A composite pigment, which consists of a colourant and a substrate, using a substrate (S) and a colourant (C), which are each coated with ions or ionisable compounds having a charge rotating in the same direction, and, if desired, of additional ions or ionisable compounds having a change rotating in the same direction as layer material (L), wherein either (I) the sign of the change of the coating of S, or the sign of the charge of the coating of S and of the charge of L, which is the same, is opposite to that of charge of the coating of C, or (II) the sign of charges of the coatings of S and C is the same and is opposite to that of the charge of L.

Adsorbent particles comprising superparamagnetic and/or low Curie Temperature transition metal-containing cores surrounded by a hydrous siliceous oxide coating can be formed by an aqueous process wherein the core is precipitated from an aqueous solution and a siliceous oxide coating is deposited thereon while complete drying of the core is avoided until after the siliceous oxide is deposited. The resulting siliceous adsorbents exhibit strong superparamagnetic and/or low Curie temperature magnetic properties with low transition metal leachability.

Magnetic particles are prepared containing a magnetic core coated with a glass layer having a substantially pore-free class surface or having pores with a diameter offices than 10 nm. The particles are used for separating biological material such as nucleic acids. A preferred process of preparing the particles is by forming a mixture of magnetic cores with a sol formed from an alcohol and a metal alkoxide, spray-drying the mixture to coat the cores with a layer of gelled sol, and heating the coated cores to obtain the magnetic glass particles. Preferably, the particles have an average particle size of less than 100 .mu.m. The magnetic core may be a composite material containing a mica core and magnetite particles immobilized on the mica core, and the glass layer may contain boron oxide. Magnetic core materials include magnetite (Fe.sub.3 O.sub.4) and Fe.sub.2 O.sub.3.

Magnetic particles are prepared containing a magnetic core coated with a glass layer having a substantially pore-free glass surface or having pores with a diameter of less than 10 nm. The particles are used for separating biological material such as nucleic acids. A preferred process of preparing the particles is by forming a mixture of magnetic cores with a sol formed from an alcohol and a metal alkoxide, spray-drying the mixture to coat the cores with a layer of gelled sol, and heating the coated cores to obtain the magnetic glass particles. Preferably, the particles have an average particle size of less than 100 .mu.m. The magnetic core may be a composite material containing a mica core and magnetite particles immobilized on the mica core, and the glass layer may contain boron oxide. Magnetic core materials include magnetite (Fe.sub.3O.sub.4) and Fe.sub.2O.sub.3.