A mixture that is close to the proper viscosity will start to run but has sufficient body finally to flow in a series of waves. A more accurate test, which can be applied repeatedly, is by means of glass viscosity tubes. If these are not available, a halfdozen test tubes of identical size (about in. X 4 to 5 in.) will prove satisfactory. Corks of identical size should also be procured. Each tube is filled with the liquid under test to within approximately У& in. of the end of the cork, which has been previously fitted and marked so that all are inserted to the same depth. When the corked tube is inverted, the air space under the cork becomes a bubble, which travels to the upper (closed) end of the tube. The time it takes the bubble to traverse the length of the tube is the measure of the viscosity of the liquid. In order to obtain a sealed sample for measuring unknown mixtures, a series of test patterns must be sprayed on wrapping paper or scrap material. The cup is filled halffull of a mixed liquid that is known to be too heavy, and its pattern sprayed. Small amounts of thinner are added successively and a pattern sprayed for each until a pattern finally registers the proper viscosity. A sample of the correct liquid is sealed in a viscosity tube as the "master" or "pilot" sample for future mixtures employing this type of solvent. Cleaning. Suctionfeed guns are a pleasure to clean. All that is necessary is to unscrew the gun from the cup, stop up the nozzle with the fingers, and pull the trigger to force out the remaining fluid. If the fluid tube is inserted in a container of solvent and the trigger squeezed several times as in Figure 5.7, the gun will clean itself. Fig. 5.7. Cleaning the spray gun. Pressurefeed guns are cleaned in the same way except for the fact that the solvent is poured into the metal, pressureresistant cup, which must then be screwed tightly in place before compressed air is triggered in. This light cleaning should be augmented by a periodic dismantling of the air cap, fluid tip, and the needle valve with its control needle, and immersing them FURNITURE FINISHING 315 in solvent. Under no circumstances should solvent be allowed to come into contact with the oil or packings of the working parts. Summary. In conclusion it should be borne in mind that in finishing fine furniture or cabinetwork, cleanliness is next in importance to a craftsmanlike finish. In the first place, the work itself must be so clean that no wood or sand dust remain to adulterate and discolor the finish to be applied. For the same reason brushes or spray guns that apply the finish should be immaculate. Finally, maximum efforts should be exerted to eliminate from the air dust whose settling would mar an otherwise perfect finish. furniture finishing technique In addition to cleanliness of the surface, tools and air and repeated stirring of the finishing fluid, following a methodical procedure in enameling or painting furniture will obviate considerable unsatisfactory retouching.