all hot surfaces of the hot plate 70 as well as the inner flask holder 2 and also provides gripping surface regions completely around the circumference 32 of the reaction block the shield has a thick annular portion with an upper surface 38 that extends inward to a cylindrical inner surface 60 that is closely spaced outwardly from the outside of cylindrical holder element 12 hence shield outer circumference 32 meets upper surface 38 at a chamfered corner and is backed by a thick annular ring of solid ptfe resin that mass further acts to retard heat transfer away from the hot flask holder 2 or the hot plate 70 and towards the circumference 32 that surrounding thick annular layer of insulating material is quite rigid and allows a very stable and continuous gripping surface region to be defined by round grooves 34 36 which act as secure finger holding surfaces that are uniform and run completely around the circumference fig5 further illustrates an intended use of the assembled reaction block upon a top surface of a conventional laboratory magnetic stirrer and hotplate 62 the shape of a round hot plate upper surface 70 being about 135 mm in diameter is schematically represented by dotted line the flat lower surface 80 of the thin wall circular horizontal element 22 is about 138 mm in diameter and will engage over and around the flat area and edge of hot plate upper surface 70 the reaction block will be located against sliding by the holder vertical element 34 which in turn is covered by.