18 J6.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



149 



PAINTING ON GLASS. 



There are tliree kinds of paintings on glass ; paintings will) (he diflVrent 

 •oloiirs on separate pieces of glass, painting on uncoloured glass, and 

 painting on crystal. The first two methods are frequently combined so as 

 to constitute a fourth kind of painting on glass. The first kind of painting 

 is inconlestably the most ancient. Glass is prepared in sheets, blue, 

 »iolet, jellow, green, and red, and after being divided into pieces of the 

 proper size and shape, the separate portions are put together by glaziers' 

 lesd. 



The preparation of purple glass has fallen into such disuse, that till very 

 recently, the art was considered to be entirely lost, but this is not the case, 

 for there stdl exist printed receipts which describe all llie details of the 

 operation. Haptiste Porta, wlio was born in the year 1540, has given one 

 of the receipts in his Magie Naturelle, and he has taken care at the same 

 time to warn us of the dilHcully of obtaining a successful result. Other 

 receipts are found in the compilations .of Neri, Merrel, and Kunckel, 

 »nd have been transferred to the encyclopa;dia. No information, however 

 is given respecting red glass. It is not prepared with the purple of gold, 

 for this substance gives neither ascarlet red nor the red, of clear wine: in- 

 stead of osyde of iron, the protoxyde of copper is used. 15ut as this last 

 produces an exceedingly deep colour which deprives the glass of its trans- 

 parency, the usual plan is to cover white glass with a thiu layer of red 

 j^lass, so as to form a kind of plated glass. The process is as follows — 

 there are placed in the furnace two crucibles, of which one contains com- 

 mon glass, the other glass of ihe same composition, but coloured with pro- 

 toxyde of copper, to which is added protoxyde of tin. This last body tends 

 to prevent the oxydation of the protoxyde of copper ivhich would have the 

 eflect of cultuiring the glass gieen. A small addition of protoxyde of iron 

 gives a scarlet red or flame colour. If the glass take a greenish tint a little 

 bi-tartaiate of potash will renew the colours by restoring the bi-ox\de of 

 copper to the state of a protoxyde. The workman commences by taking 

 (m his •' blowing-iron" a small quantity of red glass ; he then plunges the 

 tube into the white glass, of which he takes a much largerquantily, and he 

 (hen blows it out accordiug to the ordinary method of making " tables" of 

 crown glass. This n;eibod was employed for the ancient glass of church 

 windows ; at the present day this glass is manufactured at Hoirniungsthal, 

 in Silesia, by the Tyne Company in England, by Bonteinps in France, and 

 at Besan^on, 



The glass, as has been said, is cut up into coloured plates. The tints 

 and half tmts are applied by means of coloured enamels on one face or the 

 other of the glass, which is exposed to heal, and the different pieces are 

 joined by glaziers' lead according to the pattern or d^sign. If the paintings 

 be small, and designed to be viewed close, plated glass, and not glass 

 coloured throughout its thickness, is employed. Parts of the coloured 

 layer are removed at the requisite places, and on the while glass thus laid 

 bare, the colours required for the painting are applied. In this way de- 

 signs are obtained of which the colours differ altogether from the ground- 

 colour. Instead of removing the coloured layer by mechanical means, it 

 may be destroyed by fluoric acid. 



The effect of the wea'her insensibly alters the colours of ancient paint- 

 ings on glass. 



Painting on glass properly so called, that is to say, the application of 

 coloured enamels to uncoloured sheets of glass was little known to the an- 

 cient artists, and it is only in our own day that the progress of cheiuistry 

 has advanced this art to auy degree of perfection. 



Painting on uncoloured glass was executed in 1800 by Dihl ; it consists 

 in tracing the same design on two sheets of plain glass, which are sub- 

 mitted to the action of lire, and then the faces on which the designs are 

 drawn are laid one upon the other. 



To fix by heat the colours on glass without altering its form, or fusing it, 

 it is necessary to add vitreous matters, which are readily fusible, fluxes, 

 which vary according to the nature of the colours. 



Silicate of lead is employed with or without borax, minium and very fine 

 land are fused together, and different proportious of calcined silex and 

 quartz. For instance, take 



. 3 parts or Borax calcined 

 . y ,, Quartz 



1^ „ Alinium 



The quantity of flux required for each colour, so that it may have the 

 required fusibility aud clearness is very variable, the necessary proportion 

 is in general three or four parts. All colours are not adapted for the same 

 flux ; the purple of gold, the blue of cobalt, require an alkaline flux ; the 

 minium injures iliese substances, while other deep colours are not injured 

 by fiuxi-s into which lead enters. 



Some substances require to be vitrified with the flux proper to them, be- 

 fore they can be employed in painting, as the feeble heat to which they are 

 •ubsequently subject is not sufficient to develop the colour properly. The 

 deutoxyde of copper, and the yellows, blues and violets, are among these 

 »ub^^ances. \\ lib purple of gold aiidoxyde of iron on the contrary, great 

 precautions are necessary to prevent the injury of the colour by too great 

 beat. The coloured enamels when prepared are reduced to powder, and 

 preserved from the action of moisture. 



. All kinds of glass are nut suitable for painting. Fxcess of alkali is 

 destructive ; preference is therefore given to the hardest glass, which con- 

 tains a great deal of silex, aud which dues not attract muitture, as the bo- 

 keuiiau glass for lustance. 



Quartz 

 Alinium 

 Burax calcined 



d parts. 

 3 „ 

 1 .. 



Before applying the colours with the brush, they are mixed on a palette 

 with turpentine. When the painting is finished the colours are fixed by 

 heat, an operation which requires great rare and experience. Pots oi fire- 

 clay closed by a cover of the same substance are placed in a support of 

 iron, so that they can be enveloped on all sides by the flames ; the method 

 adopted in France for cooling the glass is to put it in separate furnaces 

 heated by charcoal. The plates of glass are laid one upon another on clay 

 slabs, supported on props of the same material. The heat is judged of by 

 trial-pieces, which are introduced with the rest of Ihe glass into the fur- 

 nace, and are withdrawn with a spatula. When the colours are well vitri- 

 fied, the plates are put in the annealing oven and gradually cooled. It is 

 necessary that this last operation should be conducted very gradually, to 

 ensure the permanence of the colours. 



The colour communicated to glass by the protoxyde of copper is. as has 

 been observed, too intense to be employed alone, for it causes the " metal" 

 to appear opaque of a deep brown. It is necessary for procuring a trans- 

 parent red that the glass should be extremely thin. Consequently, the 

 only means of getting red glass of a proper thickness is by covering plain 

 glass by a thin layer of red. The plated glass has the advantage of allow- 

 ing the partial removal of the red layer in order to obtain white figures or 

 add other colours. The glass of the Middle Ages shows that this method 

 was adopted by the ancients. 



In order that, when the red and white glass are blown together, they may 

 be well united and do not separate during cooling (as happened in some of 

 Engelhardt's first experiments), the " metal" of both must be the same, or 

 at least analogous. It is best to make the red a little weaker than the 

 while ; the latter must not contain any oxydising substance, which would 

 injure the red colour. 



Great care is required to avoid air bubbles in the glass. The red and 

 white must be ready at the same time, in order to work together well. The 

 beauty of the glass depends also materially on the skill of the workman, 

 for it is easy to understand that the coloured glass is always thicker near 

 the orifice of the blowing-iron than at a distance. It is on this account 

 that Ihe glass is seldom of a uniform colour, except in the middle of Ihe 

 plate: at the extremity of it the red layer is sometimes so thin that all 

 trace of colour is lost. Dr. Engelhardt has preseried several ancient 

 specimens, in which this gradation from a deep colour to a light one, has 

 been made use of in a very happy manner to produce striking effects. 

 After a certain degree of practice, the workman is able to oblain a 

 tolerably uniform colour, and Ur. Engelhardt expects to effect this object 

 completely in a gla^s manufactory where he has directed attention to this 

 particular branch of the art. 



It is sometimes necessary, when the glass has once beeii painted and the 

 colours fixed by baking, to add a second coat of painting ; and as it is then 

 necessary that the glass should be again subjected to heat, the colouring 

 matter must be lenilered so fusible by an additional proportion of flux as 

 to avoid all risk of fusing Ihe colours first painted. — Translated from tlie 

 Recue Scieyitijiqae et liulustrielle. 



CLERESTORIES IN MODERN CHURCHES. 



Christ Church, Plymouth. 



Sir, — I am really most reluctant to intrude myself again upon your 

 pages; the more so, as " Candidus" has so kindly and (of course, as / 

 think) so ably defended the principles on which I have acted. The answer, 

 however, which you have made to " Candidus" seems to require from m« 

 a word or two in explanation. In the first place, Christ Church, Plymouth, 

 isTiot l.ghted by '• clerestories only." There are windows at the east ends 

 of the aisles on the gallery floor, besides the two great east and west win- 

 dows, which literally do make the main body of the interior as " cheerful 

 as the day;" and in the darkest seats beneath the galleries, on a gloomy 

 day, you can see perfectly well. There is no part " useless" or even " in- 

 convenient," "by its darkness." The body of the church is so wide, and 

 the gallery recesses so comparatively shallow, that the "defect" which 

 you say " must exist" does not exist. Though a positive sun-beam cannot 

 " shine round a corner," it is equally certain that where there is a great 

 central reservoir (if I may use the expression) of daylight, it will expand 

 its illumination even into corners which the sun's immediate rays can never 

 reach. Again, " the extent of unbroken surface of the north and south 

 walls" is not so great as you seem to imagine. There a e recesses for 

 seats where, under other circumstances, there would have been windows; 

 and these (had our means allowed it) might have been rendered highly 

 ornate. At all events, here are admirable localities for mural monu- 

 ments. 



You are, likewise, under a misconception as to the reference made by 

 " Candidus" to the octagon of Ely Cathedral. The "lower windows," as 

 jou term them, are only "lower" than Ihe lantern, Thej are the cltrtttor^ 



