0.15 0.5 0.03 0.6 0.48
TiO
2
0.56 1.3 0.22 3.8 3.16
P
2
O
5
n.a. n.a. 0.39 0.9 0.72
Cr
2
O
3
n.a. n.a. 0.63 n.a. n.a.
SO
3
8.1 5.2 2.36 n.a. n.a.
Cl 0.5 0.6 0.53 n.a. n.a.
CO
2
n.a. n.a. 12 n.a. n.a.
Total 89 98.9 99.8 100 100.77
 Viking 2 landing site, XRF [ 13 ].
  Mars Pathfinder, APXS (normalized to a sum of 98%) [ 14 ].
   Spirit Rover, APXS data from [ 15 ], recalculated to 12 wt % CO
2
[ 16 ].
    Developed by Allen et al. [ 11 ], (XRF volatile-free, normalized).
 Simulant analyzed as delivered, XRF (volatile-free).
https://doi.org/10.1371/j ournal.pone.0204025.t001
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Martian atmosphere, subsurface, regolith and polar caps with the overwhelming majority
thought to be in the form of ice [ 19 ]. Deposits of water ice that can be > 100 meters thick have
recently been reported [ 20 ]. In addition, since the discovery of recurring slope lineae in 2013
[ 21 ] there is ongoing scientific debate whether there even is contemporary water activity in the
form of liquid water brines in shallow Martian soil.
Materials and methods
The Martian regolith analog (JSC-Mars-1A) in the size fraction < 1000 μ m was supplied by
the Orbital Technologies Corporation (ORBITEC, Colorado, USA). The chemical composi-
tion of JSC-Mars-1A resembles the Martian regoliths analyzed in the course of the Viking
lander, Mars and Spirit Rover missions ( Table 1 ).
Slurry preparation
Two processing routes to produce slurries were tested. In the first route, milling of the raw
JSC-Mars-1A material in a swinging mill was followed by fractionation of material between 25
and 50 μ m in a sieving tower. Water-based slurries with 51 w% solid load were prepared from
this fraction and roll ball milled for 48 hours with 12 mm ZrO
2
grinding balls. The material
had to be added gradually to the container because the fine particles impeded the dispersion of
the material. In another approach, the raw material was simply passed through a coarse
500 μ m grid sieve without an intermediate milling step, directly poured into the water and roll
ball milled in the same manner as in the first route. A commercial porcelain slurry was
obtained from Royal Porcelain Factory in Berlin and used as a reference.
Slip casting
The molds to slip cast rings with an inside diameter of 30 mm and a height of 18 mm were
made using casting plaster with a water plaster ratio of 4 to 5. Rings were cast by placing the
molds on steel plates and filling them generously with slurry. To obtain comparable wall thick-
nesses casting time for JSC-Mars-1A rings was set to 4 minutes and 8 minutes for the porcelain
slurry. After casting the remaining slurry and steel plate were removed and the ring mold con-
taining the wet ring was rotated for 90 seconds to generate a homogenous inside surface.
Finally, casting overlaps were cut from each side using a knife and the rings were left to dry.
To produce vases a plaster mold for porcelain vases from the Royal Porcelain Factory in Berlin
was generously filled with JSC-Mars-1A slurry and left to cast for 6 minutes (the increased
casting time compared with those for the rings was chosen to accommodate the larger vase
geometry) with small amounts of slurry added to keep the liquid level. The mold was emptied
and rotated for 120 seconds and the casting overlap cut off. After 15 minutes casts were
demolded and small casting failures were retouched using a brush and fresh slurry. After the
green body had dried retouched areas and mold burrs were sanded using sandpaper with grit
sizes of 2400 and 4000.
Sintering
For sintering three different firing profiles were applied as follows: (i) heating with 1.7 K/min
to 1000 ˚C, no dwell time, (ii) heating with 1.7 K/min to 1130 ˚C, no dwell time and (iii) heat-
ing with 1.7 K/min to 1130 ˚C with 10 h dwell time; all followed by furnace cooling. The com-
mercial porcelain slurry was sintered with the optimized schedule, i.e. heating with 2 K/min to
1440˚C. The final three sintering schedules were chosen after the side-view hot-stage micros-
copy study with the approach of having one bisque firing schedule (1000˚C) and two schedules
Towards the colonization of Mars by ISRU: Slip cast ceramics from Martian soil simulant
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in the determined sintering range. All sintering runs were performed in a muffle furnace in
standing air atmosphere followed by furnace cooling. The weight loss during sintering was
determined using a laboratory scale weighting dried slip cast green bodies and sintered parts.
Methods
X-ray fluorescence (XRF) analysis of JSC-Mars-1A powder was performed using an autosam-
pler PW 2400 sequential wavelength X-ray spectrometer with Rh-anode (Panalytical, Nether-
lands). Volatile fraction was measured by heating a powder compact prepared from a mixture
of 6 g JSC-Mars-1A powder sample and 1.5 g of Wax C (Hoechst, Germany). A melt tablet was
prepared by fused beads method with 0.6 g JSC-Mars-1A material fused into a glass with 3.6 g
of FX-X65-2 molten flux (Fluxana, Germany) using the high-frequency furnace Rotomelt at
1200 ˚C. Particle sizes were obtained with a LS 13 320 (Beckman Coulter, USA) laser diffrac-
tion particle size analyzer with exchangeable wet (for water dispersed material) and dry (for
powder samples) measurement units. The size distributions were determined for the dry as-
received JSC-Mars-1A material, two different JSC-Mars-1A slurries with different preparation
routes and a commercial porcelain slurry. Rheological properties of slurries were investigated
using a Physica MCR 301 rheometer (Anton Paar, Austria) with parallel-plate geometry (25
mm diameter and 0.5 mm gap size) in rotation mode at 25˚C. To determine an appropriate
sintering schedule for the produced green bodies, side-view hot-stage microscopy (Hesse
Instruments, Germany) was performed on cylinders (3 mm width and 3 mm height) from
ground raw material, spring pressure hand pressed with a pressure of 1.5 N/mm
2
. The mea-
surements were conducted with 3 repetitions (that all gave similar results) in air with a heating
rate of 10 K/min up to 1350˚C with 30 minutes holding time, in a tube kiln. The microscope
projects the image of the sample, irradiated from the opposite site, onto a digital image pro-
cessing system that measures geometry changes during heating as well as changes in width,
height and area of the projected images. To evaluate further the sintering behavior of the speci-
mens two parameters were applied, i.e the area shrinkage S
A
and the isotropic volumetric
shrinkage S
V
defined as S
A
(T) = (A
T0
-A
T
)/A
T0
and S
V
(T) = (V
T0
-V
T
)/ V
T0
= (S
A
+1)
3/2
–1,
where A
T0
, V
T0
, A
T
and V
T
denotes the area (A) and volume (V) at the temperatures T
0
and T,
respectively. A helium gas expansion pycnometer Pycnomatic ATC (Porotec, Germany) was
employed to determine the powder particle density for the raw JSC-Mars-1A as delivered. The
volume shrinkage B and density of green and sintered parts was determined by measuring
ring masses with a laboratory balance (RC210P, Sartorius, Germany) and ring volume by X-
ray computer tomography CT 40 (Scanco Medical AG, Switzerland). These measurements
were performed on one ring sample per sintering temperature before and after sintering for
each sintering schedule. The accuracy of the obtained density values was verified by measuring
dimensions of cast (and sintered) disks with a caliper and calculating the density using their
weight. The linear shrinkage A was calculated from the volume shrinkage B, assuming isotro-
pic shrinkage, according to A ¼ 100 ð ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
B = 100 þ 1
3
p  1 Þ [ 22 ]. The porosity ϕ of the parts was
calculated using ϕ = ( ρ
particle
− ρ
bulk
)/( ρ
particle
− ρ
fluid
) with the JSC-Mars-1A raw particle den-
sity as particle density ρ
particl e
, the bulk density ρ
bulk
and ρ
f lu id
saturating fluid density—the
mass loss of the JSC-Mars-1A powder after firing was considered for the porosity calculation.
Microstructural analysis was carried out with scanning electron microscopy (SEM) using a
Gemini Leo 1530 (Zeiss, Germany) on fresh fracture surfaces of as-slip casted and sintered
samples.
Mechanical properties and Weibull analysis were assessed on 20 identical slip cast samples
for each sintering schedule by brittle ring test in a RetroLine mechanical testing machine
(Zwick/Roell, Germany) at a deformation rate of 100 μ m/min [ 23 ]. As the calculated tensile
Towards the colonization of Mars by ISRU: Slip cast ceramics from Martian soil simulant
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strength values are only to be compared with values for materials obtained from similar ring
tests, as critically discussed by Hudson the rings made of commercial porcelain were charac-
terized in comparison [ 24 ]. Tensile strength σ
R
is determined according to σ
R
= KP /( P r
2
t),
where K is the stress concentration factor being a function of the ratio of internal (r
1
) to exter-
nal radius (r
2
), P—the applied load, and t the thickness of the annulus [ 25 ]. To calculate K we
adapt the equation for concentric rings: K = r
2
((r
1
+ r
2
)  (3  (r
1
+ r
2
)–t))/(((r
1
+ r
2
) − t)  t
2
) [ 26 ].
Evaluating this formula, we found for one of our sintered rings with a ratio r
1/
r
2
= 0.841 K
value of 233. This is in good accordance to a value of K = 232 for the same ratio read from a
diagram in Durelli and also close to a value of K = 226 extrapolated from a table in Batista
[ 27 , 28 ], hence confirming the feasibility of our approach.
Results and discussion
With future colonization of Mars in mind, our goal was to explore the simplest possible slip
casting route to ceramics without any dispersing or binding agents and with a minimum of
technological steps. We have found out that neither a milling step nor the addition of additives
are necessary for achieving good quality slurries from JSC-Mars-1A regolith simulant. Both
particle size distributions and shear viscosity properties underline the suitability of our ISRU-
processing route for formulating slurries with processability characteristics similar to those of
commercial porcelain slip ( Fig 1a and 1b ). This finding is particularly relevant, considering
that it could potentially allow the processing of slurries with only in-situ resources and avoid-
ing a time and energy consuming milling step. In the next step we produce and characterize
three sets of specimens with different complexity and shape, i.e. (i) disks represent the simplest
possible geometry, (ii) rings are used for the evaluation of mechanical properties as well as
Weibull statistics by brittle ring tests and scaling up our production route by slip casting with a
three-part plaster mold to produce a (iii) complex shapes with vase geometry. The drying,
demolding and sintering conditions for the JSC-Mars-1A slurries are explored in the next step.
By applying the square root of time law we analyzed the overall material transport process dur-
ing casting ( Fig 1c ) which is expressed by L
2
/ (Pt/ η ), where L is the layer thickness of the body,
P the differential pressure across the system, t the casting time and η the viscosity of the slurry
[ 29 ]. The slope of the linear fit in Fig 1c is representative for the casting rate, which is indicative
of the “rapidness” of the slip casting process. Thicker walls in ceramic components are
achieved with the JSC-Mars-1A slurries due to the significantly faster casting (n = 1.12 ± 0.04)
compared to an established porcelain slurry (n = 0.73 ± 0.05).
The JSC-Mars-1A casts differ from porcelain as they tended to rupture, this holds especially
for parts with long planar surfaces. To understand this behavior it is important to note that the
regolith simulant does not contain sheet silicates. These phyllosilicates or clay minerals swell
upon hydration leading to the special plastic behavior of partially saturated clays. This plastic-
ity is the most important prerequisite for traditional ceramic processing and gives cast porce-
lain bodies sufficient elasticity to prevent rupture in demolding processes. In contrast,
JSC-Mars-1A casts shows no plasticity when still wet and could not be manipulated without
breaking. Rupture of the slip cast parts during demolding could be mitigated by increasing
wall thickness and using forms with easy to demold surfaces. Hot-stage microscopy is applied
to evaluate the sintering regime and set up an appropriate sintering schedule for the slip cast
ceramics [ 30 ]. Fig 1d and 1e displays the change in the form factor and area of the dry pressed
JSC-Mars-1A pellets in the temperature range of interest for sintering, i.e. between 1100˚C
and 1350˚C, underlying several characteristic points. Changes in the form factor correlate with
changes in the shape of the sample. The temperature range in HSM experiments in which the
area of the samples decreases while the form factor remains unchanged (here 1100–1250˚C for
Towards the colonization of Mars by ISRU: Slip cast ceramics from Martian soil simulant
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JSC-Mars-1A) is the sintering range. The maximum area and volume shrinkage of 0.47 and
0.78, respectively, achievable without deformation of the sample’s shape, is obtained at
1225 ˚C. The melting begins at 1278 ˚C as indicated by rounding of the edges of the pellet.
Noteworthy (but of minor significance for our present study) are the following characteristic
temperatures in HSM measurements: (i) the sphere temperature (1309 ˚C) at which the edges
of the test piece become completely round with the height remaining unchanged. (ii) The
hemisphere temperature at which the sample forms approximately a hemisphere (1325 ˚C),
i.e. when the height is equal to half of the base diameter, and (iii). the flow temperature (1348
˚C) at which the test piece’ height is one-third of its height at the hemisphere temperature. At
temperatures higher than 1250˚C, the area of the sample stops decreasing and starts increasing
instead. This phenomenon is very well known for porcelain bodies, often referred to as “bloat-
ing” [ 31 ] and is related to the undesired release and expansion of gases when firing the ceramic
at excessively high temperatures. This bloating effect is clearly undesirable and for this reason
peak temperatures not higher than 1130 ˚C were selected for sintering schedules of the
JSC-Mars-1A samples. Therefore we set three different firing profiles as follows, sintering at
(i) 1000 ˚C, no dwell time, (ii) 1130 ˚C, no dwell time and (iii) 1130 ˚C with 10 h dwell time.
All three sintering profiles produced mechanically stable parts with significant differences in
Fig 1. Characteristics of the slurries and sintering behavior of JSC-Mars-1A materials. (a) Particle size distributions
in the raw material and in the slurries formulated from differently processed powders in comparison to a commercial
porcelain slurry. (b) Viscosities of the slurries in comparison to the commercial porcelain slurry. The small-sized
particle fraction in the pre-milled slurries causes a significant rise in the viscosity, making these slurries unsuitable for
the casting process. (c) Wall thicknesses of cups slip cast from JSC-Mars-1A and commercial porcelain slurries. (d)
Images of JSC-Mars-1A pellets at characteristic temperatures obtained with hot-stage microscopy. (e) The sintering
range of JSC-Mars-1A derived from the area of the sample image and the shape of the image.
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shrinkage, mechanical characteristics as well as colors ( Fig 2 ). The best quality parts are
obtained at 1130 ˚C with 10 h dwell time with volume and linear shrinkage as much as ~62%
and ~17% ( Table 2 ), respectively, which were significantly greater than the typical shrinkage of
phyllosilicate based ceramic material systems such as porcelain.
SEM micrographs show that in slip cast green samples larger crystalline grains are embed-
ded in a voluminous but loose matrix of extremely fine particles ( Fig 2a ). After bisque firing
changes in the microstructure seen in SEM ( Fig 2b ) are not very pronounced. For this temper-
ature treatment the overall porosity (see Table 2 ) does almost not change (62.95% to 62.78%)
while there is an increase in density (1.35 g/cm3 to 1.44 g/cm3) plus a significant volume
shrinkage (30,46%) and mass loss (22.61%). With increasing firing temperature (1130˚C, Fig
2c ) a significant portion of the sample starts to melt, which in turn leads to less pores with
increased size. If the peak temperature of 1130˚C is maintained for 10h ( Fig 2d ), the resulting
structure is a completely vitrificated matrix supporting a few idiomorphic crystals. In-depth
analysis of microstructural and mineralogical evolution during sintering of slip cast parts from
JSC-Mars-1A will be the topic of an upcoming publication. Since the sintering treatment
seems to have an effect on the form stability, we analyzed the μ CT data by dividing each data
set from top to bottom into ten sections from which we obtained ten outlines. The outer- and
innermost lines from these ten overlapped ring outlines where used to define the area of devia-
tion (green surfaces around the rings in Fig 3 ). To give an idea of the increasing sintering
deformation with higher temperatures, the areas of deviation (green surfaces) where normal-
ized by dividing them though the respective areas of ideal circles that had the averaged inner
Fig 2. Characteristics of the slip cast parts. Photographic images (left bottom) overlaid with the μ CT images (left top)
and SEM images (right) of JSC-Mars-1A slip cast rings after demolding (green body) (a) and that sintered at 1000 ˚C
(b), 1130 ˚C (c) hold 10 h at 1130 ˚C (d).
https://doi.org/10.1371/j ournal.pone.0204025.g0 02
Table 2. Shrinkage (green to sintered), density, porosity and Weibull parameters of slip cast ring samples.
Sample Shrinkage (volume/
linear), % 
Bulk density,
g/cm
3
Mass
loss, % 
Porosity,
%
Area deviation
factor, %
Tensile
strength, MPa
Weibull
parameter m
Charac-teristic
strength, MPa
JSC-Mars-1A, green body - 1.35 - 62.95 1.75 - - -
JSC-Mars-1A, sintered at
1000 ˚C
30.46 / 9.27 1.44 22.61 62.78 4.39 14 ± 3 4.2 15
JSC-Mars-1A, sintered at
1130 ˚C
52.36 / 15.07 2.23 22.68 40.35 6.61 36 ± 10 3.8 40
JSC-Mars-1A, sintered at
1130 ˚C with 10 h dwell time
61.73 / 17.38 2.65 22.76 28.23 8.89 46 ± 11 4.5 51
Porcelain sintered at 1400 ˚C - - - - - 3 ± 6 4.7 41
 from green body to sintered part
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diameters of all 20 rings at that specific sintering temperature, obtaining a simple area devia-
tion factor ( Table 2 ). While the green body samples showed a deviation of ~1.8%, the value
increases with sintering temperature and time—the sample fired at 1130˚C for 10 h has a max-
imum deviation of ~8.9% and the highest degree of eccentricity. As the analysis of the micro-
structure showed vitrification of the matrix we conclude that this is the result of partial
melting / liquid phase sintering of these samples leading to an increased sinter deformation.
As the main iron-containing phases are hematite and maghemite, these are likely responsible
for the reddish-brown colors. The color change into reddish is associated with the oxidation
state of iron in oxides typical for fired earth (oxidation of Fe
3
O
4
to Fe
2
O
3
) as described by
Sherriff et al. for ancient roman pottery [ 32 ]. According to X-ray photoelectron and Mo ¨ ss-
bauer spectroscopic characterization it could not be attributed to the change of the Fe
3+
frac-
tion in hematite. Fig 3 shows the shape of the load-displacement curves with fracture
occurring in a two-step process. During the tests we observed that the first peak is associated
with a fracture across the diameter parallel to the loading plane and the second break is due to
the transverse diameter of their outer periphery. The relatively low Weibull parameters and
high level of standard deviation could be a result of inhomogeneities in the geometry of the
rings, on the one hand in the inner surfaces of the rings from the removal of excess slip after
the casting process, as well as from sintering deformation. Overall our slip cast ceramics from
Martian soil simulant show exceptionally good mechanical properties compared to the porce-
lain reference. Of the three sintering schedules only the JSC-Mars-1A samples sintered at
1000˚C showed a characteristic compressive strength below that of porcelain (15 MPa), while
the ones sintered at 1130˚C without dwell time where similar to porcelain (40 MPa) and the
samples sintered at 1130˚C for 10 h surpassed the reference with a value of 51 MPa. This is
especially noteworthy as these samples showed a high calculated porosity (28.23%) compared
to standard porcelain, which exhibits firing temperature dependent porosity values as low
as < 5% [ 31 , 33 ], and the general rule for the bending strength of ceramics being that the
strength decreases exponentially with the increase in porosity [ 31 ].
With future colonization of Mars in mind, we verified the applicability of our processing
route by using a three-part plaster mold to slip cast a vase geometry ( Fig 4 ). We found the
developed casting system to be easily scalable to this bigger size and once we took care not to
damage the geometry during the demolding process, we were able to cast intact vases that had
a height of 12.5 cm before drying. After being treated in the above developed firing conditions,
Fig 3. Mechanical properties of slip cast parts. a) Characteristic shape of load-displacement curves obtained by
diametral compression (brittle ring test) of four different slip cast ring samples at 100 μ m/min. b) Probability of failure
as a function of stress (lines), calculated from the Weibull parameters, compared with the experimental values
(symbols) and their corresponding probability of failure.
https://doi.org/10.1371/j ournal.pone.0204025.g0 03
Towards the colonization of Mars by ISRU: Slip cast ceramics from Martian soil simulant
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the vases showed the same mechanically stable characteristics as the simpler ring forms, dem-
onstrating that our simple production route can be scaled to produce stable complex shapes
that might be used in possible colonization scenarios of Mars.
Conclusions
In our work we successfully fabricated mechanically stable ceramic geometries from exclu-
sively Martian resources. Our developed production route is simplistic, yields ceramics that
meet the requirements of everyday use and could therefore serve as a starting point for future
Mars colonization. We demonstrated that the wet processing of Martian surrogate material via
slurries into solid ceramics presents a promising alternative to the multitude of dry consolida-
tion approaches that are presented in literature. Recently there have been reports on the pro-
cessing of JSC-Mars-1A using additive manufacturing [ 34 , 35 ] and our findings could similarly
pave the way for such novel techniques relying on materials in water-dispersed form, such as
additive manufacturing technologies like layer wise slurry deposition (LSD) or laser-induced
slip casting (LIS) that can be controlled remotely and would enable a production of complex
shapes without humans being present on Mars.
Acknowledgments
The authors wish to thank the Royal Porcelain Factory (KPM) in Berlin for providing the vase
mold as well as the reference porcelain slurry. Furthermore, we wish to thank Petra Marsiske
for the XRF measurements.
Fig 4. Vases slip cast with the JSC-Mars-1A slurries. Top panel: Slip casting fabrication procedure of vases. Bottom
panel: Vases, from left to right: directly after demolding, dried green body, 1000˚C without dwell time, 1130˚C without
dwell time, 1130˚C with 10 hours dwell time.
https://doi.org/10.1371/j ournal.pone.0204025.g0 04
Towards the colonization of Mars by ISRU: Slip cast ceramics from Martian soil simulant
PLOS ONE | https://doi.org/10.1371/journal.po ne.0204025 October 11, 2018 9 / 11

Author Contributions
Conceptualization: David Karl, Aleksander Gurlo.
Funding acquisition: Aleksander Gurlo.
Investigation: David Karl, Franz Kamutzki, Andrea Zocca.
Methodology: Andrea Zocca, Oliver Goerke.
Resources: Andrea Zocca, Oliver Goerke, Jens Guenster, Aleksander Gurlo.
Supervision: Aleksander Gurlo.
Visualization: David Karl.
Writing – original draft: David Karl, Franz Kamutzki.
Writing – review & editing: David Karl, Franz Kamutzki, Andrea Zocca, Jens Guenster, Alek-
sander Gurlo.
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Why institutions use Plag.ai for originality review, entry 21

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