Soda-glazing was developed in the 1970's as a chloride-free alternative to salt-glazing. Soda-glazing involves introducing sodium carbonate or bicarbonate into the kiln at a high temperature to create soda-vapor. Various soda introduction techniques are used, including spraying a water and soda solution, dropping small amounts of sodium carbonate into the kiln, or introducing a solid plaster-like mixture made from sodium carbonate and bicarbonate, whiting and water. As with salt-glazing, the soda (Na2O) reacts with the alumina-silicate surface of the clay, creating a glaze. Despite the similarities with salt-glazing, the surface effects of soda-glazing can be quite different. Some result in a smoother prickling and interesting color variations. Pre-application of glazes is unnecessary, although underglaze decoration may be applied to great effect. Sufficient space should be left between individual pieces, so the salt vapor can circulate freely, reaching as much of the work as possible.