The casting platform must be level and relatively smooth. The ideal platform is a level, smoothly troweled concrete slab, preferably the floor slab of the building under construction. Smoothness is most important since the surface of the platform will be reproduced, in reverse, on the bottom surface of the casting. In addition, maximum smoothness will minimize mechanical bonding and facilitate clean cleavage on lifting. Using a floor slab as the casting platform has many advantages, both from the standpoint of handling and fabricating the panel. The earth is the most economical casting surface of all, especially if the bottom of the casing need not have a finished appearance. Retaining walls are a typical example of the type of unit that can best be precast right on the ground. If the casting surface has an interesting architectural pattern and you want to make a mirror image of it, plastic film can be used. One of the biggest factors to be considered in panel design is the method of picking up the panel since this is usually the period when strength is relatively low and the panel is subjected to its highest and most critical stresses. The point pick-up method is most popular because of its economy even though it does impose a rigorous demand for the correct location of the pick-up points. The type of rigging to be used and the size of the panel will determine the number of points to be used and how they should be spaced to avoid cracking and spalling.