JOURNAL OF COSMETIC SCIENCE 338 oven (Philip Harris Ltd, Shenstone, England) at 60°C for 72 h followed by milling and sieving through a 1-mm sieve to produce “dry powder” grade or by autoclaving the mud for 15 min at 121°C to produce “sterilized mud” grade. Two formulas (K10 and K5B5G) were prepared using the treated mud (i.e., both dry powder and sterilized mud grades). These samples were evaluated by the same procedures and at the same stability conditions parallel to untreated mud formulations. STABILITY STUDY Formulations were stored in plastic jars under the following conditions and testing performed at the specifi ed time points: —Room temperature 25°C analysis after the 14th and 28th days of storage. —Elevated temperature 45°C analysis after the 14th and 28th days of storage. —Freeze–thaw cycling -20°C/24 h and room temperature/24 h analysis after two temperature cycles. The formulas were tested to provide a baseline value after 24 h of preparation to ensure complete hydration of the formulation. Samples stored at 45°C were left for 2 h at room temperature before testing and room temperature samples were considered as control samples. Samples were evaluated visually, and tested for spreadability, extrudability, percent mois- ture content (w/w), rate of drying [percent (w/w)] at 32°C, pH, separation percent (w/w), and rheological properties. Visual assessment. The visual appearance of all formulations evaluated during the study period included color, phase separation behavior, and any other distinct behavior. Spreadability. The parallel plate method (10) was used to evaluate spreading behavior using an in-house spreadability testing apparatus that consisted of two smooth wooden slides to hold the mud samples and a pan to hold a weight during measurement as illustrated in Figure 1. Figure 1. In-house spreadability testing apparatus used in the study.

PHYSICAL PROPERTIES AND STABILITY OF DEAD SEA MUD MASKS 339 Excess of the mud sample was spread on the surface of the lower slide, then the upper slide was used to cover the sample and a 100 g weight was used to compress the sample between the two slides for 2 min to obtain a layer of uniform thickness. The time (in seconds) required for the upper slide to move away to the edge of the lower one when the 100 g weight is placed on the pan was recorded and spreadability was calculated using to the following equation = × l S m t (1) where S is the spreadability (kg cm·s-1), m is the mass placed on the pan (0.1 kg), l is the length of the slide (cm), and t is the time (in seconds) required to move the upper slide (10). The test was performed in triplicates and data are presented as mean ± standard deviation (SD). Extrudability. The extrudability was evaluated using the method described by Purushothamrao et al. (11) after modifi cations. The test was performed using a clean pouch (PET/PE, 500 g capacity) with a tip opening diameter of 1 cm. The pouch was half-fi lled with the mud formulation, then for each formulation the extrudability was evaluated in triplicate by measuring the weight of mud ejected from the pouch opening on pressing with fi ngers, while holding the pouch in hands. The test was performed in triplicates and data are presented as mean ± SD. Percent moisture content (w/w). Duplicate measurements of percent moisture content (i.e., Loss on Drying percent LOD%) were performed using a moisture analyzer (Adam AMB 310 Adam Equipment, Kingston, United Kingdom). The test was performed on 1–2 g samples at 60°C until a constant weight was achieved, and the percent moisture content was calculated as percent (w/w). - initial weight final weight % moisture content= 100 initial weight × (2) The test was performed in triplicates and data are presented as mean ± SD. Drying rate at 32 °C. This test was performed to evaluate the drying behavior of different formulations under conditions mimicking the application of a mud mask onto human skin (12). Accurately weighed samples of about 1 g were spread on square pieces of aluminum foil (5 × 5 cm) using a spatula to get a uniform layer. Samples were then placed in a laboratory incubator (IN-010 Gemmy Industrial Corporation, Taipei, Taiwan) maintained at 32° ± 1°C to simulate human skin temperature and monitored for 20 min, which is the applica- tion time suggested by manufacturers of facial mud masks. Four samples were prepared from each tested formulation one sample was withdrawn and weighed using an analytical balance (Shimadzu AY120 analytical balance Nakagyo-ku, Kyoto, Japan) every 5 min up to 20 min. The drying percentage at each time point was calculated using the following equation original weight weight after incubation % drying = 100% original weight - × (3)