One problem. Two billion people rely on firewood for cooking. Woods are chopped down, depleted, overexploited and finally left to erosion. Arable land is laid waste and left arid. The consequences are frightening. The expanse of barren, arid land is growing every day with people forced to roam farther and farther from home to collect wood. Most cooking is done over a three stone op en fire - with an 80% loss of energy. It is a hazard to children, a serious fire risk and fills the lunge with acrid smoke. At the same time, cooking fuel is the largest, single element in the budget of a typical low-income family. A closed hearth could solve the problem, reduce the energy loss to 20-30% and reduce the need for wood to one third. However, the poor have no means to finance a proper cooker. One solution: A fireplace - mould - bag. The cheapest hearth can be built of clay surrounding a mould which can be used again by others in the community. Not a mould of steel, which is awkward and difficult to work with in its many parts, is expensive and liable to rust and will cause the clay to tighten and crack as it dries. But a mould of plastic could be the ideal solution. An inflatible plastic bag which will allow individual freedom in designing and constructing the hearth and its opening. Rotational moulding is the only realistic production technique for solving the problem. The plastic chosen is approximately 1.5 mm thick sort PVC with a shiny surface. Optimal softness and thickness should be tested on the finished product. The air valve is a simple bicycle valve. Advanced technique, a simple product and accessible to the poorest societies - a plastic product which will help to protect and preserve woods and nature. The use: One mould-bag is sufficient for each village. Donated or purchased. The plastic mould is unrolled and then inflated by each individual user. Transportation/ storage could be the mould rolled around a bicycle pump and placed in a plastic tube with lid - easy to transport and might safe storage. The form is placed where the hearth is to be and a 5 inch drain pipe or smaller tin pipe is attached to the roof or the wall above the conical shaped top in the one end of the mould. The hearth is shaped according to where and for whom it is built with clay, laterite or earth mixed with a little cement. The three round bulges must be kept clear from the sharp edge. If cast in cement, the mould could be filled with water and shuttered. When the clay is dry, the air is released and the mould can be removed, to be passed on to the next family. Final finishing touches and repairs can be done with fine clay in suspension, as with clay flooring. If using a single opening, the other is closed with a flat stone or a small pot.