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ANYAGTUDOMÁNY
Remnants of organic pore-forming
additives in conventional clay brick
materials:
Optical Microscopy and Scanning
Electron Microscopy study
FERENC KRISTÁLY Institute of Mineralogy and Geology, University of Miskolc askkf@uni-miskolc.hu
LÁSZLÓ GÖMZE A. Department of Ceramic and Silicate Engineering, University of Miskolc
femgomze@uni-miskolc.hu
Szerves pórusképző adalékanyagok maradványai a hagyományos agyagtégla anyagban:
Optikai- és pásztázó elektronmikroszkópos vizsgálatok
Agyagtégla-gyártási vizsgálati anyagmintákat kísérleti égetésnek vetettünk alá a szerves
pórusképző adalékanyagok viselkedésének tanulmányozása céljából. Fűrészport, napraforgómag-
héjat és rizskorpát adagoltunk a nyers agyaghoz, és azt 900°C-on kiégettük. A nyers- és az égetett
anyagok ásványi összetételét RTG por-diffrakciós (XRPD) eljárással határoztuk meg. A nyers-
a nyagok termikus viselkedését derivatográffal (DTA) vizsgáltuk. Optikai mikroszkóppal (OM) és
pásztázó elektronmikroszkóppal (SEM) tanulmányoztuk a kiégetett anyagok mikroszerkezetét.
Az OM és a SEM kimutatta a növényi eredetű anyagok mikroszerkezetét megőrző maradványok
jelenlétét. A jelen tanulmány fő célja a szerves adalékanyagok maradványainak morfológiai jel-
lemzése, és azok kialakulási folyamatainak a felderítése.
Ferenc Kristály
Graduated in 2005 with Master of Science
equivalent geologist diploma, at the Babes-Bolyai
University (Cluj Napoca, Romania); diploma
thesis “Characterization of mineralogy and
microstructure of C111 type silica porcelain”.
In 2005, admission in the PhD program at
the Department of Mineralogy and Petrology,
University of Miskolc (Sámuel Mikoviny
Doctoral School of Earth Sciences). Scientific
activity – conference participating: 20-24
September 2004, Miskolc (Hungary): 2nd
Mid-European Clay Conference 9-10 March
2006, Miskolc (Hungary): 3rd Mineral Sciences
in the Carpathians Conference: member of
Organizing Committee, Co-editor of Abstract
Volume 12-18 August 2007, Miskolc (Hungary):
6th International Conference of PhD Students
Scientific Papers: Gorea M.–Kristály F.–Pop, D.
(2004): Characterization of Some Kaolins Used
for Producing Electric Insulator Ceramic. Acta
Mineralogica-Petrographica, Abstract Series, V.
4., 2004, Szeged Egyetemi Kiadó, pp.44. Gorea,
M.–Kristály, F. (2007): Study of the Distribution
and Shape of the Pores in Silica Porcelain. Rev.
Chim. (Bucharest), Vol. 58/2, pp. 146-150.
Dr. Gömze A. László
1973-ban szerzett gépészmérnöki oklevelet
a Moszkvai Építőmérnöki Egyetemen.
Szilikátvegyész oklevelét 1979-ben kapta a
Mengyelejev Kémia-technológiák Egye temen.
1985-ben a műszaki tudományok kandidátusa - kitüntetéssel. Szakmai pályafutását az
Épületkerámia-ipari Vállalatnál kezdte, ahol részt vett a Kerámia Téglagyár, a Padlólap II. és az új
Őrbottyán II. Téglagyár tervezésében. 1977-ben már a KEVITERV Egyedi Gépek és Létesítmények
tervező osztályát irányította. Még ebben az évben tanársegéd lett Szaladnya Professzor mellett
a Miskolci Egyetemen. Ugyanitt 1999-től a Kerámia- és Szilikátmérnöki Tanszék vezetője. Több
szabadalom és találmány szerzője. Hazai és külföldi publikációjának száma meghaladja a 200-at.
Introduction
Conventional clay bricks are the most frequently used
building materials in the past few centuries of human history.
Production of clay bricks does not desire special raw material
processing, preparation process or ring techniques, the bricks
can be prepared from the raw clay, in producing facilities set
up in the nearby of raw material source, decreasing the costs of
production. e clays used in brick production are a mixture of
plastic, hydrated minerals - the clay mineral fraction, and non-
plastic minerals. e plastic minerals are clay minerals such as
montmorillonite, vermiculite, chlorites, illite and kaolinite. e
non-plastic part of clays is made up by micas, most frequently
muscovite and biotite, feldspars and quartz. Carbonates like
calcite and dolomite, or siderite also are present in variable
amounts. As impurities, iron oxy-hydroxides and detrital
organic matter are present.
During time, clay brick production techniques were adapted
to t the requirements of new product types. Where buildings
were exposed to high humidity media, the resistance of brick
was increased by rising the ring temperatures. Excessive
plasticity of raw clays was treated by the addition of vegetal
materials, usually straw. e same technique was applied when
an increase in the dry mechanical resistance was desired.
Possibilities of improving the mechanical properties of red
products by the addition of organic materials of vegetal origin,
like rice husks [1] or derived products of these, like sawdust
ash [2] or rice husks ash [3] were also investigated.
By the increasing energy demand of heating of the buildings
for human living, the attention of brick producers was directed
towards the thermal isolation capacity of building materials. In
the case of traditional clay brick, the solution for increasing the
thermal isolation capacity (thus decreasing heating costs) is the
arti cial increase of porosity. Beyond the primary porosity of
red clay material, caused by the decomposition of carbonates,
contraction of clay minerals and organic matter combustion,
the introduction of pore-forming additives contributes to the
increase of porosity of bricks. Di erent inorganic (calcite,
dolomite [4]) and organic (wastes from di erent industrial
activities [5], [6], [7]) types of additives were tested for suitability
in brick production. In published papers, the attention of brick
producers is directed towards the ceramic properties of materials
with additives, such as compressive and bending strengths,
capillary water up-take and capacity of heat conductivity, which
are the main properties that characterize building materials [8],
[9]. However, the transformation reactions of the additives and
their interactions with the transformations su ered by the clay
during ring are also important to know, from the point of view
of microstructure stability and mineral phase equilibrium in the
red products [10], [11].
In this study we discuss results of observations made on
the transformation of sawdust, sun ower seeds hull and
ungrounded rice husks in red laboratory samples.
Materials and Methods
e samples considered in the present study were obtained
from clay used in brick production, mixed with the three
organic pore-forming additives: sawdust, sun ower seeds
hull and rice husks. Sample preparation and ring was done
in the micro-pilot laboratory of the Department of Ceramic
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Materials and Silicate Engineering, at the University of Miskolc.
e admixtures were homogenized in a Koller homogenizer
and cylindrical samples of 3 cm in diameter were extruded
by vacuum extruder. Extruded samples were owen-dried and
red at 900˚C in oxidative atmosphere, with linear heating up
and a soaking time of 2 hours.
e determination of mineralogical compositions and
observations on the microstructure of raw and red samples
were performed at the Department of Mineralogy and
Petrology, University of Miskolc. e compositions of raw
materials pore forming additives and red samples were
determined by X-ray Powder Di raction (XRPD) on a Bruker
D8 Advance di ractometer in Bragg-Brentano geometry
equipped with Cu-Kα radiation source. Clay and red
samples were prepared by grounding in agate mortar, while
samples of additives were ground via liquid N
2
freezing.
ermal behavior of clay and additive samples was tested by
Derivative ermal Analysis (DTA) on a MOM Derivatograf
C PC handled apparatus. Characterization of microstructure
of raw additives and red samples was performed by Scanning
Electron Microscopy (SEM) and Optical Microscopy by plane-
polarized light (OMPL). OMPL observations were carried out
on a Leitz -Wetzlar microscope, on thin sections prepared from
red samples, in order to distinguish between amorphous and
crystalline phases associated with grains of additives. Samples
for OMPL were embedded in acrylic resin and polished with
diamond paste. Parallel with SEM observations, the chemical
composition of di erent phases was checked by Energy
Dispersive Spectrometry (EDS) on a Jeol JXA 8600 Superprobe
at 15 kV and 15 and 20 nA. Back-scattered Electron (BSE)
imaging technique was applied to enhance chemical contrasts
in the microstructure. SEM analyses were carried out, with
graphite coating, both on fracture and polished surfaces. For
the later, samples prepared for OMPL were used.
e mineralogical composition of clay, by XRPD, is given by
illite, kaolinite, vermiculite and chlorite, among with important
amounts of quartz, calcite and dolomite. Presence of muscovite
and feldspars is signi cant also. Pyrite and goethite are present
as accessory minerals.
Characterization of raw additive samples
Fig 1. XRPD pattern of additives. Cellulose is the main component.
1. ábra Az adalékanyagok röntgen-pordi rakciós felvételei. A fő alkotó a cellulóz.
Based on XRPD analysis the organic pore-forming additives
are composed mainly of cellulose (Fig 1).
e DTA analyses revealed the main thermal domains
in which the organic matter of additive samples su ers
transformation reactions (Table 1). e rst domain is
characterized by a strong endothermic reaction between 50 and
190 ˚C, associated with a weight loss of ~ 7% due to the loss of
adsorbed water and volatile compounds.
e second domain
is determined by the strong exothermic reaction between
190 and 390 ˚C, associated with weight losses in di erent
percents for the di erent samples. is reaction is due to the
oxidation of hard organic compounds from the composition of
vegetal matter, like lignin and cellulose. A er this reaction, the
cellulose framework of organic matter decays. Following the
second domain a series of smaller endothermic reactions can
be observed, with a continuous weight loss up to 15%. ese
reactions can be associated to polymorphic transformations of
phases of residual carbon and inorganic compounds from the
vegetal matter [12].
Thermal
domain
Parameters Sawdust Sunflower
seeds hull
Rice husks
1
∆TG wt% 7,66 6,91 7,4
DTA T
0
52 49 47
DTA T
max
116 113 131
DTA T
1
184 186 190
A
DTA
-5,421 -6,959 -9,925
2
∆TG wt% 51,25 53,2 41,95
DTA T
0
170 191 191
DTA T
max
295 276 285
DTA T
1
385 376 379
A
DTA
11,921 14,055 14,736
Table 1. e main thermal reactions of vegetal additive material
1. táblázat A növényi adalékanyagok fontosabb termikus reakciói
Observations by SEM on the raw additive material helped
to understand the structure of vegetal materials used and to
link the observed remnants to the original materials (Fig 6
and 7). e composition of vegetal samples was checked for
cations and mineral matter content by EDS measurements. e
grains from sawdust have a brous structure, with bers empty
on the inside. e material building up bers has a massive
structure; contents of Ca, Mg and locally Si were detected. e
structure of material building up the hull of sun ower seed
has a more porous structure, longitudinal channels can be
observed. Porosity of material is increasing towards the interior
of the hull. e outermost sheet has a compact structure and
is enriched in K (the lighter sheet of the structure showed in
Fig 6, image from the center). e structure of rice husks is
compact, and towards the inner side a gradual enrichment in
Si is observed. e innermost sheet is very rich in Si and has
an uneven surface.
400
300
100
organic additives
200
0
7 10 20 30 40
2-Theta - Scale
Lin (Counts)
rice husks
sunflower seeds hull
sawdust
cellulose
cellulose
dichloro-
glyoxime
oxalic acid
cellulose
Characterization of red samples
A er ring, all the samples presented the characteristic
homogenous “brick” colour, indicating the uniformity of
ring. e semi-quantitative mineralogical composition of
the red samples determined by XRPD is listed in Table 2.
e composition is that characteristic for clay bricks made
from clay with carbonates [13], characterized by the presence
of newly formed minerals such as diopside and gehlenite. e
gehlenite is present as an intermediate member of the gehlenite
-ackermanite series. Muscovite and quartz are preserved from
the clay material, and a neo-formation of feldspars is observed.
e mineral phases present do not defer between samples with
di erent additives, only variation in their relative percentage
can be observed.
Phases F(%) R(%) N(%)
Albite calcian
(NaCa)AlSi
3
O
8
27 23 29
Gehlenite
Ca
2
(MgAl)(Si
2
AlO
7
)
11 5 7
Quartz SiO
2
32 29 29
Augite
MgCaFeSi
2
O
6
81012
Hematite, syn Fe
2
O
3
332
Muscovite
KAl
2
(Si
3
Al)O
10
(OH)
2
586
Microcline maximum
K(AlSi
3
)O
8
10 11 7
Anhydrite CaSO
4
340
Pseudowollastonite
Ca
3
(Si
3
O
9
)
102
Akermanite
Ca
2
Mg(Si
2
O
7
)
176
Table 2. Semi-quantitative mineralogical composition of the red samples based on
XRPD
2. táblázat A minták félmennyiségi ásványtani összetétele röntgen-pordi rakciós
vizsgálatok alapján
A. OMPL study. Observations by optical microscope with
plane-polarized light in thin sections were
performed to study the relations of the remnants of pore
forming additives to the matrix of the samples.
Fig 2. OMPL image of sawdust remnant grain in transversal section to the wood
ber (le image at II N, right image with xN)
2. ábra Fürészpor szemcse maradványának polarizációs fénymikroszkópos (PFM)
felvétele, a rostok irányára merőleges metszetben (jobbra II N, balra xN)
Based on the optical properties, the material building up the
remnants PA is of inorganic nature, with amorphous structure
and colored by hematite identically to the matrix of the samples.
e di erent PA generates remnants in di erent relations to
the matrix. Contraction of the mineralized remnant relative
to the original size of additive grain can be determined as the
distance between the remnant and the pore enclosing it. In
the case of sawdust, the material of remnants is mostly jointed
with the matrix, without large separation surfaces surrounding
it (Fig 2). e bers from wood materials inner structure are
preserved, replaced by the inorganic, mineral matter, lled or
empty in the inside (Fig 2, le image).
Fig 3. OMPL image of remnant from the hull of sun ower seed. e remnant
preserves the geometry of pore (le image at II N, right image at xN)
3. ábra Napraforgómag héj maradványának PFM felvétele. A maradvány megőrzi a
pórus geometriáját (balra II N, jobbra xN)
e external walls of bers are totally preserved and not
deformed. e remnant is made up by amorphous material.
e circular section of bers suggests an elastic behavior during
the shaping process, the extruding do not cause irreversible
deformation of exible vegetal structures. e absence of
separation surfaces indicates the good adherence of sawdust
grains to the clay particles, and a low rate of contraction during
combustion of organic compounds.
Fig 4. OMPL image of rice husks remnants. A symmetrically arranged network of
amorphous phase is formed (le image at II N, right image at xN).
4. ábra Rizshéj maradványának PFM felvétele. A maradványt egy szimmetrikusan
elrendezett amorfanyagból álló háló alkotja
e samples with sun ower seeds hull show less developed
remnants, a more pronounced contraction of organic matter
before mineralization is observed, materialized under the form
of inner rings, with identical geometry to the pores created by
the combustion of organic matter (Fig 3). e higher rate of
contraction is due to the lower cellulose contain of sun ower
seeds hull than of sawdust grains. e morphology of pores
enclosing remnants is similar to that of raw additive grains, thus
we have to deal again with elastic behavior during extruding
process. e sameness between pores and remnant grains
geometry suggests a low rate decomposition process, which
allows contraction without skewness. e remnants observed
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in samples with rice husks are a glass like, slightly colored
matter, forming a network structure that preserves the walls of
cells from the rice husks. From point of view of contraction and
amount of remnant phase formed, these samples could be placed
between the former two (Fig 4). e shape of pores from the
honeycomb-like structure is similar to the structures observed
in the raw samples (Fig 5). is suggests the elastic behavior of
grains again and a high content in mineral components of raw
organic structure. Small quartz-like crystals can be observed
in the glass like phase.
Besides the parallel orientation of elongated grains in the
matrix to the PA, due to pressed-processing of samples, no
mineralogical, chemical or grain-size zoning is observed
around grains of PA.
Fig 5. e network of glass-like material from remnant of rice husk, on the le (OMPL
image, II N). e Si-rich areas in the raw husk, right image (BSE image)
5. ábra Az üveges rácsszerű anyag a rizshéj maradványaiból, a baloldali képen (PFM
felvétel, II N). Si-ban dús zónák a rizshéj szerkezetében, jobb oldali kép (Pász-
tázó elektronmikroszkóppal, visszaszórt elektron kép)
B. SEM study. SEM
observations were
carried out on both raw
samples of additives, and
the remnants of additive
grains in red samples.
In order to prepare
polished surfaces,
the samples were
embedded in synthetic
resin, both the raw and
the red samples. is
way the preservation
of structures for the
remnants was assured. In
Fig 6, the representative
structures are shown for
the three additives. e
question of the origin
of remnants was solved
by matching the SEM
observations made on
the raw samples with
that from red samples.
In Fig 7, remnants of
additives from red
samples are shown in
order to emphasize the perfect preservation of structures. Due
to the application of BSE imaging, we were able to characterize
the chemical homogeneity of samples, too. On the raw samples,
one can easily distinguish the enrichment of K in the case of
sun ower seeds hull and the Si for the rice husks. is chemical
zoning is not visible on the remnants, most likely because these
parts richer in mineral matter survived the ring mainly. In the
case of samples with sun ower seeds hull we can observe an
oriented distribution of mica akes, parallel to the wall of pores
(as seen in central image from Fig 7, white elongated grains
represents the transformed mica lamellae). is means that
sun ower seeds hulls are stronger in raw state than the forces
applied when samples were extruded, and created oriented
structures in the microstructure.
Fig 7. BSE images of the red samp
les, on polished surfaces. On
the 1., samples with saw-
dust, in the 2. sample with
sun ower seeds hull, on the
3. sample with rice husks.
7. ábra Polírozott felületen készült
BSE felvétel. 1. fűrész port
tartalmazó minta,
2. napraforgó maghéjat
tartalmazó minta,
3. rizshéjat tartalmazó
minta
On fractured surface, observations have revealed the 3D
shapes of remnants of PA and con rmed the replacement of the
organic matter by an inorganic one, during the process of ring.
e shapes are preserving the morphology of hard parts of the
di erent organic additives, practically replacing the organic
components. Samples with sawdust are characterized by the
presence of remnants built up by parallel bers of ~30 µm in
diameter (Fig 8), partially lled with inorganic substance.
Fig 8. Secondary electron image on remnants of sawdust grain. On the le image,
totally preserved bers from the structure of wood. On the right, detailed im-
age of bers
8. ábra Fűrészpor maradványának szekunder elektron képe. A baloldali képen a
teljesen megőrződött rostok láthatók. Jobboldalon a rostok részletes képe
Fig 6. Representative structures for the di erent additive materials. BSE images, 1. saw-
dust grain, 2. fragment of sun ower seeds hul, 3. fragments of rice husks.
6. ábra A különböző adalékanyagokra jellemző reprezentatív nyers szerkezetek.
Visszaszórt elektron kép, SEM felvétel. 1. fűrészpor szemcse, 2. napraforgó
maghéj töredék, 3. rizshéj töredékek.
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e remnants of organic additives can be characterized by
OM, SEM and EDS. e presence of the remnants of organic ad-
ditives in the microstructure of brick materials may have several
e ects on the properties of bricks produced from the test mate-
rial. Since the grains of pore-forming additives are replaced by
a solid substance in di erent percentage, the e ect of additives
over the heat conductivity capacity should not be the one expect-
ed. To understand the manner in which the addition of vegetal
materials to brick materials in uences the production process,
and properties of red products, further research is made.
Although the question of applying organic additives in the
brick production have been investigated from the points of
view of ceramic properties (raw plasticity, hardness, apparent
porosity, bulk density, compressive strength), the matter of
replacement of the original organic compound by a secondary
inorganic compound, with the preservation of the initial
morphology, wasn’t discussed. e importance of this process
in the thermal isolation and mechanical properties of bricks, as
well as the impact on mineralogical composition of samples, is
subject for further investigation.
e phenomenon of replacing organic, vegetal with inorganic,
mineral matter could have importance in archeological studies.
References
[1] G.W. Carter, A.M. Cannor, D.S. Mansell (1982): Properties of bricks incor-
porating unground rice husks. Building and Environment, Vol. 17/4, pp
285-291.
[2] U. E. Augustine (2006): E ect of addition of sawdust ash to clay bricks. Civil
Engineering and Environmental Systems. Vol. 23/4, pp 263-270.
[3] M.A. Rahman (1988): E ect of rice husk ash on the properties of bricks
made from red lateritic soil-clay mix. Materials and Structures, Vol. 21/3,
pp 222-227.
[4] G. Cultrone, E. Sebastian, M.J. de la Torre (2005): Mineralogical and phy-
sical behavior of solid bricks with additives. Construction and Building
Materials 19, 39-48.
[5] Ismail Demir (2006): An investigation on the production of construction brick
with processed waste tea. Building and Environment 41, 1274-1278.
[6] W. Russ, H. Mörtel, R. Meyer-Pittro (2005): Application of spent grains to
increase porosity in bricks. Construction and Building Materials 19, 117-
126.
[7] I. Demir, M. Serhat Baspinar, M. Orhan (2005): Utilization of kra pulp
production residues in clay brick production. Building and Environment 40,
1533-1537.
[8] V. Bánhidi, L.A. Gömze, P. Pázmándi (2007): Modi cation of heat conduc-
tivity in commercial brick products by recharging bio-waste material additi-
ves. MicroCAD 2007., Materials Science and Material Processing Techno-
logies, pp. 1-4, 2007
[9] P. Pázmándi, V. Bánhidi, I. Kocserha, L. A. Gömze (2007): Investigation of
the e ect generated by the mixing ratio of pore forming additives in tradi-
tional brick products changing the pore texture and mechanical properties.
MicroCAD 2007., Materials Science and Material Processing Technolo-
gies, pp. 73- 78, 2007
[10] Kristály, F., Zajzon, N. (2007): Behaviour of organic pore-forming addi-
tives in traditional clay-bricks: e ects on mineralogy and microstructure.
6
th
International Cnference of PhD Students, 12-18 august 2007. Natural
Science section, pp. 65-70.
[11] Kristály, F., Zajzon, N. (2007): Clay-brick fabrication and metasomatic
processes, a comparison of natural to synthetic mineral transformation
reactions. 6
th
International Cnference of PhD Students, 12-18 august 2007.
Natural Science section, pp. 71-77.
[12] Robert C. Mackenzie (1957): e Di erential ermal Investigation of
Clays. e Central Press, Aberdeen. 457 p.
[13] M.M. Jordan, T. Sanfeliu, C. de la Fuente (2001): Firing transformations
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Clay Science 20, 87-95.
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In samples with sunflower seeds hull, remnants are
characterized by the presence of polygonal plates of 10 µm wide
and 5 µm long (Fig 9, le image), arranged in multiple rows, in
elongated shapes, lled with inorganic substance, separated by a
narrow line at joints (Fig 9, right image). e remnants represent
the K-rich parts from the original material of the hull.
Fig 9. Secondary electron image on remnants of sun ower seeds hull. On the le im-
age, partially preserved structure from the hull. On the right, detailed image
of the structure
9. ábra Napraforgó maghéj maradványának szekunder elektron képe. A baloldali ké pen
a részlegesen megőrződött héj látható. Jobboldalon a szerkezet részletes képe
Sample R is characterized by the presence of a cellular
structure, with rhomboidal cells of 10 µm wide and 25 µm long
(Fig 10), inside the remnants of the husks. ese structures are
the remnants of Si-rich components inside the husk, mostly
the cellular membrane.
Fig 10. Secondary electron image on remnants of rice husk grain. On the le image,
totally preserved network structure from the interior of husks. On the right,
detailed image of cells
10. ábra Rizshéj maradványának szekunder elektron képe. A baloldali képen a teljesen
megőrződött sejtszerkezet látható. Jobboldalon a sejtek részletes képe
From SEM observation on polished surface no presence
of reaction rims around the remnants of organic additives
was observed, indicating the homogeneity of the material.
e chemical composition of remnants was tested by Energy
Dispersive Spectrometry measurements. In the case of F sample
the composition is dominated by Mg, Ca with Si and Al and
small amounts of Fe. e N sample is similar to the F sample,
with variable Mg-Ca ratio, while in the case of R sample the
composition is SiO
2
with minor Fe present.
Conclusions
e main mineralogical composition of red samples is not
a ected by presence of the remnants from organic additives.
e degree of delity at which the structures from vegetal
materials are preserved is unique. is is showing a slow and
linear decomposition of organic compounds, that allowed the
non-organic fraction of material to rearrange and replicate the
structure.