Title

Various Modes of Recrystallization of Ice

Author(s)Luyet, B.

Citation

Physics of Snow and Ice : proceedings, 1(1), 51-70

Issue Date

1967

Doc URL

http://hdl.handle.net/2115/20285

Typebulletin (article)

Note

International Conference on Low Temperature Science. I. Conference on Physics of Snow and Ice, II. Conference on
Cryobiology. (August, 14-19, 1966, Sapporo, Japan)

File Information1̲p51-70.pdf

Instructions for use

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

Various Modes of Recrystallization of Ice

B. LUYET
American Foundation for Biological Research, Madison, Wisconsin, U.S.A.

Abstract
The term recrystallization has been applied to structural changes occurring in aqueous solutions
solidified at low temperatures, in which the crystallizable water has either crystallized, or has remained
amorphous, or is partly crystalline and partly amorphous. -These changes, most of which areillustrated in photomicrographs, are classified into four main types: (1) the abrupt turning opaqueof the preparations (irruptive recrystallization); (2) the slow coarsening or their structure (slow-pacerecrystallization); (3) the growth of some crystalline particles near the melting point (premelting
recrystallization); (4) a transition from one crystalline structure to another (reorientative recrystallization). The first type comprises four subtypes classified on the basis of the conditions underwhich the opacity is produced and the kind of crystallization units affected. -The temperature atwhich irruptive recrystallization occurs depends primarily on the molecular weight of the soluteand varies only slightly with its concentration. Slow-pace recrystallization occurs in the range
between the temperature of irruptive recrystallization and the melting point. -The mechanism:
involved in the changes reported appears to consist in a transition either from the amorphous tothe crystalline phase (a true devitrification), or from a partly crystalline to a more completely
crystalline composition (a crystallization), or from one crystalline formation to another (a true recrystallization). The latter may take place in two ways: crystalline particles of a given size may
grow larger, at the expense of small ones by migration of m01ecules from the small to the large
ones (migratory recrystallization); or a reorientation may be involved, as in the transition fromcubic to hexagonal ice (reorientative recrystallization).

I. Introduction
On several occasions, when I brought up for discussion with physicists the subject"recrystallization", I was asked what I meant. When I presented the facts, I generally
found an interested audience to which many of the phenomena described appeared as
new, as they could not be identified with those generally discussed in treatises of physics.
A consultation of encyclopedic dictionaries indicated a rare use of the term recrystallization by physicists, except in some unrelated sense, such as, to designate a crystallization repeated for the purpose of purifying a product. The metallurgists, however, havebeen using the term to designate a process which is closely related to some forms of thephenomena that I am to discuss here. The subject of this lecture being, thus, one which
has not been recognized as a distinct item in the field of physics, my first task is torelate the facts on which the notion of recrystallization is based. My lecture will thenconsist (1) in a presentation of the factual data, and (2) in a discussion of these data,m which an attempt will be made at obtaining an insight in the basic concepts involved.
Since the nature of the phenomena described is still to be established, I use theterm "recrystallization" on a tentative basis, without commitment on its final retention,

52

B.LUYET

although, III using it, I anticipate the conclusion of this paper, that the use of the term,in its broad sense, is justified.
Unless indicated otherwise, the fin'dings to be surveyed are the result of investigations
on aqueous solutions carried out by my collaborators and myself during the last 30 years.
Most of the observations reported were made on thin layers of solutions mounted
between two cover glasses, and refrigerated by/immersion in a precooled bath. Photographs or motion pictures were then made through our cryomicroscope assembly (developed by Luyet and Rapatz, 1957, and by Rapatz and Luyet, 1957).

II. The Fundamental Facts
Before presenting the facts about- the structural changes grouped under the name"recrystallization", I will describe the main types of crystallization units which are toundergo the changes in question. The presentation of the facts will itself consist of
two parts: a descriptive expose of the changes encountered, and an account of the temperatures at which they occur.
PRINCIPAL TYPES OF CRYSTALLIZATION UNITS

The conditions under which a specimen is frozen, in particular the freezing rate,
determine the crystalline :pattern obtained. For the purpose of the present survey threecharacteristic patterns should be mentioned; they are shown, respectively, in I, 2 and
3 of Fig. 1. When the rate of crystallization is low, one has well-formed hexagorial
structures (Fig. 1-1), in which six axial columns of ice originate from a center of crystallization, at 60° angles from each other, and grow branches at 60° angles from them;
then secondary branches arise at 60° angles from the primary, and so on. (One shouldnote that, since we are dealing with solutions, the structure is not a solid crystal, but adendrite bathed in a concentrated solution.) When crystallization is faster, the dendrite
acquires an irregular structure. As shown in 2 of Fig. 1, the number of axial columns
arising from the center is variable and so are the angles between them, and the regularity of the branching is not maintained. When the freezing rate is increased further,
the number of axial columns arising from the center of crystallization is enormous andno branching can be distinguished (Fig. 1-3). The structure obtained is typically what
the crystallographers call a "spherulite". I shall designate the three patterns described
as: "regular dendrites", "irregular dendrites" and "spherulites".
At still higher freezing rates, the radii of the spherulites become so thin that they
are no longer visible and the structure is transparent in ordinary light. In polarized
light, between crossed polarizer and analyzer, it shows the Maltese cross, as illustrated
in 4 of Fig. 1. Since the spherulites vanish out of view at high freezing rates, I named
them "evanescent spherulites".
A st.udy at higher magnifications of the structure of the spherulites showed thebranching. Thus, the three crystallization units described have essentially the samestructure; what differs is the regularity in the arrangement of the parts, the size of theparts and the frequency of the dichotomies.

RECRYSTALLIZA nON OF ICE

53

Fig. 1. The principal types of crystallization units obtained when glycerol solutions ofvarious concentrations are frozen at various rates. X 95. I: Hexagonal form, G M solution,- 35°C; 2: "Irregular dendrite", 1 M solution, -GO°C; 3: Coarse spherulite, 6 M solution,-GO°C; 4: "Evanescent" spherulite, 3 M solution, -80°C (I and 3 from Luyet and Rapatz,
1958; with permission of Biodynamica)
1.

PRINCIPAL CHANGES OBSERVED IN FROZEN PREPARATIONSUNDER VARIOUS CONDITIONS

The changes to be examined will be classified into seven series. In the fourth
series I gathered a variety of changes of which some are readily recognized as belonging
to Series 1, 2 and 3, while others are not sufficiently known to be assigned to a particular
category.

1.

The turning opaque, upon rewarming, of rapidly grown spherulitic structures

As illustrated in I and 2 of Fig. 2, an evanescent spherulite (transparent area in I)
turns rather abruptly opaque (Fig. 2-2) when its temperature is raised to a certain value(-lOOe in the case of gelatin solutions).

54.

B. LUYET

Fig. 2. The three main cases of formation of opacity clouds during the freezing or the
rewarming of solutions. X 95 I: An evanescent spherulite (white area) formed in thefreezing at -35°C of a 35% solution of albumin; 2: The same area as in I after the
preparation had been rewarmed to -·5°C; 3: An irregular dendrite (rosette) formed in
the relatively slow freezing, at -30°C, of a 50% gelatin gel; 1+: The same rosette as in
3 after the preparation had been rewarmed to -10°C; 5 : Large opaque borders of evanescent spherulites developed during the freezing, at -30°C, of a 300/0 gelatin gel; 6:
Development of dark areas at the meeting points of three spherulites during the freezing at -40°C of a 500/0 sucrose solution (3 and 1+ from Rapatz and Luyet, 1959; 6 from
Luyet and Rapatz, 1958; with permission of Biodynamica)

RECRYST ALLIZA TION OF ICE

2.

55

The turning opaque, up~n rewarming, of some slowly grown irregular dendrites

The change just described in spherulitic structures takes place, under similar conditions, in irregular dendrites (such as the rosette represented in 3· and 4 of Fig. 2),
when the temperature is raised to the same point as for the spherulites.

3.

The turning opaque, during cooling, of the' borders of spherulitic structures

The borderlines between two evanescent spherulites, and the areas where three ormore adjacent units come in proximity or in contact, may turn opaque in the course of
the cooling process, as shown in I, 5 and 6 of Fig. 2.

4.

Miscellaneous cases in which specimens turn opaque under various conditions

The behavior of solutions of various solutes, at various concentrations, subjected to
rapid or slow freezing and rewarming, is very complex. In an attempt at a systematic
study of that behavior, I tabulated (Fig. 3) the patterns obtained with gelatin gels ofconcentrations varying from 35 to 65%. The concenfrations studied are given in Column
1, and the treatments applied are indicated at the top of the subsequent columns. These
treatments consisted in: (a) freezing at a moderate rate in a bath at -30°C so that the
crystallization units obtained be of the type irregular dendrites (Column 2); (b) freezing
quite rapidly, in a bath at -100°C, so that the units obtained be evanescent spherulites(Column 3), although they are identifiable as such only at the lower concentrations; (c)rewarming to - 30°C (Column 4); and rewarming to -10°C (Columns 5 and 6).
The facts reported in Fig. 3 can be classified as follows:(A) Cases in which the preparation turns opaque (six cases).
(1) when rapidly cooled
solutions of concentrations varying from 35 ~o 40% are rewarmed to -10°C (Column 5,horizontal Rows 1 to 3); (2) when rapidly cooled solutions of 45 to 50% concentrations
are rewarmed to -10°C (Column 5, Rows 5 and 6); (3) when slowly cooled solutions of
35 to 65% concentrations are rewarmed to -10°C (Column 6); (4) when a 42% solution
is rapidly cooled by immersion in a bath at -100°C (Column 3, Row 4); (5) when arapidly cooled 55% solution is rewarmed to -30°C (Column 4, Row 8); and (6) when arapidly cooled 53% solutic:m is rewarmed, first, to -30°C, and then, to -10°C (Column
5, Row 7). Another case, not represented in Fig. 3, should be mentioned, namely, when
a 30% solution is rather slowly frozen at -25°C (see Fig. 6-3).
(B) Caces in which the preparation turns partly opaque (four cases).
(1) whensolutions of concentrations varying from 35 to 40% are rapidly cooled to -100°C(Column 3, Rows 1 to 3); (2) when a 45% solution is cooled rapidly to -100°C (Column
3, Row 5); (3) when a 53% solution, cooled rapidly to -100°C, is re~armed to -30°C(Column 4, Row 7); (4) when 60 to 65% solutions, cooled rapidly, are rewarmed to-30°C and to -10°C (Columns 4 and 5, Rows 9 and 10).
(C) Cases in which the preparation remains transparent (tv.:o cases).
(1) when a
50% solution is rapidly cooled to -100°C (Column 3, Row 6); (2) when 53 to 65%
solutions are cooled rapidly to -100°C (Column 3. Rows 7 to 10). Another case, notmentioned in Fig. 3, should be added, namely, when solutions of concentrations higher
than 65% receive any of the treatments used in this series of experiments.

5.

Slow coarsening of the structure at gradually rising temperatures
Material frozen into irregular dendrites, which either has a finely granular structure,

56

B,·LUYE1'

1
Cone entration
0/

,0

2

3

4

5

Frozenat _30 0'c

Frozenat _100°

Frozenat _100 0
Rewarmed
to _30 0

Frozenat _100 0
Rewarmed
to _10 0

f

1

35

2

38

N

0
C
H

A
3

40

4

42

5

45

6

50

7

53

8

55

9

60

10

65

N
G
E

0
>

i

\,

,>
',\

i)

6
Frozenat _30 0
Rewarmed
to_lO°

RECRYST ALLIZA TION OF ICE

57

Fig. 4.
Coarsening of the structure in a solution of lOra albumin rewarmed to various temperatures after having been frozen at -40°C. X 95. I: Specimen frozen at _40DC and warmed to-7°C; 2: Same field as in I after the temperature was raised to -3°C; 3: Same field after
the temperature was raised to - 2°C; 4: Same field after the temperature was raised to - PC(from Luyet, 1966; with permission of Eiodynamica)
Fig. 3.
Photomicrographs of the patterns of ice formation observed in gelatin gels of various
concentrations (as indicated in Column 1), (A) after rapid freezing at -lOO°C (Column 3), andrewarming to -30°C (Column 4) or to -lOOC (Column 5); or (E) after slower freezing at -30°C(Column 2) and rewarming to -lODC (Column 6). Magn.: X 37, except for the balls in columns
2 and 5 which are magnified X 75 (from Luyet, 1966; original data from Persidsky and Luyet,
1959; with permission of Biodynamica)

58

B. LUYET

or m which no granularity can be detected, acquires slowly a coarser structure when
the temperature is raised above a certain value. This is illustrated in Fig. 4, which
represents a solution of 10% albumin frozen at -40°C and warmed to -7°C (Fig. 4-1),
and further warmed, at a rate of the order of a degree per minute, to -3°C (Fig. 4-2),
to -2°C (Fig. 4-3), and to -1°C (Fig. 4-4).

In this latter photograph, the frozen

specimen consists of large plates of ice (the white areas).

6.

Crystal growth taking place near the melting point

When the ice from a frozen solution is melting, and the preparation IS maintainedat or near the melting point, one observes a growth of some of the larger particles atthe expense of the small ones, which disappear. This is illustrated in Fig. 5, in which
1 and 2 show the transformation, in 10 minutes, of a preparation crowded with small ice

Fig. 5. Growth of large ice particles at the expense of small ones, in a solution of 10;6
albumin maintained close to its melting point. x 440. I and 2: Two frames from a timelapse motion picture, separated from each other by a time interval of 10 minutes. 3,
4 and 5: Three frames of a time-lapse motion picture, the first of which preceded thesecond by 20 seconds, and the third by 25 seconds (from a paper in press by Luyet,
Gehenio and Sager; with permission of Biodynamica)

59

RECRYST ALLIZA TION OF ICE

particles of vanous shapes, into one with a few large spheroidal particles, and 3, 4 and5 show the gradual decrease in size and disappearance of small particles (such as theone in the ~enter of 3 and 4).
A motion picture film (projected at the Conference) showed that the disappearanceof a small particle near a large one coincides with a growth and bulging out of thelarge particle toward the small one, indicating 'that the two phenomena, melting of one
particle and growth of the other, are correlated.
The growth of large particles and the melting of small ones, and the opposite process, the melting of large particles and the growth of small ones, were also shown inmotion pictures to alternate when the temperature was alternately raised and loweredby means of a microheater in the preparation.
Another motion picture film, taken by Drs. MacKenzie and Rapatz of our Laboratory,
showed a passage from polyhedral ice crystals to particles of spheroidal shape, and viceversa, when the temperature was, respectively, raised and lowered. The changes werealternated several times in succession.

7.

Changes interpreted as indicative of a reorientation of the crystalline structure

Several investigators have reported that pure water crystallizes into cubic ice at very
low temperatures and that, upon warming, a reorientation of the structure takes place;
resulting in the formation of hexagonal ice. Dowell, Moline and Rinfret (1962) reported
the same change in the case of gelatin gels. (Since the conclusion that cubic ice ischanged into hexagonal ice involves an interpretation of the factual data, I wish to pointout· that only the changes reported that is, the X-ray or electron diffraction data could
be classified under the title "Fundamental Facts", not their interpretation.)II.

TEMPERATURES OF OCCURRENCE OF THE CHANGES DESCRIBED

1. Solutions frozen into evanescent spherulites.
The temperatures at which solutions
frozen into evanescent spherulites (Fig. 2-\) turn opaque (Fig. 2-2) on being rewarmed
are only slightly affected by the solute concentration and are greatly dependent on thenature of the solute; they are apparently related to its molecular weight. Thus aIM
sucrose solution turns opaque at -31.4°C, and a 2 M at -31.8°C, whereas the temperature of the change for glucose (2 M) is -40.6°C, and that for raffinose (1 M) is -27.2°CcTable 1.

Temperatures at which thin layers of aqueous solutions of concentrations
between 25 and 50%, rapidly cooled, become intensely opaque in one minute
upon being rewarmed

SoluteTemp.
°C

SoluteTemp.
°C

SoluteTemp.
°C

Hemoglobin-3.5

Gelatin*-11.5

Dextrose-38

Soluble Starch-5.0

Polivinylpyrrolidone-14.5

Glycerol-58

Albumin-5.3

Raffinose-25.4

Ethylene Glycol-63

Dextin-9.9

Sucrose-30.5

Acetamide-65

Formaldehyde-72* A thin layer of gelatin solution turns intensely opaque in one minute at -11.5°C, and in a
few seconds at -lOoC. This last figure will be the one generally used in the present paper

60

B. LUYET(for details, see Luyet, 1939).
I summarize in Table 1 the essentials of the information now available on the temperatures at which thin layers of solutions, of concentrations between 25 and 50%, of
solutes of a wide range of molecular weights, turn opaque in one minute. For convenience, I will tentatively designate the figures tabulated as "recrystallization temperatures".
2. Solutions frozen into irregular dendrites.
These crystallization units (for instance,
the rosette shown in Fig. 2-3) turn opaque (Fig. 2-4) at the same temperature as theevanescent spherulites.
3. Solutions turning ojJaque durmg cooling.
The dark borders of the spherulites
are wider when the temperature of the freezing bath is closer to the "recrystallization
temperature". They may become so wide that they occupy the entire spherulitic areas,that is, the entire preparation becomes opaque. This gradual increase in opaque areas
is illustrated in Fig. 6 in three cases in which the freezing bath temperature was -40,
-30 and -25°C (I, 2 and 3, respectively).

Fig. 6.
Decrease in transparent areas (white areas in photographs) in preparation 100 micra
thick of a 300/0 gelatin gel, when the freezing temperature approached the "recrystallization temperature". -Freezing bath temperatures: I, _40DC; 2, -30°C; 3, -25°C.
x85 (from Luyet and Rapatz, 1958; with permission of Biodynamica)

The temperatures at which the speci4. Solutions subjected to various treatments.
mens subjected to the various treatments reported in Fig. 3 turn opaque, are indicatedon the top of the columns. One should note that gelatin gels of different concentrations,cooled rapidly, become opaque when rewarmed to two different temperatures, -10 and-30°C. (The temperatures at which the changes took place in one minute were actually-11.5 and -37°C; to produce the changes in a shorter time, we set the baths at,
respectively, -10 and -30°C; see Gehenio and Luyet, 1959.)
5. Preparations acquiring slowly a coarser structure.
The transformation describedin Fig. 4 begins to take place when the slowly rising temperatures reach the values
given in Table 1 (that is, the values at which transparent spherulites turn opaque); thenit continues at an accelerated pace in the range between those values and the melting

RECRYST ALLIZA TION OF ICE

61

Fig. 7.
Changes produced in frozen gelatin gels by short exposures at or above the "recrystallization temperatures", and absence of change after long exposures to temperatures a few degrees below. x60. I: Preparation consisting of a 30% gelatin gel, frozen
at -40°C into transparent spherulites (white areas with dark borders). 2: Same field as
in I, unchanged after a 5-hour exposure to -15°C. 3: Same field, after the spherulites
have turned opaque, following a one-minute exposure to -10°C. 4: Preparation consisting of 10% gelatin, frozen at -20°C into irregular dendrites. 5: Same field as in 4,
unchanged after a 5-hour exposure to -15°C. 6: Same field, after the preparation
has turned granular, following a I-to-2-minute exposure to -5°C

point. Figure 7 il1!.lstrates the facts (1) that, a few degrees below the temperature of
the onset of opacity, transparent spherulites remain transparent for hours, while, abovethat temperature, they turn opaque in times of the order of seconds (I, 2 and 3 ofFig. 7), and (2) that crystallization units of the type irregular dendrites maintain theirfine-grain structure for hours a few degrees below the same temperatures, and acquire acoarser grain in a relatively short time above those temperatures (4, 5 and 6 of Fig. 7).
6. Crystal growth near the melting point.
The phenomena described in Fig. 5 takeplace one tenth, or a few tenths of a degree below the melting point.

7. Transition from cubic to hexagonal ice.
The temperature at which the transition
is reported to have occurred in pure water varies from - 80 to -100 o e (Blackman andLisgarten, 1957; these authors survey the earlier literature on the subject). AccordingB. LUYET

62

to Dowell and Rinfret (1960), the rate of the transformation IS related exponentially tothe temperature; what is done in one and a half hour at -soae would take about 17
hours at -lOOoe.

III. Discussion
I shall examme first a process which seems to take place in most of the changes
reported, "migratory recrystallization". Then, I will consider separately each of the sevenseries of changes listed in the first part of this lecture.
1.

MIGRATORY RECRYSTALLIZATION

The first and outstanding change reported, the turning opaque of transparent spherulites (Series 1 in the list), has been shown to result from a transformation of invisible
ice particles or spears (too small to be seen) into particles of the order of a few micra,

Fig, 8.

Structure of the opacity cloud and changes that it undergoes at rising temperatures.
Portion of an evanescent spherulite (the white area) formed in the freezing. at -35°C,
of a 30% gelatin gel; parts of the opaque borders are shown at left. 2: Area represented in I, after the temperature had been raised to -DOC. 3: IVlagnified view of the
opacity cloud formed in the central region of 2. 4: Same area as in 3, after the temperature had been raised to - 3°C. Magn.: 1 and 2, X 100: 3 and 4, X 990 (from thefiles of Luyet and Eapatz)
1:

RECRYST ALLIZA TION OF ICE

63

which make up the opacity clouds. This is illustrated in Fig. 8, where Phot. I shows
an evanescent spherulite in the transparent state (white area), Fig. 8-2 the same field
after the formation of the opacity cloud (the dense central area), and Fig. 8-3 a portionof the cloud at a higher magnification. Thus, the phenomenon as a whole consists ina passage from a large agglomeration of small particles into a smaller agglomeration oflarger particles. The turning opaque of slowly grown units, such as that shown in 3
and 4 of Fig. 2 (Series 2), and the darkening of the borders of the spherulites duringcooling (Series 3) shown in I, 5 and 6 of Fig. 2 involve probably a change of the same
nature and of the same order of magnitude. The coarsening of the grain and the sub~sequent formation of plates of ice represented in Fig. 4, and the increase in size anddecrease in number of particles when the temperature is raised in the specimen shownin 3 and 4 of Fig. 8 (Series 5) are themselves direct illustrations of the passage froma large number of small parti~les to a smaller number of large blocks; and even more
so is the change occurring at the melting point, illustrated in I and 2 of Fig. 5 (Series
6). Although the transformation seems to be of the same general nature in these fiveseries of events, the particles are of quite different orders of magnitude, as shown in
the photographs cited.
What all these cases apparently have in common is the instability of particles of agiven size at a given temperature. The smaller particles having a higher energy content
become unstable at rising temperatures. The process which has important applications
III metallurgy, is designated by the professionals in that field as "recrystallization" andis described graphically as follows: "a few of the crystals slowly grow larger at theexpense of many smaller crystals" (Holden, 1963). Specifically, in the case of ice, thehigher vapor pressure at the surface of smaller particles causes the migration of themolecules from them to the larger ones. Dorsey (1948, p. 412), speaking of ice, called
this phenomenon, "migratory recrystallization". Although it may not give the final andcomplete answers to our questions, migratory recrystallization seems to play a majorrole in the changes reported.
II.

1.

ANALYSIS OF THE VARIOUS CHANGES REPORTED

The turning opaque, upon rewarming, of rapidly grown spherulitic structures

In the case of evanescent spherulites rewarmed after rapid freezing, other phenomena
than migratory recrystallization probably take place. A study of the changes in volume
undergone by solutions of polyvinyl pyrrolidone of various concentrations, in the courseof rapid cooling followed by rewarming, indicated that an appreciable quantity of water
was prevented from crystallizing during cooling, and crystallized during rewarming. The
findings are plotted in Fig. 9. The percent by weight of the solution which was changed
into ice during cooling, Ie, is the amount below the curve connecting the triangularmarks; the percent which was left in the amorphous state after rapid freezing and which
crystallized upon rewarming, IR' is the amount between that curve and the one connecting
the circular marks. The data reported in Fig. 9 indicate (1) that a part of the water
does not crystallize during the rapid freezing, and (2) that it does crystallize during the
rewarming, when the temperature reaches the "recrystallization" point.
The opacity cloud resulting from the growth of ice particles to visible size may,64

B. LUYET
100
90
80
(J)

PVP

!z70
w
Z

~60
~

0

<.)

50
lJ...

0

w40
<!)

j:!

i5 30
<.)

0::
W

a.. 20
10
0
0

10

20

30

%

40

50

60

70

80

90

100

CONCENTRATION

Fig. 9. Diagram showing the percent by weight (1) of the ice formed during rapid cooling,Ie, (2) of the ice formed during rewarming IR' (3) of nonfrozen water W, and (4) of solute(PVP), in solutions of the initial concentrations given in abscissa (from a paper in press
by Luyet, Vehse and Gehenio; with permission of Biodynamica)

thus, be formed in three ways: (1) preexisting crystals may grow larger at the expenseof the material left amorphous in the previous rapid freezing; the process would thenbe a resumption of an interrupted crystallization; (2) new crystals may originate in theamorphous material and grow at its expenses; the process would then be a true devitrification; (3) the larger of the previously formed crystals may grow further at the expenseof the· small ones; which is the process described as migratory recrystallization.
Using the term recrystallization in its broad sense, which includes the resumptionof an interrupted crystallization, I designated as "irruptive recrystallization" the abruptchange in opacity of the sort described here, occurring upon rewarming, in solutions
which had undergone a previous crystallization.

2.

The turning opaque, upon rewarming, of slowly grown irregular dendrites

The changes of the type represented in 3 and 4 of Fig. 2, which are very similar
to those represented in land 2 of Fig. 2, are probably explainable on the same basis,
although we have no evidence that some material remains amorphous after the relatively,
slow crystallization of the water into irregular dendrites (rosettes in this case).
Like in the preceding series (under No.1) the change would be an "irruptive recrystallizatz"on" .

3.

The turning opaque, during cooling, of the borders of spherulitic structures

Apparently, the heat of crystallization released while the spherulite is formed (thatis, during rapid cooling) is enough, mostly along the borderlines of two or three adjacentRECRYSTALLIZATION OF ICE

65units where growing spears of ice converge, to raise the temperature locally to the point
at which the opacity clouds develop (the temperatures given in Table 1).
To test this interpretation, we froze layers of 30% gelatin gels at temperatures 30,
20 and 15 degrees below that at which the opacity cloud would appear upon rewarming,
and we observed that the dark bands between adjacent spherulites were broader at higher
temper'atures, until, at the highest temperature mentioned (15 degrees below that of recrystallization, at -25°C) the entire preparation turned opaque. This gradation is shownin the three photographs of Fig. 6 (Luyet and Rapatz, 1958).
Whereas the turning opaque of preparations described in Series 1 and 2 was "induced"
by rewarming, that described here, which takes place during cooling, appears to be aself-induced phenomenon and is called "spontaneous irruptive recrystallization".

4.

Miscellaneous cases in which specimens turn opaque under various conditions

Let ~s now examine the 14 cases of changes undergone by specimens treatedin various ways, as stated in the description of the changes of Series 4. The 14 cases(12 of which are represented in Fig. 3) were classified into three categories, dependingon whether the preparations (a) turned entirely opaque, (b) turned partly opaque, or (c)remained transparent.
Cases 1 and 2: Solutions of(a) Preparations turning entirely opaque (7 cases).
35 to 40%, and of 45 to 50% concentrations, exposed to -100°C, turning opaque whenrewarmed to -10°C (Columns 3 and 5, Rows 1 to 3 and 5 to 6 of Fig. 3). These cases
are interpreted as being typical representatives of irruptive recrystallization, induced by
the rewarming to -lOoC, of spherulites grown at -100°C. The spherulites, well identifiable in polarized light, at lower concentrations, become so numerous and so small from38 to 45% that they can no longer be recognized as such by that criterion. But their
behavior is practically the same as that of spherulites.
Case 3: Rosettes or balls, slowly grown at -30°C, turning opaque when rewarmed
to -lOoC (Columns 2 and 6)'. The process is classified as irruptive recrystallization ofslowly grown units (rosettes or balls).

Cases 4 and 7.-Case 4: A 42% solution turning opaque during cooling to -100°C(Column 3, Row 4). The change is interpreted as being a spontaneous irruptive recrystallization. The dark borders of the numerous spherulites apparently cover the entire- preparation and make it solidly black.-Case 7: a 30% solution, turning opaque duringcooling to -40, -30 or -25°C (illustrated in Fig. 6). The change is classified in thesame category as that of Case 4, though only a few large spherulites were involved in
the spontaneous recrystallization.
Case 5: A 55% gel, exposed first to -lOO°C, becoming opaque when rewarmed to-30°C (Fig. 3, Column 4, Row 8). This case probably belongs to a quite different category. The fact that the preparation turns opaque at -30°C, while those of 35 to 50%
concentration did not change at that temperature, suggests a different type of crystallization units. The fact, furthermore, that, at high concentrations and low temperatures,the rate of crystal growth becomes very low suggests that the crystallization units, which
may be balls, have to be warmed to -30°C to grow sufficiently to become visible in an
appreciable time. If so, the process leading to the formation of opaque preparations

66

B. LUYET

would be a mere acceleration of the growth process. Whereas, at concentrations. from35 to 50%, the spherulites could grow fully at -100°C until their growth was stopped byneighbour spherulites, thus leaving no unoccupied territory, the balls formed at -lOO°C
in 55% gels grew only to a certain diameter, leaving unoccupied territory between them.
In the former case, resumption of growth (at -lO°C) would be at the expense of interradial material within the spherulites, in the latter (at -30°C), at the expense of thematerial in the unoccupied territory, between the balls.
Case 6: A 53% preparation, exposed to -100°C, then to -30u C, then to -10°C,
becoming completely opaque at this latter temperature (Column 5, Row 7). The crystallization units, assumed to be of the type balls, would grow to a limited extent at-30°C, rendering the preparation quite opaque (Column 4, Row 7), and these would undergo an irruptive recrystallization (of slowly grown units, the balls), rendering the fieldentirely opaque (Column 5, Row 7).
(b) Preparations turning partly opaque (4 cases).
Caces 1 and 2: Solutions of
35 to 40% and of 45% concentrations, turning partly opaque when cooled to -100°C(Column 3, Rows 1, 2, 3 and 5 of Fig. 3). The partial opacity seems attributable tothe darkening of the borders of the spherulites (spontaneous irruptive recrystallization).
The gradual increase in opacity from 35 to 42% would be due to a gradual increase in
the number of spherulites; at 45% their number would still be higher than at 42%, but,on account of the corresponding decrease in size, many of them would be too small to
be seen.
Cases 3 and 4: Solutions of 53% and of 60 to 65% concentrations, exposed to-lOO°C, turning partly opaque when rewarmed to -30°C (Column 4, Rows 7, 9 and 10),and turning more opaque when rewarmed to -lOoC (Column 5, Rows 9 and 10). The
partial opacity acquired at -30°C is probably due to a resumption of growth by crystallization units of the type balls which were formed at -lOO°C but could not grow atthat temperature and remained invisible. The degree of. opacity is apparently related
to the number of units which could reach visible size. Upon a further rewarming to-lOoC, the crystallization units darken (irruptive recrystallization of slowly grown units)
but the background, never nucleated, remains transparent.
(c) Preparations remaining transparent (3 cases).
Case 1: A 50% solution cooled
rapidly to -100°C and rewarmed to -30°C (Columns 3 and 4, Row 6). The preparations
exposed to -100°C would be filled with fully grown spherulites, too small to be seen;
at -30°C nothing would happen since the water between the spherulites is already frozen;
at -10°C the preparations would turn black as a result of recrystallization and/or of thecrystallization of the water left unfrozen within the spherulites.
Case 2: Solutions of 53 to 65% concentration cooled to -lOO°C (Column 3, Rows
7 to 10). The preparations would contain quite large numbers of crystallization unitsof the type balls which grow too slowly at -100°C to become visible, but are able togrow to a certain degree when the temperature is raised to -30°C (in the 55% gel, theywould be just of the number and size to fill the preparation and render it entirely opaque).
The spaces between the balls, which were never nucleated, would remain transparent
even at -10°C.
Case 8: Solutions of concentrations higher than 65% (not represented in Fig. 3).

RECRYSTALLIZATION OF ICE

67

The material would remain entirely transparent, as no structural change would take placem it, no matte:r:: what treatment it received.
The processes encountered in the "many cases of Series 4 which have been reviewed
are: irruptive recrystallization, induced or spontaneous, of rapidly or of slowly grownunits and resumption of an interrupted crystallization of either the type spherulites or
the type balls of crystallization units.

5. Slow coarsening of the structure at gradually .rising temperatures
The changes reported in Fig. 4-2, 3 and 4, and in Fig. 7-6 would be accountableprimarily by the process described as migratory recrystallization. The change would
begin, upon rewarming, at the temperature that I designated as "recrystallization temperature",(of which I listed a few in Table 1), it would proceed at an accelerated paceat higher temperatures and become precipitous near the melting point. To distinguish
this process, which is relatively slow over the major part of its range, from the irruptive
process discussed above, I called it "slow-pace recrystallization".

"6.

Crystal growth taking place near the melting point

The growth of large crystals at the expense of small ones, near the melting point,
illustrated in Fig. 5, is a clear case of migratory recrystallization. To distinguish itfrom other recrystallization phenomena which may be fundamentally of the same nature,but occur at djfferent temperatures, I designated it as "premelting recrystallization".
The passage from spheroidal to polyhedral, or from polyhedral to spheroidal forms
at different temperatures, so well illustrated in the motion picture film by MacKenzie
and Rapatz (1966) is probably attributable, to a considerable extent, to the differences inheat transfer due to the different geometrical configuration of corners and rounded surfaces, as analyzed by Vogel (1921).
I wish to note that migratory recrystallization may tell only a part of the story.
The multiple processes that I assemble under the name of premelting recrystallization
have aparently several characters in common with those reported by others, under various
denominations, such as, regelation, coalescence, sintering, ripening, formation of colloidal
ice, formation of quasifluid interfacial film on ice crystals (cf, in particular, Barnes, 1928,
p. 7; Jellinek, 1959, 1961; Kingery, 1960; Nielsen, 1964).

7.

Changes interpreted as indicative of a reorientation of the crystalline structure

The transition from cubic to hexagonal ice at rising temperatures, reported by several
investigators in the case of pure water, has been claimed to take place also in solutions,in particular, in a 50% solution of gelatin (Dowell, Moline and Rinfret, 1962). Luyet,
Tanner and Rapatz (1962) who also investigated, by X-ray diffraction, the same solution,
proposed a different interpretation of the data. But, since this problem is to be examinedin the other paper (Luyet, 1967), I will not discuss it any further here. At present, weconsider "reorientative recrystallization" in solutions as a possibility.

IV. Supplementary Notes
Recent investigations revealed some new aspects of the recrystallization phenomenonor suggested new approaches to its study. I shaH mention here briefly three suchinvestigations.

68

B. LUYET(1) The method of differential thermal analysis permitted us to distinguish two steps
the process, in the case of a 45% solution of glycerol: (a) a resumption of an interrupted crystallization at about -112°C and (b) a recrystallization proper around -60°C(Luyet, Rasmussen and Kroener, 1966).
(2) Determinations of the dielectric constant of a solution of 38.4% PVP, at various
temperatures, suggested that the recrystallization temperature may correspond to an incipient melting (Amrhein and Luyet, 1966);(3) Studies with the freeze-drying microscope (developed by Dr. MacKenzie, 1964)of the changes occurring during the sublimation of the ice in frozen solutions, indicateda "collapse" of the structure when the temperature of the specimen reached the recrystallization point (MacKenzie and Luyet, 1965).
The first two of these investigations were presented in some details in the otherpaper (Luyet, 1967 a) in this volume; the third has been briefly outlined in a paper in theother volume of the Proceedings of Sapporo Conference (Luyet, 1967 b); the reader willfind more information about it in a survey paper by Dr. MacKenzie (1965).

In

v. General Conclusion
Altogether, in this survey, we encountered two main processes: (1) a passage fromcrystalline particles of one size into those of another, and (2) a resumption, during
rewarming, of a crystallization interrupted during cooling. There is also the possibilityof a direct passage from the amorphous to the crystalline state in the absence of preexisting crystallites, a true devitrification, although we have no evidence that such a
passage does actually occur in solutions. In fact, the only solutions in which we could
prevent the formation of crystalline ice were those of very high concentrations (generally
above 60%) and, in those, no change took place upon rewarming.
The first of the two processes mentioned is a recrystallization in the proper senseof the word; the second could be called a recrystallization in the sense that it is a resumption of crystallization.
Accepting that broad definition, I will summarize the analysis of the changes observedor reported by distinguishing four types of recrystallization: irruptive, slow-pace, premelting and reorientative, and by subdistinguishing four kinds of the type irruptive:
induced or spontaneous, affecting rapidly frozen or slowly frozen ice formations, as
shown in Table 2.
Table 2.

Types of recrystallization(A)

Irruptive(a) Induced or spontaneous(b) After rapid or after slow freezing(B) Slow-pace(C) Premelting(D) Reorientative

A process which may playa predominant Tole in each of the first three types (A,B and C) is "migratory recrystallization", which may be defined summarily as the growthof large crystalline particles at the expense of smaller ones.

RECRYSTALLIZATION OF ICE

69

Acknowledgments
I ·wish to express here my gratitude to my colleague Dr. G. Rapatz whom I consulted
on some of the data reported in this paper and who supplied or assembled most of thephotographs.

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