USEFUL PROJECTS. 



653 



and repeat this just as often as 

 you wish to procure different de- 

 grees of colour. 



Every time you take off the 

 aquafortis the plate must be wash- 

 ed twice with soft water, and then 

 set to dry as before. 



To ascertain the depth of your 

 work you should rub a small part 

 with a piece of rag dipped in tur- 

 pentine, and then apply the finger, 

 or a piece of rag rubbed on the 

 oil rubber, to the place so cleared, 

 and it will give you some idea of 

 the depth. 



The walling-wax is taken off 

 by applying a piece of lighted 

 paper to the back of the plate all 

 round the opposite parts of the 

 margin where the wax is placed : 

 then let the plate cool, and the 

 whole of the grounds, &c. will 

 easily come off by washing the 

 plate with oil of turpentine, which 

 must be used by passing a rag 

 backwards and forwards until the 

 whole dissolves : it is then to be 

 cleaned off by rags ; and care 

 must be taken that no part of the 

 turpentine is left hanging about 

 the plate. The plate should only 

 pass once through the press. 



. Directions respecting Grounds. 



No. 1. — The ground in hot 

 weather must have an additional 

 one-third of spirits of wine added 

 to it for coarse grounds to repre- 

 sent chalk ; and one-half added 

 to it for fine grounds, to represent 

 black-lead pencil ; and always to 

 be kept in a cold place in summer, 

 and a moderate warm situation in 

 winter. 



N. B.— If any parts are not bit 



strong enough, the same process 

 is to be repeated. 



Gum-water must be made in 

 the proportion of half an ounce of 

 gum-arabic to a quarter of a pint 

 of water. 



Turpentine-varnish iscomposed 

 of an ounce of black rosin to an 

 eighth part of a pint of spirits of 

 turpentine : if the weather is ex- 

 cessively warm, it ought to be 

 made with the sixth part of a pint 

 of spirits of turpentine. 



Tracing-rag should be made of 

 a piece of Irish linen, not too 

 much worn, the surface of which 

 is to be rubbed with another rag 

 dipped in sweet oil, just sufficient 

 to retain a small portion of ver- 

 milion or pounded red chalk. 

 This must be placed with the co- 

 loured part towards the ground of 

 the plate, and the drawing or 

 tracing laid upon it, which must 

 be traced very lightly with a blunt 

 point or needle. 



Method of preparing Ox-gall in a 

 concentrated state, for Painters, 

 and for other uses. By Mr. 

 Cathery.* 



[From the Transactions of the Society of 

 Arts, &c. Vol. LVIII, for 1810.] 



It has long been a desideratum 

 to find out a method of preparing 

 ox-gall for the use of painters, so 

 as to avoid the disagreeable smell 

 which it contracts by keeping in a 

 liquid state ; and at the same time 

 to preserve its useful properties. 

 I have invented a method of doing 



The Society voted ten guineas to Mr. Cathery for this communication. 



