46 JOURNAL OF COSMETIC SCIENCE
of these particles depends on the compos1t10n of the solids and liquid, and it will
determine the plasticity of the final product (1). Some substances, known as hydrophilic
thickeners, such as gums, can also change plasticity and alter the application character-
istics of the final product. Moreover, such substances may be useful in stabilizing the
dispersion of solids and, consequently, in preventing gradual phase separation, which is
occasionally observed during the shelf storage of clay masks (2). Clay facial masks should
be also formulated so as to prevent a slight or even complete dehydration of the
formulation under aging. Therefore, the inclusion of humectant substances, like glycerin
and propylene glycol, would avoid such dehydration (1).
Clay liquid dispersions involve an assortment of different stages including incorporation,
wetting, the break-down of particle clusters, and flocculation of the disperse particles.
Usually, the stability of the clay dispersion is affected by interactions between liquid and
particles and also by attractive interactions between solid particles (7).
Several authors have studied the effects of size, particle shape, mineralogy, and chemistry
of clays as well as the effects of pH, salt concentration, and mixing conditions on the
final dispersion properties (7-12). To our knowledge this is the first study focusing on
the physicochemical stability of clay mask preparations. Therefore, this study aimed at
predicting the physicochemical stability of clay masks under storage conditions at
different temperatures. This prediction is important in defining the components of
formulation, packing material, cosmetic forms, and preparation techniques (13).
MATERIAL AND METHODS
FORMULA TIO NS STUDIED
The clay facial mask formulations were developed with grade cosmetic raw materials
selected in agreement with technical and scientific specifications suited in relation to
particle size, microbial load, color, and odor.
The formulations were developed with kaolin (30% w/w) and montimorillonite (15%
w/w) (Alban Muller International, Vincennes, France). Formulations F2 and F3 also
contained magnesium aluminum silicate (5% w/w) (R. T. Vanderbilt Inc, Norwalk,
CT). The hydrophilic thickeners were hydroxypropyl starch phosphate (2.0% w/w)
(National Starch &Chemical Company, NJ) (Fl) and xanthan gum (0.2% w/w) (Rhodia
SA, Boulogne-Billancourt, France) (F3). Glycerin (4% w/w) and propylene glycol (4%
w/w) were used as humectants in all formulations.
As the way of preparation has a great influence on the degree of clay dispersity and thus
on the physicochemical stability of the final product, all samples were prepared in the
same way. The clays were weighed and sprinkled into distilled water (at 75 ° -80 ° C)
during continuous stirring for 30 min using a high-shear mixer (model 252-21, Quimis
Ltda, Sao Paulo, Brazil) at 8000 rev/min -1 .The hydroxypropyl starch phosphate gum
was hydrated prior to its addition to the clay dispersion. In the F3 formulation, mag-
nesium aluminum silicate and xanthan gum were blended and then sprinkled into
distilled water in the same manner. The humectants were added in sequence.

CLAY FACIAL MASKS 47
PRELIMINARY STABILITY TEST (PST)
The formulations were left at rest for 24 h before testing to ensure full water adsorption
(11,14). After this period, 5 g of each visually stable formulation (homogeneous visual
aspect) were submitted (three replicas) to centrifuge (model 208N, Fanem Ldta, Sao
Paulo, Brazil) testing. The formulations were evaluated 1000, 2000, and 3500 rev/min- 1
rotor speed, during 15 min at each velocity (15).
The formulations were classified, after centrifuge testing, according to liquid phase
separation volume: 7-10 ml (IM), intensely modified 4-6 ml (M), modified 1-3 ml
(SM), slightly modified and those without phase separation (N). Formulations classified
as normal were submitted to the accelerated stability test.
ACCELERATED ST ABILITY TEST (AST)
Yhe formulations were submitted to the stress conditions of temperature in a short
storage period. The amount of the formulations submitted to AST was 30 g, considering
that on each day of analysis there was a sample that did not return to storage conditions.
Samples were stored in polyethylene packing material. Temperatures, storage periods,
and days of analysis were (13,15):
(a) 45.0° ± 0.5°C 14 days analysis at the 1sr, 3rd ,7t\ and 14th days.
(b) -10.0° ± 0.5°C/24 h and 45.0° ± 0.5°C/24 h (temperature cycles)
12 days analysis at the 6t h and 12th days.
(c) 5.0° ± 0.5°C 14 days analysis at the 15\ 3rd ,7t\ and 14t h days.
Yhe formulations were first acclimatized for at least 1 h at room temperature
(24° ± 2.0°C) prior to testing, considering the first day of analysis (t1) as 24 h after the
preparation of the formulations.
Yhe physicochemical characteristics evaluated, on each day of analysis, were: organo-
leptic characteristics (visual aspect, color, and odor), pH value, and apparent viscosity
(13,15). The formulations were dispersed (1:10) in distilled water in order to measure
pH values. The apparent viscosity of the formulations was determined using a rotational
viscometer (model Visco Star R, Fungilab S.A.). The apparent viscosity measurements
were obtained under the following experimental conditions: 24° ± 2.0°C, 18 g samples,
TRll spindle, and rotation speed of 100 rpm. Analyses were accomplished in parallel
with a reference sample, stored at a controlled room temperature (22° ± 2°C).
RESULTS AND DISCUSSION
PRELIMINARY STABILITY TEST (PST)
The formulations presented a red color, an earthy characteristic odor, and pH values
ranging from 6.8 to 7 .1.
After centrifuge testing, the F2 formulation was intensely modified with a considerable
phase separation. Yhis instability was attributed to an insufficient viscosity of the
formulation to keep a high content of soiids dispersed. The clay types differed from each
other mainly by their mineral composition, which has an influence on the viscosity and