301 Lamia T Rejab BDS, MSc (Assist Lect) Department of Prosthetic Dentistry College of Dentistry, University of Mosul Sawsan H Al–Jubori BDS, MSc (Assist Lect) ABSTRACT Gypsum products used in dentistry are in the form of α and β calcium sulfate hemihydrate as dental stone and plaster. The aim of the study is examination of the particle’s shape, size and distrib- ution of the different commercial dental gypsum products. The particle’s shape was examined microscopically with trans- mitted light microscope instead of ref- lected light microscope or electron micro- scope. Analysis of the particle’s size distribution and the measurement of the powder particle’s fineness was performed by sieving procedure using sieving machine. The data was collected and calculated and then compared with the ADA Specification No. 25 for gypsum products. The results showed that the micro- scopical examination with transmitted light microscope give clear accurate and precise image about the different shapes of the stone and plaster. The results of the analysis for particle’s size distribution showed that there is a difference in the distribution between the different tested commercial dental gypsum products and also showed that all products have par- ticle’s size larger than the ADA Speci- fication No. 25. Key Words: Gypsum products, micro- scopical examination, particle’s size. خلصة ال  بيسدتعسل في من الجبس الوعان هشاك ن  ني ولسي شان اα  ني و وβ ي اندي س لي مان وال .ستر" الدشبل ن" و "ال "الدتو  وكيملل ومجيك كي اسيف البحث هو در هد  جي بس لني وا ال ي اب لسدي حو ك الجسيو مجي توزي م وياب الجسيل كي اسي تسيل در. لتيد مختل.ي تجاري ادزوء العت مجهر الستخدام بادل من مجهير ي يد تيك ولت لقتروني السجهير اوس جو الزيوء السيشع ياس نعومي ياب وك الجسيو مجيل توزيمي اء تحل إجير متدياعيد اشخيلح وا ال اشي ماسيتخدام با السديحو  السعي تسدس يلستاي م اارنتي ها مسي ل مت تي ات الشت (  ا عاليسيADA Specification No. 25 .) يور الر صيو ووي ديات جظهيرب الشتل لذي ن" و الجي بس "الدي توي ج لشي وعسختلب الوي ا الجسي  ادييزي وء العت مجهي ر الواسي ض "بلسي تر "ال وي اب جن ك الجسي مجي ل توزي مي تحليات وجظهيرب نت  لسي اد لتجاري ا لمني وا التوزي م في هشي اك في ر  اسي الدر هيم تيك فحريها في والتي الجيبس الديش  ستلي ت ي م الني وان جس جي ات جظهيرب الشت وكي مل مجي كب السعي تسدم.ي ابي ر مي ن السيواب جاوي ا جسي (  ا عاليسد والسحيدADA Specification No. 25 .) INTRODUCTION Gypsum products used in dentistry are based chemically upon calcium sulfate hemihydrate. (1–3) Depending on the method of dehydration from natural occurring calcium sulfate dihydrate, different forms of hemihydrate can be obtained which are α– and β– forms. β–hemi-hydrate forms known as dental plaster is irregular, porous and spongy; while α– form known as Al–Rafidain Dent J Vol. 3, No. 2, 2003 Examination of the particle’s shape, size and distribution of the commercial dental gypsum products