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A ground joint is a machined or close-fitting joint that makes a tight fit without packing. In masonry, it is a structurally cohesive joint made by two blocks, usually without the use of mortar. A metal piping ground joint is one that makes a hermetic seal without the use of washers or gaskets. A glassware ground joint is made by grinding glass with sand or emery to make a tight-fitting plug for a bottle or flask. The term "ground" refers to the grinding of the material that forms the joint.
A masonry ground joint can be stronger than one held together by mortar. A famous example is the enigmatic stone walls of the ruins in Peru's Macchu Picchu, formed by rocks ground so precisely that the joint is too narrow to accommodate even a sheet of paper. This type of joint, made with little or no mortar, is more common in fine stonework than in bricklaying or rough masonry. Components of industrial ground slabs are often fitted together with ground joints because the tight-fitting, mortarless joints are are resistant to breakage, or spalling, from the heavy loads such slabs have to withstand.
Unions for joining steel, iron or copper pipes that incorporate a malleable ring of copper or brass at the joint, ground to form a hermetic seal when the union is tightened, are called ground joint unions. A union is a fitting that allows threaded pipes to be joined without disassembling and reassembling them, and it is necessary when a component, such as a meter or regulator, has to be installed in a line for gas, water or compressed air. The precisely ground ring makes a more reliable and tighter seal than a rubber or plastic washer. A ground joint union is not the same thing as a ground union, which is an electrically grounded fitting that allows static electricity to discharge and is used for propane connections.
Spherical or conical glassware ground joints make a bottle or flask gas-tight, liquid-tight or vacuum tight. They typically consist of a a ground aperture, plug, plastic sleeve and a clamp. The glass is ground with a combination of emery and oil or fine sand and water, and the sleeve typically is made of polytetrafluoroethylene (PTFE) resin. Ground joint containers have many applications in pharmacy and medicine as well as in fields of chemical and biological research.