The diatomaceous earth (DE), collected from the Mariovo region in North Macedonia, was characterized and thermally modified. The material represents a sedimentary rock of biogenic origin, soft solid that can be easily disintegrated, with white to grayish color, with bulk density of 0.51–0.55 g/cm³, total porosity of 61–63%, and specific gravity of 2.25 g/cm³. The chemical composition is as follows: SiO_2,86.03; Al₂O₃, 3.01; Fe₂O₃, 2.89; MnO, 0.06; TiO_2,0.20; CaO, 0.76; MgO, 0.28; K₂O, 0.69; Na₂O, 0.19; P₂O₅, 0.15; and loss of ignition, 5.66 (wt%). The mineralogy of the raw DE is characterized by the predominant presence of amorphous phase, followed by crystalline quartz, muscovite, kaolinite, and feldspar. Significant changes in the opal phase are observed in the 1,000–1,200°C temperature region. At 1,100°C, the entire opal underwent solid–solid transition to cristobalite. Further ramp of the temperature (1,100–1,200°C) induced formation of mullite. Scanning electron microscopy (SEM) and transmission electron microscopy depict the presence of micro- and nanostructures with pores varying from 260 to 650 nm. SEM analysis further determined morphological changes in terms of the pore diameters shrinkage to 120–250 nm in comparison to the larger pores found in the initial material. The results from this investigation improve the understanding of mechanism of silica phase transition and the relevant phase alterations that took place in DE upon calcination temperatures from 500 to 1,200°C.

In understanding the catalytic efficacy of silver (Ag⁰) and gold (Au⁰) nanoparticles (NPs) on glass-ceramic (GC) crystallization, the microstructure–machinability correlation of a SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ system is studied.

Legislative framework addresses the issues of alloy corrosion, demanding the restricted use of probable carcinogenic, mutagenic, and toxic-for-human-reproduction (CMG) metals like nickel, cobalt, and chromium and demanding the development of new biomaterials. The aim of this research was to evaluate and compare the ion release of standard dental alloys and their hypoallergenic equivalents. Six types of orthodontic alloy wires (nickel–titanium (NiTi), coated NiTi, stainless steel (SS), Ni-free SS, and cobalt–chromium (CoCr) and titanium–molybdenum (TMA) were immersed into artificial saliva of pH 5.5 and 6.6. Release of metal ions was measured by inductively coupled plasma–mass spectrometry after 3, 7, 14 and 28 days. The data were analyzed using analysis of variance, and results with p < 0.05 were considered significant. NiTi released more Ti and Ni ions compared to the coated NiTi; SS released more iron, chromium, and nickel compared to the nickel-free SS. CoCr released cobalt in a high concentration and low amounts of chromium, nickel, and molybdenum compared to the molybdenum and titanium released by TMA. Release of metals from dental orthodontic alloys in vitro was overall lower at pH 6.6 and for the hypoallergenic equivalents when compared to standard dental alloys.

Trepel is the local name for a mixture of diatomaceous earth and clay minerals. It represents a greyish, soft, very light, weakly cemented, fine biogenetic sedimentary rock. The studied material is taken from the vicinity of Bitola city (Republic of Macedonia). Here, trepel was treated up to three temperature intervals (800, 1000 and 1200C) for a period of 1 hour. The X-ray powder diffraction results indicate the presence of both an amorphous phase and the following crystalline phases: quartz, feldspars (plagioclase), mica (muscovite) and chlorites. The results of SEM analysis revealed skeletons of alga Diatomeae with nano-pores. By thermal treatment of the samples, a gradual change in color as well as higher bulk density and compressive strength was observed. The increase of the temperature, in addition, affected the mineralogical composition and increased the presence of the amorphous phase (aluminasilicate glassy phase). SEM results of the thermally investigated samples depicted morphological changes expressed by shrinkage of the pore diameters in comparison to the initial material. The major and minor constituents were established by chemical analysis revealing the following chemical composition of raw trepel: SiO₂ (63.65 wt%), Al₂O₃ (11.76 wt%), Fe₂O₃ (5.93 wt%), MnO (0.13 wt%), TiO₂ (0.63 wt%), CaO (1.42 wt%), MgO (2.22 wt%), P₂O₅ (0.11 wt%), K₂O (1.63 wt%), Na₂O (0.92 wt%), LOI (11.50 wt%).