June, 1922
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
oxide improves quite markedly the workability and enhances the whiteness to some extent, but has little effect on the resistance to weathering. The mixture of borax and phosphate is no improvement over phosphate alone. Numbers 152 and 153 proved to he quite satisfactory formulas, being equal to 29 or 87n in permanence and slightly better in workability, the ammonia-casein solution being slightly more fluid than that obtained with phosphate. The effect of formaldehyde in this mixture is noticeable only in its slight improvement in workability. Other modifications of original formulas illustrated by 164 to 160 show no improvements. Of the variations of the glue preparations, 161 to 171 and 176 to 181, Formulas 161, 162, 168, 169, 171, 176, 177, and 180 are all very, decided improvements over those of the first series. It is quite evident that formaldehyde, phosphate, ammonia, sodium salicylate, kerosene, salt, and alum each make the paint more resistant, the glue-phosphate and glueammonia mixtures being especially durable. The effect of alum on the permanence is not so marked, but it does considerably improve the working qualities of the mixture. The lighthouse formula is repeated as 172 and with the addition of formaldehyde as 173. with a substantiation of the earlier results. Formaldehyde improves this formula, but not sufficiently to justify its use in this combination. The results of variations in quantities of calrium chloride
511
(182 and 183) indicate that the proportions used in 105 were close to the optimum. In general it may be said that whitewash formulas containing a definite siccative are very much more permanent than those without such an ingredient, and that for exterior use casein and linseed oil are the best binders tried. Those which are water-soluble, such as glue, are not so good for exteriors. The addition of substances, such as formaldehyde, which harden or render the film less soluble, improve considerably the resistance of most films. Of the alkaline salts used for accelerating the solution of casein, trisodium phosphate proved to be the best, its advantage probably depending upon its mild alkalinity. Ammonia also proved to be a satisfactory substance for this purpose. Of the addition agents used with lime in the absence of sicratives, common salt proved to be somewhat more effective than the others. Tests of the best formulas described above by painting them on stones showed they were as permanent on the stone as on wood and that the order of merit was the same under both conditions. I t is planned to extend the investigations to include tests of other formulas and of the same formulas on other materials, such as plaster, brick, and metal. The development of formulas for use on interiors will also be undertaken.
T h e Manufacture of Reflecting Telescope Disks' By Donald E. Sharp and Walter H. Rising SPENCER LENS COMPANY OF BUFFALO, OPTICAL GLASSPLANT, H A M B U R G , N.
Rejecting telescope disks may be made by casting glass into a suitable mold and annealing. The annealing schedules of Adams and Williamson halre been found to be satisfactory. Electric or gas-fired furnaces may be used for obtaining the uniform temperatures necessary, but the gas furnace has been found to be the more reliable. Special equipment is required for obtaining accurate cooling rates, and the Leeds and Northrup electrical control and Robertshaw gas regulator are suitable when slightly modified. If reflector disks are made of low expansion glass the change of figure in a parabolic mirror due to change of temperature can be minimized.
K
I
the past Europe has been the only source of large telescope disks. Recently, however, several large disks have been made in the United States. Reflecting telescope disks have been made by the casting method, but, refractor disks are made by molding, for experience has not yet shown whether or not poured disks can be obtained free enough from striae to permit their use for refractors. Moreover, although sufficiently large blocks of homogeneous glass have been obtainable in this country, the principal difficulty in making refractor disks has been the lack of facilities for proper annealing. Although the theoretical requirements for annealing glass to any degree of freedom from strain were understood in the United States, their practical application was not attained until recently because the literature is devoid of reference to the methods and apparatus employed abroad in the manufacture of telescope disks. It had been supposed in this country that gas furnaces could not be used for the careful heat treat,ment necessary in the annealing of large disks; consequently, an electric furnace was designed by an American manufacturer for 1
Received March 3, 1922
Y.
this purpose. This furnace, as redesigned by the authors, has proved a success. However, results have demonstrated that a gas-fired furnace is fully as satisfactory as the electric furnace, and is simpler to control. Both furnaces are described in this paper. The procedure to be outlined here is that actually used in the manufacture of 40-in. reflector disks. Briefly, it consists of pouring molten glass from the pot into a mold, and annealing the glass by means of a specially equipped furnace. This method was devised because the pressing process, although possible for disks up to about 12 in. in diameter, is obviously impossible for those 40 in. in diameter. The annealing treatment discussed herein with reference to reflector disks is applicable also to refractor disks, the manufacture of which will be treated in a subsequent paper. For casting glass, in addition to the ordinary equipment used in an optical glass factory, apparatus for raising and tipping a pot of glass is required. The contrivances used for pouring plate glass would be ideal, but in their absence the desired results were obtained by employing a bail attached to a chain fall. The pot was held by an adjustable wrought iron ring suspended from the bail. On opposite sides of the ring, short square rods projected through the eyes of the bail. Handles, six feet in length and terminating in a cross bar, were fitted to the square rods, and provided a means for tipping the pot. Except in its simplicity, the equipment differed little from that used in casting plate glass. The mold into which the glass is to be poured should permit the disk when finished to be easily removed, and must be so made that the glass will not adhere. These requirements were met by a mold of ordinary split firebrick (9 in. X 4 . 5 in. X 1 25 in.). The bottom of the mold consisted of a heavy iron casting on which the brick were laid.
The wall consisted of brick laid inside an iron ring or hoop. The iron casting served not only as a support for t,he mold, but also as a means of obtaining oniformit,y of temperature. A layer of talc over the lining of the mold prevented t,he glass from adhering.
Fro
I - E L B C T ~ C F u a r ~ r eA S ~ I I U I N ACL OL N SV I I V C I B D . RIIBOSTAT *ND L&BDS A N D X"RT"."I CONTROLLE. IN B I C I C R O U N D A I LBPT
In order to insure the iuiiform temperature conditions that, are necessary for good annealing. a cast-iron cylinder of t,he same diameter as tlie heavy plate mentioned above was employed. The cylinder, together nit,h a cover of the same size as the bott,om plate, made it poseible, after the disk was poured, to enctLse the whole mold in iron. The higher thermal conductivity of the metal casing as compared to that, of the brick and gl'ass miniinized temperature inequalities in the disk. Inasmuch as the existence of a tempmiture gradient. during the first part of the annealing result,s in an unsymmct~ricalstrain in the finished article, it is nec,essary to take part,i(!ular pains to obtain uniiorm tamperalure conditions. The annealing schedule for a disk 40 in. in (iiarncter and 8 ill. in t,hickness calls for holding the glass 7 days at, a mistant. temperat,iire, am1 cooling aftrr thc expirat,ion of this time at an initial rate of 0 . 2 " C. per lr. Thc rate may gradrially increase, so that thc total annealing time need not greatly exceed 2 wks. Twi types of furlrace ha~-ebeen suc~:cssfiilly employed for t.his ~i-ork,at1 electric iuriiace ~:ontriillcrI I>? a standard G. E. panel, and equipped with a Lecrls and Sortlirrip recorder; and i i gas furiiace consisting oC a sliplitlj~ alterid pot arch provided with a modified Rolicrisliaw ternperatke rrisulator. The gas furnace proved :is satisfactory as the electric iirriiace, and linil the nilded idwmtngr thut it reqiiired a lcss skilled operator. It shoiikl he o b served, inoreovw, that p m c r iutcrrilpiion- may p v x e n srrioun c1is:icIv:mtaee i t i tlic i i s e of an clrrtric iiii.nace.
T11r ~ : L E r r a r i .1~r:RS.icE The herhn,eelciiieiit of the clcetric furii;ice (Fig. 1j originally coiisisted of a heavy ribtion of iiiihronic alloy 11eid 1:, porcie lain hooks. niirl \wmid u p and d o ~ mthroilghoiit the iiitrrim Tliis rlispo,sitio*iof rtory, as lient. was rge. Conieqitcntly, possible entirely to surround t,he cliarge x i t h heat try transferring n part of the resistance ribbon from tlw side walls to tlie inside oi the bot,toni and cover of tile furnace, in a nimner resembling
the spokes of a wheel (Fig. 2). To permit t,he furnace to be opened, tbe cover was designed as a separate heating element capable of being connected iii series wit,h the other element wheri the furnace wa,s in oprration. The lining of the furnace u w of firebrick 4 . 5 in. in thickness, the inside diameter of the cylinder being 48 in. The insulation surrounding the lining consisted of a 9-in. d l of Sil-0-Cel brick, and the insulation in turn was protected by a heavy iron casing. The bott,om of the furnace was of the same material and thickness as the sides. The top consisted of a %in. firebrick crown built up on a heayy circular cast-iron ring, and was of necessity movalile in order t,hat the funiacc might be filled and emptied. .isheet-iron cylinder, covering, tightly the outer furnace wall, was placed over the firebrick top. The cylinder was about 24 in. high and was filled with sand or Sil-0-Cel powder in order to insulate the firebrick top. The temperat,ure of the furiiace \vas eontrolled by a 59stern of relays operated by the potentiometer recorder, t,he latter bcing connected bo two Hoskins t,herrrioroi~pl thermocouples measured beniperatures at trro piiiibs: the one, with its jonctiou adjacent to ttie heaiing clement, recorded fliictuations in teinperat,ure of the heating medium supplying heat to t,he furnace, and t,he other, with it,s junction extending into the interior of t,he iurnare wlit~e it touched the cent,er of the charge, provided a record of t,lie lieat t.reatment of the disk. Ry maintaining, at the outw thcrrnocouple a cert,ain tnnpr"rature, which incident,aliy may fluctuate inore or less to either side of the desird degree, a remarkably steady temperature, slightly lower than ths,t at the renistance element, was srlstaincd in the furnace. Thc proper temperature \vas maintained by means of t h e control mechanism of the recorder. Two fiber disks, A and R, ahout, 6 in. in iliamrt,er, i w r e mounted behind t,he recording instrumeiit; t h c x rli4ks were so arranged as to rotate with the potentiometer nliclr wire, and. accordingly, any movement from the zcro of the potentiometer was folloived by a similar inovrmcnt of tlie fiber control disks. On the edges of each control disk \vert monnted two brass strip contact,s, and x stationary finRer fixed to tire frriinc of the iiintrumerit made contact with the brassskips of each disk. An additional contact point., whirh ~ n adjusi,s able. alsi~ riinde cont,aot, nith the strip.; 011 disk R. Tlicse disks am1 fiiigrrs play an inipertarit part in the oper:ttion of tlic iiirn:we, for by inakius and Iircakinp l l t c proper circuits, rclays are opcratcd ~ l i i c l i throw t,hr rurrent on and off tlw furIUICC. Of the threc r o 11 t :L c t finis one serves to clcierruiue t,be maxiiouni t,rmperat,uro to irliich the rcsis; tar will lie dloi\-cd to rise, another serves to operate
THE JOURNAL OF INDUSTRIAL A N D ENGIJEERING CHEMISTRY
June, 1922
the closing relay whenever the charge cools off below its scheduled temperature, and the third, or the adjustable finger, serves to make and break the circuit between certain limits, thereby holding the resistance element at the proper temperature above that scheduled for the disk. I n order to maintain
513
mit a strong car to be rolled in and out of the furnace. The car served as a carriage for the mold. The auxiliary aection of the track was so made that it could be moved about on a transveyor truck. The temperature of the furnace was controlled by a special Robertshaw regulator. The Robertshaw regulator (Fig. 4), consists of a valve actuated by the differential expansion and contraction between a carbon rod and a copper tube. I n this installation, the expansion tube extended t ugh the crown and into the interior of the furnace abo in. A worm gear and handle served for adjusting the regulator valve and for determining the temperature. The device has an arbitrary scale, and its calibration must be determined for each installation.
THECASTING PROCESS
4 - 5 k FIG.3-cROSS
SECTION THROUGH C s N T E R OF
"i GAS FURNACE,
SHOWING
LOCATION OF DISK THERMOCOUPLES, REGULATOR, ETC.
the proper temperature in the charge, it is necessary, before setting the adjustable finger, to determine the temperature difference between the charge and the ribbon. The control for determining the maximum temperature of the ribbon is desirable, in order that the loss by radiation may be supplied from a source only slightly higher in temperature than that of the charge; but for the rapid heating of cold molds, and in order not to lose time, it is desirable to heat the ribbon to a high temperature. In addition to the automatic controller, a large adjustable rheostat was used, capable of reducing the furnace input from its maximum of 24 kw. to about 5 kw. The use of the rheostat permitted greater flexibility in the operation of the automatic control, for when the input was cut down by the rheostat to a degree that maintained the furnace temperature a little hotter than the required schedule, the "make and break" in the relay system became less frequent. The apparatus described above served very well to hold the furnace a t a definite temperature, but for cooling the furnace at! a predetermined rate, adjustments had to be made by hand. For cooling with comparative speed, it was possible either to utilize the rheostat, or repeatedly to set the adjustable contact finger of the recorder mechanism, or to use both: However, for very slow cooling rates it was necessary to rely upon the automatic mechanism alone, and by regular adjustments obtain the average cooling rate required. These adjustments required, by reason of their frequency and nicety, the constant attention of a skilful operator, and to obviate this burden an automatic device m7as developed. This deyice was merely an adjustable backgearing mechanism for altering the setting of the adjustable finger, and, in conjunction with the apparatus described above, provided any desired cooling rate from 0 . 2 " C. per hr. to l o C. per hr. Higher cooling rates could be obtained by the use of the rheostat. As previously stated, the electric furnace was not as satisfactory as the gas furnace.
THEGas FERXACE The gas furnace (Fig. 3) consisted of a regular pot-arch enlarged to accommodate the iron cylinder and mold. The fuel used was natural gas burned behind a breast wall by means of three burners of the Bunsen type. A small track was laid down on the floor of the pot-arch, and a short movable section of track was so arranged outside as to per-
The glass was melted and fined in accordance with the 24-hr. process,z which has been so fully treated in the literature as to require no discussion here. While the glass was being melted, the mold was heated in an auxiliary furnace for a period of 5 hrs. a t 100' C., and was then raised to about 500" C., by which treatment the moisture was entirely removed from the mold. This point was reached, and the mold removed from the furnace, several hours before the former was required for further process. The glass was cooled in the usual manner, and when the melt reached the proper temperature the stirring machinery was taken away. The pot was a t once removed from the furnace on a pot carriage and placed upon clay blocks immediately beneath the crane. The hoisting apparatus was adjusted, a cover of heavy a s b e s t o s aymJ3 board placed on top Cam Rdlumment of the pot, and the melt permitted to chill for several minutes. The manner and extent of cooling are important, for if the glass is too hot when poured, bubbles will form when it strikes the mold;and, on the other hand, if poured too cold, the glass flows slowly, and by layering entraps air. In fact a successful pouring depends to a very great extent upon F t G . 4-CROSS Sl%CTlONT H R O U G H CENTER O F URE the decision and &ill ROBERTSHAW T E M P ~ R A TRECUL.4TOR with which the operation is performed. I n this connection, it is to be noted that a measurement of the actual temperature of the glass is difficult because of the apparent opacity of the surface of the glass to the radiation of the hotter glass beneath. A relative measurement is unsatisfactory, affording no means of control by reason of the practical impossibility of exactly duplicating conditions. Accordingly, the exact temperature a t which the pouring should be made has not been determined, although the moment for pouring was successfully ascertained in each case by an experienced workman, who determined the proper viscosity by applying an iron rod to the surface of the glass. Further experimentation will undoubtedly establish a method for determining, in a more
/
0 -1
2
G. W Morey, J . A m . Cevam. Soc., 2 (1919),146.
514
THE JOURN.4L OF INDUSTRIAL AND E.VGIiVEERI.VG
accurate and satisfactory manner, the exact temperature at which to pour. The proper moment for casting having arrived, the mold was placed close to the pot, which was raised and slowly
CHEXISTRY
1'01. 14, s o . 6
The annealing schedule actually used on 40-in. disks was longer than was necessary. Aceording to the work of Adams aiid Williamson: a slab of borosilicate glass 20 ern. thick should be suitably annealed if held for 7 days a t the proper annealing temperature, and then cooled according to the following schedule:
" C.per Hr
RATE
Initial. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.......................
Aftei20" .............................
0.2 0.3 0.4
o..5 Affer30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . After 40" ............................. 0.6 0.8 After50 O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alter 60'. . . . . . . . . . . . . . . . . . . 1.1 After 70'. . . . . . . . . . . . . . . . . . . 1.5 After 80'. .......... . . . . . . . . . . . 2.0 After 90D.. ......... . . . . . . . . . . . 2.8 After100' ............................ 4.0 kiaximum cooling rate 6" C. per hr.
Fro. S---.Casrr~cA
R I P ~ . B C I O RD I S W
tipped through an angle of about 120' (Fig. 5). The relative posit,ions of the mold and tlie pot were then carefully adjusted so the molten glass struck the mold a t its very center and slowly, and without folding, filled out the mold to its edges. When sufficient glass was poured, the pot, was so quickly tipped back to and beyond its origiiial position that the rapidly diminishing stream of glass was caught upon the side of the pot, thereby greatly minimizing the number of bubbles formcd from folds in tlic surface of the glass. When the disk was annealed in the electric furnace the mold was immediat,ely transferred to tlie furnace, where it was carefully raised and lowered into the interior. The cast-iron cylinder and cover were then hoisted into place, as was also tlie heavy furnace cover after the therinocoupies had been placed in their proper position. When the gas furnace was employed, the cast-iron cylinder and cover were immediately put into place, the transveyor truck moved to it,s position at the front of the pot-arch, and t.he mold carriage rolled in on its track. A wall of brick and clay was quickly built. up in front of the disk, and the furnace filled with sand to a depth over the mold casing of approximately 6 in., lewing a space, however, of about a foot be-
CN
Fro. 6-C"~.m D, T m o n n r i c n ~S c m ~ u COR ~ i A~.vr,ir.i-.cA SLAB8 IN. THICKNESS C u x v i A. S c m n i r ~ nACTUALLY Us~n 09 4 0 . 1 ~ Disrs .
neath the CTOIT-II for the passage of heRt from the burners. Thus the disk inside the casing was insulated with sand on all sides. Two tlrerrnoeouples were arranged. the one with its hot, junction in cont.aet with the center of the iron plate above the disk, and the otlicr projecting a few inches inside the door of the furnace. The fire was a t once turned on full until the temperature of tlic furnace was from 50" to 100" C. above that of the disk, and was then gradually lowered until the disk reached its annealing temperature.
Tile annealing temperature used was deterinined froin n sample taken from a previous melting. To allow for a SUEcient fact,or of safety, the disk was held at its aniiealing temperature for 14 days, a period twice as Ion,* as was iieccssary. This temperature for glass 8 in. in thickness W M 104" C. Cooling followed in accordance with the scliedule of A d a m and Williamson as outlined abovel althoush t h e maximum cooling rate was retarded so as llot greatly to exceed 1" C. per hr., a precaution doubtless unnecessary. IIowever, below a temperature of 100" C. it would be difficult tu cool t,he furnace a t n more rapid rate; rapid cooling accelerated, for example, hy opening t,he furnace door, must he aroided by reason of the probability of uneven cooling setting up dangerous temporary strain. Theoretically, to be sure, the tensile strength of the glass should permit a disk to he cooled toward the last at a rate even in excvss of 0" C. per hr., and such a rate would permit a very appreciable and import,ant saving of time; but, practically its use invites disaster, and it was, therefore, deemed ad1 to employ a slower, and, consequently, a safer cooling rate. A coinpurison of the two curves shown in Fig. R will show the time which might possilAy be saved by using the thcoretical schedule. It is unnecessary to state that the ease with wfiiclr tho scliediiles were maintained resulted from the automatic controlling devices. For example, when operating the electric furnace, the temperature of t,he ribbon, and coniequently of the furnace and its charge, was detcrmiiicd hy the setting of the recorder, for when the temperature of tlie ribbon reached the inaximum limit as set by the recorder, the current was automatically broken, and then, as the temperature of the ribbon lowered and reached the minimum set by the recorder, the automatic control operated to couplete t,he circuit. The difference in temperat.ure between t,he maximum and minirnurn limits was constant, eorresponding to tilo distance between the two brass strip contacts mounted on the fiber disk 13. By this inems, just enough heat was added to supply that lost hy the furrrnce and disk, and a remarkably constant temperature wils tlicreby inaintained in the latter. By lowering the setting of the instrument. the two limits were lo~vercdequelly, :rud i'oiisequently the temperatures of bhe ribbon, furnace, and dish lowwed accordingly. Tiirou~lioutthe upper mngc both limits were set ahore the temperature of the disk, and irk tlic lower range gradually approached its t,emperatim until at about 7.5" C . all temperalures pr:~cticiilIy coincided At t,his point, the irce cooling rate of the furnace bcrarrii: required liy the schedule, so the power was shut, off. Control of the gas furnace followed somewhat different lines. In the gas furnace heat was supplied coiitinoou,4y, 1
J. Fronklin I n ~ t . , 190 (l!l20), 850.
