NE\VS LETTER 13British Technical Committee, Coryw Vitrearum Mtdii Aeri

Physics, University of York 2l February 1975 Ret. TC17512

1

2

3

4

CONTENTS

GENERAL

DOES CLEANING AFFECT WEAT'HERING

PROTECTIVE GLAZINGS

NEW ABSTRACTS

1 GENERAL

1.1 ORGA-IIIZAT]ON OF CVMA DUR]NG 1975

A neeting of the Bureau of Uni-onAcad6nique fnternationale, at r^rhich Conitdfnternational drHistoire d.e lrArt represent-atives vere present, va$ heJ-d, in Paris on22nd January and it vas agreed that, for thepresent, Professor Louis Grodecki shoul-d. takecare of the general organization of CVMA and-of the art-historical aspects, ancl I shouJ-dco-ordinate those international aspects vhiehare of a techni-cal nature.

The 9th Colloquiur of the CVMA, due to behel-d in September 1975, is sti[ pla€ued byfinancial d.ifficulties but it is hoped. that asmalJ- speeialist colloquium can be held inParis to vhich a l-inited number of contrib-utors may be invited.. The meeting voul-d bedevoted soleJ.y to the mechanj-sm of theweathering of glass and the appropriatenessof various conservation proced.ures. It ishoped that, at the sa.ne time, a meeting of theChair:nen of the National Cornnittees can beheld to recommend a successor to ProfessorHahnloser.

I.2 REPORT OF M. BETTEMBOURG'S VISIT TO

BRITAIN

Ing. J.M. Bettenbourg calne to Britainbetveen 23rd September and. 6th October 197L ona grant from the British Council, and hevisited Cambrid-ge, Canterbury, London(Victoria and Afbert Museum), Norwich,Sheffieldn Wells ancl York. He has prepareal arlinteresti.ng five-page report, the highlightsof which are sunmarised. here.

He particularly eonnents that the diverg-encies in methods of conservation, vhich henoticed tvo years ago vhen he attended the1!J2 Synposium in York and Canterbury, havetend.ed. to d-isappear as conservation techniquesbecome based- more and more on a scientificunderstand-ing of the basic problerns. (ttod.oubt these Nevs Letters have assisted thisprocess!) Thus, although the doctrines andphilosophies regard-ing restoration differfron coirntry to country, such differences intechniques as remain arise largely frbm thedifferences in the states of d-eterioration ofthe vind.ovs, for exa.nq>l_e the corrosion is moreserious at Canterbury tharr at York. Otherd.ifferences concern questions of replacingbadly-corroded- ned"ieval glass by mod.ern glassor by other ancient glass of the same colour,and vhether the replacement glass should beleft plain, be painted, or be given a patin-atecl appeara^nce.

Epoxy resins for ed-ge-joining brokenglass are being replaced by silicone adhes-ives, folloving the French vork on accel,eratedageing of adhesives (nef.B of British AcadenyBibliography). He remarks that, in CanterburyCathedral, one vindorr contains 120 pieces ofglass vhich have been t?d-oubl_ed-rt vith clearglass thus adding to the veight of the vindowvhereas, in Frarice, the tend.ency is nov toprotect cleaned glass vith a resin layer inpreference to ttd.oublingtt.

He remarks that he had profitabl-e dis-cussions rrith everyone he met in Britain andthat such exchanges, especial\r as betveencraftsmen, should. continue because they canbut improve the general l-ife of meilievalwinclows everJrwhere.

1.3 LASERS FOR CLEAN]NG GLASS?

Item 1.5 of N.L. lio,l-1 (p.3) carried anote by Professor 0"S, Heavens about thepossibility of cleaning crusted medieval glasswith l-aser be:ns. Dr John F. Asmus of theUniversity of California, San Diego has novsent a connento given at (i) belov and-Professor Heavensr reply is at (ii), Dr Asmushas al-so sent me a preprint of his paper, onthe use of lasers in the conservation ofstalned gfass, to be given at the Stockhol-mIIC-iiK!' Joi"nt International- Congress (seeAbstract No.U9 on page T ).

(i) Comment by Dr John F. Asmus

WiLh the exception of his closing para-graph, Prof. O.S. Heavensf concfusions inttl,asers for Cfeaning of Gl-ass?tt (N.L. No l-1)are entirely consistent vith our analyses andexperimentaJ observati-ons, Unfortr"rnately, inhis final paxagraph concerning costs he seemsto have confused. the d-istinction betveen thepe-+- p.u-f.se. power and the continuous pover ofa laser. In fact "a 10-jou1e per pulse norn:al-mod-e l-asertt at tt5o pul-ses per minutett repre-sents a peal< (not ttaveragett) power of 10 kil-o-watts. Its average pover is a much morenodest 10 watts. This is 1OO times sma^llerthan the rrl kifovatt CW Nd-YAG laser systemrtfor vhich he quotes a price of t42r000. Inour work on stone and other materials weachieve coverages of some tens of squarecentimeters per minute as Prof. Heavens spec-ulates. Iiovever, the average povers employed-range from J to 2J vatts and the laser systemscost tlrOOO - 81000. There are nrunerous con-cepts for reducing the costs of these lasersquite substantially includ-ing direct poverline excitation of flash lamps, plastic pump

cavities, and flowing dye active media. I^lhen

the market for such prod,ucts grows a bit morej/ie may anticipate theiv commercial availability.

(ii) Repfy by Professor 0.S. Heavens

Professor Asmus is perfectly right incal.ling attention to the absurd, error in my

reference to the costs likel-y to be invol-ved.in faser cleaning of glass. The aE)rage poverrequi,red for a reasonable rate of cleaning isindeed in the tens of vatts, rather than in thekilowatts region. Unfortunately, this unfor-givable error of a factor of about a thousandin pover is not reffected. in the price factorwhich, as Professor Asmus points out, is in theregion of onJ-y 5 to B. f r,ras certainly pessi-nistic, but vhen labour costs and overhead"sare taken into account, it stifl seems asthough one is talking in terms ofT, i1-2 perdmz (t100-1200 per ma) for the cleaning process.When the cost of removing and replacing thevi-ndovs is taken into account, I believe thatnnotwithstanding my embarrassing l-apse, my finalconment, that Itj.t is l-ikely that the operatingcosts at present woufd. be prohibitively highrt,remains vafid.. The situation may change. AsProfessor Asmus indicates, there are possi-bilities for red.ucing the costs of lasers anda substantial red-uction can be expected- vhenthe lO-vatt l-aser becomes an essential- in everykitchen or motor car. (The first, prototype,

cofour television sets surely cost more thanthe present l-O-r^ratt laser!). Laser cleaningoffers an exciting prospect and it vould beenormousJ-y satisfying.if it could rapidlybecome a normal- and easily accessibl-e technique

: df ?estoration.

I.\ HEAT]}JG OF MDIEVAI Gi,ASS!

My provocative remarks in item 3 ofN.L. No.l-2 have already brought a comnent froma read-er. Mr Frederick W. Col-e RF r}4GP, whois in charge of the glass restoration studiofor Canterbury Cathedral, has vritten to pointout that, vhen ed.ge-bonded pieces of ned.ievalglass are heated to accelerate the curing ofthe ad.hesive, it is important to heat theglass up sJ-owly and cool it dor^m again slovly.He says, ttT'he glass should be put into a col-d-kiln and al-loved to vann slovly up to therequired. temperature, and then to cool viithoutopening the kiln; this calr take 2)+ hours.rr.He goes on to say that this procedure wasfoll-oved vith the rnid-l-5th c. head ofEl-izabeth Woodville, from CanterburyCathedral, and the glass is in good condition.

I.5 A H]STORY OF GI,ASSMAKING

From time to tine f am asked- to recornmendrra book on glass technoloryrr which vould be ofuse to art-historiansn architects or craftsmenvho vish to gain some und.erstanding of glasstechnolory vithout getting too involved viththe complexities of chenistry. f know of onlyone such book and f have no hesitation inrecomnend.ing ItA History of Gl-assmakingtt byProfessor R.W. Douglas and. Miss Susan Frank,publi.shed in L972 by G.T. Foulis & Co. Ltd..of Henley-on-Thames, Oxfordshire, at a priceof t)+. 50.

L.5 WAR-TIME STORAGE OF MEDIIVAI GLASS

In response to my appeal for inforrnationon this subject, Mr B.J. Ashwell, the Archi-tect for Gloucester Cathedral, has kindJ-yconsulted two retired vergers, Mr Moody vhoretired in l9T0 and Mr Fred Berry vho retiredin L956 and is nov aged 82.

Brieflyn some of the glass from the GreatEast Windov was removed in 191+0 and part of itvas stored. in crates in the cel.lars of Miserd.enHouse, near Stroud. The remainder of the glasswhich vas taken out (some of the upper ligbtswere left in) vas stored in the crypt vherethe coronation chair and the Effiry of RobertDuke of Noflralidy vere sa.ndbagged. for protection.The crypt becarne dartp and moul-d grev on thevood.en objects. So some ventilation r^ra,s pro-vided betveen the sandbags.

Thus this seems to be a situation vorthyof further study, some of the glass being leftin place, some being stored in apparentfy drycel-lars (probably vrapped in nevspaper andstrav) , and some being stored. in a damp cryptfor J or B years. If ve can discover whichglass was gi.ven each rrtreatmentrt it vil1 beof great interest to compare the present stateof the panels.

2 DOES CLEANII{G AFFECT WEATHERING 7

2.1. MORE RESULTS T'ROM THE M(PERIMM{TS

ften 2 of N.L. No.f2 d-escribed the startof a series of experiments on a smal-l pieceof green l-2th c. glass from York Minster.Stages (a) to (d) vere listed in the tabt-e atthe top of page 5 and it was concl-uded thatthe crust on the surface (n) Aia not eonferprotection against accelerated. veatheringbecause the loss of K2O (in 50h at B5og ittr^rater) vas O.O22%, and that airbrasive treat-ment of the whole surface (") aia not increasethe rate of veathering (conpared. with thecrusted. salrple) because the loss of K2O wasthen O.O2I%" MechanicaJ- polishing (d),hovever, improved the durability, the loss ofK2O then being reduced to O.OL2/".

Subsequently (Stage e) the salrple vasheated in an oxy-gas fJ.ame to give a fire-finished surface. During this strong heatingit cracked- into three pieces anct the smal-l-estr,ras lost. The remaining tr^ro pieces measured-about 1\ x 1l+ x 2 rm and 16 x'l x 2 mmt havinga volr:ne of about O,65 nt ancl a surface areaof about pOO n",m2; together they veighed101) ng.

After Soxhlet extraction for !O hours atotal- of 0.35 nS of K2O hacl been extracted,equivalent to a loss of O.O22% by r,reight.This is about the same as vas found- at stages(b) ana (c) a.na night at first suggest thatthe flane-polishing had not been properlyeffective. ft does, hovever, seem thatSoxhlet extraction of these tvo very sma11pieces gives anomalous\r high results andtherefore both pieces rrere airbrad.ecl on bothsurfaces (stage f). The nev total veight vas

l-!8) ng and" 50 hours in the Soxhlet gave anextrgction of O.5O ng of K2O. This corres-pohds to 0.038% antl is nea-r1y tvice the figurepreviously recorcled for the effect of air-brasion (stage c).

lJe thus seem to have confirrnation thatairbrasion recluces the d"urability of a pol--ished" surface, the loss of K20 increasing byabot;t 7o% in both cases f(r)/(e) = O.O_38 /O.O22- 1.7 and (d)i (c) = 0.021/ 0.012 = 1.7] but itis not clear vhy the extraction rates bn thesetwo rather snall pieces is about 8OZ, higher[(f)/(d) = 0.038/_0.021 = ].8 and" (e)/(c) =O.O22/O.OI2 = I.B-l than it rrras on the originallarge piece.

Finally, the tvo small- airbracled. piecesof glass vere sent to Mr Frederick Cole to beacicl-pol-ished by the procedure vhich he usesat the Canterbury Catheclral Glass RestorationStudio, i.e. ten minutes in 40% HF. The acid-attacked- this poorly-clurabl-e glass rathervigorously; the tvo pieces fost about hal-ftheir veight ancl one of them broke into tvo!At this stage (g) the combined r^reight of thethree smal1 pieces vas only JBI+ ng andSoxhlet-extraction gave a loss of O.275 rl'e.This is not a lover va]ue and. thus it seemsthat of all three kind.s of polishing of theglass (nrechanical, flame a.nd. acid.) the firsttwo improve the d.urability of the surface, butacid polishing seems not, on the basis of thissingle experiment, to give an improvement indurability, but it certainly does not nake itworse !

To sr::marise the nev clata in the sameform as the table at the top of page 5 of N.L.No.12 ve get:

Proced.ure Weight ofs anples

(rne)

Percentageloss of K2O

Coments

(e) Extraction afterheating in af].ame

(f) Extraction afterairbrasion

(g) Extraction afteracid polishing

L6V

1585

?Birn

o.022

0.038

0.035

(fhese extraction rates are about(80% higher than those obtainetl(.on the original large sample,(uut ttre extraction fron the air-[brad.ed. samples is about 70%(irigber than from the polished(sarnples, as before.

Acid polishing gives no improve-ment

x The acid has attacked this poorly durable glass rather strongly end it is evident that one mwtttknov onets glassrt before using this useful aid to restoration. Fortunately the Canterburyglass is general\r much nore tlurable than this sample, vhich was chosen for its lack ofdurability.

2,2 COTVMENTS FROM A R&ADffi

Following ny provocative remarks in N.L.No.12, Frau Dr Eva Frocll--Kraft has kindlywritten to d.rav ny attention to the articleabstracted as item No.fB3 on page 9 . Inparti-cular she points out that the J-oss of theied deposi"t fron the flrst stripe (ffi'IE'E-

3 PROTECTIVE GLAZINGS

3.1 THE ISOTI{ERMAL GLAZII{G D(PER]MM{T ATSHU'FIELD

Tlris ex5reriment has nov been completed.ahead of schedule and a report is being pre-pared for the Build.ing Research Establishment(BRE) which gave the contract to BGIM onbehalf of the Department of the Environment.One of the many conclusions is that is,ot!e14a1glazing is nuch more "efficient" ttrai-iffifrthought because air velocities in the inter-space have sometimes exceed.ed. 1 metre persecond, corresponding to 6O0 changes of airper hour in the cavity.

The BRE has now agreed. that the unspentmoney from the contract can be used. for asinilar ex;leriment at York Minster, using awinclow (No.s VIII) on the south choir aislevhich alreaqy has external protective glazingin position. Some a4iustable external-ventilation of the interspace vi1l be intro-duced so that the themal, air-flov, andhunidity conclitions can be studiecl in thisnuch cheaper alternative system of protectingancient vindovs. a

Figs. 1 a^nil 2 shov, by pertission of theBRE, the arraogement which has been in use atSheffield, in the "BGIM catheclra1rr, theletters ind.icating the positions of thevarious sensors.

Five kinds of sensor vere used.: (i) 14chromel--alumel thermocouples all connected toone ice/vater cold junction, ind.icated. inFigs. 1and.2 (and in the list bel-ow) by theletters A to P; (ii) 3 chronel-al.umel-ttthermopilestt for measuring temperaturedilference-s between afiacent points, i.e.ffia-the alifference in l".peraturebetween the outsitle air a^ntl Face 2, W

neasurecl the difference betveen Face 2 andFace 3, and X measurecl the tlifference betweenFace 3 anal the air insid.e the rrcatheilralrr;(iii) 4 Pl-aster of Paris gauges vere used. torecord. vhen incipient cond.ensation vasoccurring (or vhen actual conclensation hadoccurrecl) and these are shown by the lettersQ to T; (iv) a lithiun chloricle brudd.itysensor at U; and 3O anemometer sanplingpoints (five are shown at Y in Fig.l) where a

of N.L. No.12) night represent only part of acrusting-d.ecrusting Brocess, as occurred. atJud.enburg, ancl not an improvement ind.urability. I agree that this is a possibilityantl that the sa:nple shoulaL be airbrad.ed a third

'tibe-iand re-submi.ttecl to acceLerated. veather-ingi an experiment on these lines vill becamielout.

DISA hot-wire vibrating lov-velocity anemometercouldpbe inserbed. {}

Ttre key to therletters is:A = outsicle airB = Face 2, topC = Face 2, bottom| = interspace, topE = interspace, nid.dleF = interspace, bottomI = Face 3, topH = Face 3, niddaeJ = Face 3, bottomK = Face l+, topL = Face 4, uiddleM = Face 4, bottonN = tfcathedral airrr, top| = trcathedral airtt, bottomQ = Face 2t topR = !'ace 2, bottonS = Face 3, topT = Face 3, bottom

It nil1 be seen from the diagrans thatthe inner glaaing, rrhen first set up, con-sisted of three penels of float glass rrithtvo panels of 19th c. stainecl glass below them.Ttre stainecl glass was replacetl by float glasswhen it hatl been established. that the stained.glass becane rrarner than the float glass if itwas irracliatecl from a clear st<y (the lrinctowfaces north vest).

A11 the sensors wiIL be teken to YorkMinster and. installed on one of the threelancets of vind.ov sVIII, in alnost exactly. thesame clispositions as in I'igs. 1 and 2 (therrindbw is about 5.5n tall) except that one ofthe incipient condensation gauges (n) wiffnow be placecl on Face lr instead of Face 2 andthere lrill probably be only one hol-e for theenenometer instead of 3Ot

Both of these experiments nifl be of theutmost value to architects a.ntl stainect glassconservationists everlnrhere. All C'\MAvorkers rrill- owe a debt of gratitucle to BRE,antl to the Departnent of the Environment, fortheir foresightedness in financing the experi-ments; aLso to the Dean antl Chapter of YorkMinster for so generously naking rrindow sVIIfavaitable fctr this inporta^nt er4>eriment.

F--r.o6 m

-_s.i

)

II

I

5'26 m

FIXED LADDERS

OUTERwrNDow Pa IIOODEN

OUTERFRAME

\

FIG 2.'CATHEDRAL,,SIDE ELEVATION

F IG. I "CATHEDRAL,,- FRONT ELEVATION

Figs. land2 Adiagrammaticrepresentationofthepositionsofthe23sensorsusedontheBGlRAsimulated'Cathedral'tostudythemechanicsofan isothermal glazing system. Only five of the 30 anemometer sampling points have been shown in the front elevation (Fig. 1) and only

one of the 30 in the side elevation (Fig. 21. ln Fig. 2 the four incipient condensation indicators have been shown by simple arrows.

LANDING

I I l+{ocm-tlll--r'tv m----=_.-1

A to P =14 thermocouoles' Q RSI = lncip.cond.indic.

U = Humidity sensorVW X =3 thermopiles

EDGE =I NSULAT

mff

9(

T-Loo.

I

rNsur

I

8Bm I

I

I

+-OING

6'd1 m

3.2 EXTERNAI PROTECTIVE GLAZINGS

We need. to knov vhich nedieval vrindovshave been protected. for several d.ecad"esagainst rain, stones etc. a^nd. I have thereforeasked readers to send me alry information theyhave.

I am ind"ebted to Mr Peter Gibson of Yorkforbhe folloving information about the firstext,ernal. protective glazing to be used" at YorkMinster.

(a) Yorkshire Gazette 29 June 186t, p.tO.

The artiel-e discusses the heating to beinstalled- j.n York l.{inster; J2 stoves at atotal- cost of t6l+5 lrere expected to guaranteea temperature of 10oC. TLre temperature atSt Paults Cathedraf in London vas stated tobe l-5oc but the greater expanse of glass atYork Minster makes this difficult to achieve.Horr'ever the rrDean and Chapter have d.eternined-to glaze the outsid"e of rbhe fj.ve sisterstv'indov, in the North Transept, vith plateglass n to obviate the great draught of col-dair through (sic) tfrat e)q)anse of glass; thisvork vill- also have the additional advantagethat it vil1 protect the beautiful stained.glass vhich in heavy gales from the north isin d.anger of sustaining consid.erable damagett.The article goes on to state that thescaffolding is in place for d-oing this vorkand hence the ttFive Sistersrt vindov seems tohave been given external proteetive glazingin the niadte of f86f and it may veII havebeen the first large wind.ov ever to be pro-tected in this way.

(b) York Herald. a l-etter to the Editor d.ated.rt .luFT6f6p1ains about the t'Deanrs so-caJ-l-ed improvements ... particular\r coveringthe Five Sisters a"nd. the great West vindor+with plate glass vhich takes ar^ray the depthof slay of the mull-ions and riehness ofeffect, . . . besid.es foruing a space for d.ustto lodge in .....". Thus both the FiveSisters and the West rsind-ov of the Navealrea$r had external glazing by the middl-e of1862.

(c) The Ni.nth OccasioJraJ- Paper on theRestorati-on of York lvlinster, datecl Jr:ne 1906conments (p.5), rrAs regards the West vindov,large plates of rHartleyrs rough patentglassr had been inserted in ord-er to protectthe medieval painted glass, but these pl-ateshad been fastened into their position vithiron bars, vhich had contracted and expanclealaccording to the varrying temperature to r,ihichthey were exposed., breaking the rough platesof glass and splitting the stone mullions ....the rough glass (is) replaced by a conrpleteskin of clear rcrovn glassr in clia.nonclquarries, sinilar to the work clone at theChapter l{ousett. A letter to the Dean from theClerk of Works, dated. 22nd May 1!06 (on p.31;cornnented that the old- plate glass vas in avery bad- state and. onl"y one panel remainedwhole. The new glazing vas 89 sq.m (958 sS..ft).

(a) fhe Tenth 0ceasional- Paper on the Restor-

ation of York l4inster, datecl Ju\r 1!0J,conments (on p.2l-) that the iron stanchionstthave bnohen the thick plates of glass put upyears ago .... an outer covering of plainvhite glass in quarries vith lead glazi.ng (vil16b) substituted for the present thick greenplates . . . . tt . As regard.s the Gre at Ea.stWindov .... ttthe substitution of vhitequarried glass for the green plates shouJ-d notbe clelayed longer than i.re ca.n helptr.

(e) The Eleventh Occasional Paper (aatea.O.ugus n glass hasnov been removed- from the ftr'ive Sisters I

rsind-ov" ft is al-most inpossible to exaggeratethe effect al-reaftr prod.uced. upon the o1dglass, vhich nov glistens vith a lustre, andexhibits a cl-elicacy and beauty which rrered,inmed and al-most effaced- by the l-ate exberaal-covering .... The oJ.d gJ-ass has become sovery thin and- ilelicate that it requires themost constant care anil attention, but as manyof the thick green plates had. split a,nd vereinsecure ...., irreparable injury miglrt haveensued. at any time ....tr.

Thus the three major vindows at YorkMinster have had- some external protection forabout 113 years a"ncl it seems that the glassof the ItFive Sisterstt vas frvery thintr in 1905.Large plates of glass (size unknovn) areclearly a danger, as a^re iron stanchions, butthe il-lustration in N.L. No.ll shovs that thebuild-up of clust, feared by the writer of theletter in 1852, did not materialise.

l4r Dennis King, of l{orwich, has kindlypointed, out that Willian Peckitt had useclouter protective glaaing on his vindov atAudley End in Essex, and. Mr Gibson thereforenost helpf\rlly carried out an exam'ination ofWillia.n Peckittrs Conmission Book, nov helclby the York City Art Ga11ery. He d.id. not infact find. any reference to external protectionat Aud.ley Enc[, but h. l1!1 find the entryreproduceal here in fig.3. This is a facsimile ''

of part of p.31 (March 1782) of the book'reproduceal here by courtesy of lvlr J. Inga^me11s

of York City Art Ga11ery. It records thepurchase of nine panes of strong glass forfixing behind ttre painted. glass in the fra.mefor the panel made for Sir John Ransilen, HiglrSheriff at Byram Hal1, near FerrybridgetYorkshire. The building has, howeverr beenctemolished so that ve cannot stu{y the results !

Mr Dennis King has also alravn ny attentionto the situation at Cothele, Cornvallr whereexternal protective quarry glazing was

rerroved i-n fBBO. It is not knowa vhen the;ffi-.r grazing was instal]-ed (the 1l+80

vinaloli is knonn to have been altered in 1540)but the lead caroes of the outer d.iamondquarries have proilucecl a dienontl-shapeclcorrosion pattern on the outsicle of themedieva.l winciorr. It is hoped to give moreinformation, ancl a photograph, in a subsequentNens Letter. He also points out that theCollins-Martin vindov at Redbourne, Lincs.hatl outer glazilrg in iron franes" set upbefore ]-B!5 (J.Brit.Soc.Ma"ster Glass Painters8, l+o8 ) .

6

Z, Ia 7"{",. 0f*,Do^fr*!g**J

- ''.:-"-1-

t *rrrirorrr/^* 'rfr"- of nood"t. ,/:f --'-2 /r,i

U, Jbb "{4* ffu,r*4 fu,- cre,(r,ffi

iul/:{ f"fu;,eA*.). t 2,,:

g p.neatryW%yt'fuf'AD

Fig. 3 A facsimile of part of page 31 of William Peckitts Commission Book, for March 1782, reproduced by kind permission of Mr J. lngamells ofYork City Art GallerY.

3,3 USE 0F A DTFTUSTNG SCR@N

Dr G. Frenzel tells me that he first usecl-

a diff\rsing screenr between the protectiveglazing ancl the isothelsal\r glazed metlievalilassr-at Henfenfeltlr near Nuremberg, in t967.Ttre systen vas again used at Ingolstad't inl:972. It successful\r recluces the effect ofshatlovs, on the metlieval- glassr from theleading of the external glazing.

4 NEW ABSTRACTS

I79. ASMUS, John F. (l:gl>) "Use ofiaseriin tiffiffion oi sta.rned grass -.

Preprint of his paper for the ItrC-NIG JointInternational Congress, Stockholn, I-f June1975, 6 pages of typescript and L illustra-tions.

Ttre introduction to this paper mentionsthe many applications of lasers to mocLerntechnological- problems and cliscusses thecontrol of the size of the laser bea.n (andhence the area cleaned), the vavelength of therad.iation (hence the depth of penetration intothe material) a.nd. the pulse length (hence thelocalisation of the effects of the heating).As regarcls medieval glass he suggests that

3.I+ E}IVIRONMENT OF TI{E GREAT EAST WINDOW ATYORK MINSTER

I am happy to report that the cost of theercperimental work, reported in itero l+.2 ofN.L. No.12, (vhich suggested that the inter-space rras "l-eokytt to the outsicle air in aclesirable m".oer) has been net fron thePilgrin Trust Special- Grant for the stud.y ofgla.ss or vindovs harring special scientific orconservational interest .

laser bee.ms may be used (i) to remove opaquesurface crusts by volatilisation or (ii) toftpolishrt the surface by localised melting.

TtreoreticaJ- considerations are usecl toindicate that surfaee nelting to a depth of0.1 m needs a pulse length ot Z.> x fO-3secontls whereas 1/\s vould be needed for 1 mm.

TLre vapourization of a surface crust voul-dneecl shorter times than these. He carrietl- outsome tri als on baclly-crustecl metlieval^g1ass(probab\y 1l+th c.) using a single 1O-Js pulsefrom a ruby laser a.nd his photographs shorrthat the crust was vapourized, or meltecl, togive a clark spot where the beam hatl inpinged.

Further tests are planned to identif! theenergy thresholds vhich night danage medieval-glass. The present ruby laser, horrever, hastoo short a pulse length €lld a neodymiun YAG

rod r,ri1l be used to increase the pulse length.

]80. BAql{ER, E.(rSf)+) "Ein uittelalter-lieher Gl-asmalerei-fund in der Wienerlvlichaelerkirchert (A medieval- painted. glassfind in St Michaelrs Church in Vi.enna). OZI(D,

During the 1973-7)+ restoration vork insidethis church it was realised that a medievaleast v-indow had been bricked- up during theBaroque period. The authorities thereupondecided to move the great late-Baroque a,ftarvhich filled- the vhole apse, and threewindorrs vere then exposed., one of which con-tained late 13th c. g1ass. ILre paper givesdetails of the complicated" technique vhich hadto be adopted. to move the glass safely (tvolancets O.3B m high and O.52 m vicle, and aquatrefoil o.83 m d.iameter) but the presentabstract is devoted. to the state of corrosionof the glass which has not so far been founclin Austria.

Starting from the outside, vhich is eom-pletely smooth and. shovs no corrosion d,a;nagen

the process of decomposition is recogrrisableas a milky-opaque layer which extencls morethan half the thickness into the g1ass. Aninvestigation of the phenomenon is in progressa^nd. it is possible that it has been caused bythe special environment due to the valling-up,vith wide differences in temperature betweenthe cold wa1l on one side and the roof spaceon the other, vith its extremes of temperaturevariation. fhe transparency of the glass isalso impaired by residues of vhitewash andplaster on the insitie vhich could not be com-pletely removed. without end.angering the paint-vork.

181. BETT .(197)+f"Re;G;rmMethod-es n test6es par 1e Laboratoire d.e

Recherche d-es Monuments Historiquestt (Therestoration of ancient vinclows; nethodstested by the LRMH). 1O pages of typescript.

The docuuent points out that the presentstate of ned-ieval wind.ovs demancls urgentmeasures to save them. The researches under-talcen by LRMH are part of an internationalprogra.unne ilevised by the CVMA. Cleaning is thebasic problem beeause jointing, surface pro-tection ancl re-attachment of paint can becarried out only on cleanetl g1ass. The resultsd.escribed. in the report are based. on researchescarried out in the fiel-ds of A, cleaninglB, aclhesion of broken glass; C, protection bydoubling; D, protection against attack byatrrospheric agents; E, re-fixing of paint;F, cementi-ng of the panels with a poltrrmericmaterial. The state of the a.ncient glassshould be consid.ered very carefully beforeembarking on a,ny treatment and the LRMH vi,l-Igladfy give technical assistance to any vork-shop vhich requires it.

A. Cleanilg

( a) Ueche4lcal _qle-aning is generallytaboo, except for the careful use of sma1l

: "'giass-fibre brushes, obtainable from suppliersto painters and restorersr eg. Soc. Adan, Blcl.Edgar Quinet, Paris. (t) ultrasonic cleaningis particularly useful for nuch-pitted glassbut it shoulct be used, onJ.y vith care. Paintshould be perfectly aclherent or it nay beconed.etached. The use of liquid"s other tha.nvater can endanger the glass, and the liquids(generally based s4 nnm61i4) offered' by thesuppliers of ul-trasonic equipment are taboo,Suppliers are Soc. Ul-tra-sons Annemasse S.A.,T6 ter, rue Etienne Do1et, pl+-Cachan, FrancelSoc. Dentalex, 2\ rue Goclot d.e Mauroy, Paris 9e.(c) Chenrical cleaning. fhe use of any acid,or strong base such as arn.oniar is taboo buttvo chemical agents are aclvocated. vhich have arather slow action; this enables the cleaningto be observed a.nd to be stopped. vhenever itis clesired. (i) a nixture of LO/o sodiumthiosulphate ancl 5% sodium pyrophosphate (seea]-so N.L. No.J, iten Z.\(a) vhere fuflertletails are giveng there is an error in thatiten ancl the seconcl foramJ-a shoulclt of course,be Na4P2O7.1O H2O) permits the slorr progress-ive removi,l- of the crust vithout unduescraping, but it is not so useful on highlypitted. glass because the time required is thenvery 1ong. (ii.) a mixture of 30 e/l of EDTA

arrd 30 g/t ot arn*onir:m bicarbonate (see N.L.No.l, iten 2.)+(b) for extra detail). This hasa faster action than (i) and it can thereforebe useful for pitted gIass. The cleaning canbe clone either vith soaked pacls of cotton vool'renevecl from time to time, or entire panelscan be inrmersed in a bath, provid-ing the paintis properly aiiherent. One disadvantage ofcomplete i..ersion is that the various piecesof glass night need different periods oftreatment. Care shoufd- be taken to examinethe outsitle surface of the glass because itvas often painted (contrary to previousbelief) and hasty cleaning coulal damage thenoclelling appJ-ied to the outside.

B: Adhesion of broken glass

Ttre authors rssenmsnal replacing jointing-leacls by eclge-glueing vith silicone ad]resives(epoxy resins d.o not resist accelerateclageing). Suitable materials are CAtr'.3(transparent, grey or vhite) from BhonePoulencn Ave. Montaigne, Paris 8e (sritisttagents are R,W. Greeff, Acorn House, VictoriaRoad, London, W.3); Beriglace (transparent)from Soc. Bericol, 3\ rue Clement, Marot,l,yoni Workey M.S.W. (transparent) from Soc.Vetter et fi1s, 5 rue C. d-e Wett, Villeurbanre.

fhe broken eclges should be cfeaned vithacetone, a,nd a thin layer of adhesive spreaclwith a spatula. fhe bare edges should remainin hrrnid air for about 15 rninutes beforejoining under slight pressure. Keep the jointin place rrith aclhesive paper and- avoid ha,nd-ling for about 2 hours. Excess dry aclhesivecan be removeii vith a sharp knife or (beforehardening) by viping vith alcohof.

c. !:=ti@The use of a thin (1.5 nm) sheet of

modern glass has tvo ad.vantages. (a) ff there,are mar\y very sma1l fragments, their ed.ge-jointing may not alvays be feasible but theycan be stuck, piece-by-piece, on the backingsheet using a polyurethane resin (aery1ic,vinyl and epoxy resins d.id not vithstand. theaccelerated ageing test). The process is noteasily reversibl-e because much time isrequired for a solvent to penetrate, but thismay be the only practicable way of saving thefragments. Ttre pollmethane materia1 testedwas Bo% Viacryl 1/C353 and 20% D?s:arodry;r 75

"The former is obtainable from VianovaKunstharz, 1)+ Johannesgasse, 1010 Wien,Austria (ttre British agents are Berger IChemical-s, Resinous Chemical-s Division, IWellington l{i1l-s, Dunston-on-Tlme, Gateshead,Co. Durha,n) and the l-atter from Soc. Bayer,92 Neuil-Iy, I'rance (or Bayer UK Ltd..n BayerHouse, tB/Z\ paradise Roacl, Richmond., Surrey,*'

'?:] o* ,,n:;J,?""f"tJ7Y,*'f"Imu.ch weakened. by veathering (due to loss ofthickness or risk of breakage) or one vhichhas been ttstarred.tt by vandalism, can be plated,using a border of sil-icone to ensure a vater-tight sea1, e.g. Seelastrip fron Eq)and.ite Ltd..or Arboseal- from Aclshead. Ratcliffe, Belper.if the meclieval glass is not f1at, the platingglass should be shaped to fit it, using arnould of 1 part plaster and l-.2 parts grog andheating to 550-7OooC.

D, Pr.otegtion aga_ingt qtlack .by atnosphericagents

The background to their recommend.ationsis given in iten 1TO on p.B of N.L. No.lt.0n1y Viacryl VC363 mixed, vith Desmoclur vasfound satisfactory (see C above). The cleaneciglass should be rinsecl vith acetone to removetraces of hunidity a.nd preferably removed fromthe leads so that the eclges can be properlycovered-. Any tools can be c1eaned., before thez'esin hardens, by using ethyl acetate. Afterhard.ening the filn can be tlissolved in CitalL2-I2, liquid or paste, obtainable from Sod.iema397 ter, rue tle Vaugirard., Paris 1le.

E. Re-fi{ne gf paint

This can be done vith the ViacryIDesmoclur mixture quoted in C(a) above afterdiluting with ethyl acetate (:-:t) to attoveasier infiltration untler the loose or Borouspaint. Appty vith a capi11ary tube while thepaint is being cleanecl so that eleaning a.ndfixing can be carriecl out simultaneously.Before complete tirying, any excess resin canbe vipecl off vith etlqrl acetate. The prnocessshould be repeatecl 3 or l+ tines until thepai.nt is filr1y fixed..

F. Cementing meclieval panels

The background. to their recommend"ationsis given i-n iten 159 on p.? of N.L. No.l-l-.Silicone mastics give better resistance toaccelerated ageing than does the traditional-cement or putty but their application is not

quite so easy. Further experiments are inhand on improvecl nastics.

182. CoLLONQUES. professor. anclivine PEEEZ Y JoBBA (fSf :) ItSu; tt ph6nonene de;ffiffi-ffiraux (part r). -mcsp paris,March 19?3. (This is Part I of the paperabstracted as item 1?l- in N.L. No.11).

This earlier paper gives full details ofthe stuily of the veathering proclucts on nearly!0 medieval glasses from Amiens (12 sanples,13th century); Aube (B sanples, 15th to lSthcentury)g Beauvais (2 s"nples, 1!th century);Brennel-is (\ sanFles, 15th century) I Evreux(5 saq'Ies, l-3th century);, Le Mans (3 sarnples,13th to 15th century) ; Quinper (1 sanpre ) ;Rouen (9 sanples, 15th century) anci St Denis(2 senples, 12th century).

No less than 13 separate crystallinematerials were ici-entified in the veatheringcrusts:- srpsun (casot+ .zBzo); syngenite(t{zsot+.caso\.H20); silica (as o quartz andas I cristobalite); caleite (caco3); fiveunidentified. materials (vhich ggl begorg6yite, K2Ca5(S0\)5.ttzo; schoenite,t<zMe( sq!)z .5ttzo; leonite, K2Ms(S0L )2.1+u2o ;polyhalite, K2MgCa2(SCt+)L2H2g; and sixkieserites, MgSO4 vith 1r212)r\,6 or fmolecul-es of H2O); there vere e.lso anotherthree unidentified materials vhich vere found-only on the grisaille wind-ovs at Evreux.

Gypsum occurs most frequently. Syngenitedoes not occur in the absence of glpsr:m andthen only on glasses which are rich in K20 andhighly corrodecl. Cal-cite occurs about ad-oaen times and- on glasses vhich have a highCaO content (tZ% to 20% by veight), except tvovind.ows at Aube but these are 1ov in afkali.

Some of the corrosion proclucts atlherevell- and. are difficult to.remove, vhereasothers are easily detached.. Clea^ning pro-cedures coulcl be related to the analyticalresults (nCW - but it voulcl probably be tood"ifficult for the conservators to obtain thenecessary aaalytical results ) .

TLre paper encls with some speculationsabout the effects of atmospheric agents anclsome experiments vere done in an environmentrich in SO2. fhe glasses vere readilyattackecl but the corrosion products observedhave not yet been identified a.ncl in no qased.o they correspond vith those found. onancient glasses. (nCtl - this erperienceemphasises the problem of clevising suitableaccelerated tests of corrosion to simulatethe long-terru corrosion; probably the S02was too strong.)

1B:.-. r'FopL-gRAITr Eva (r9Tl+) ttMitte]--al-terliche glasmalerei - erforschung,restaurienrngtr (Medieval painted. glass -research and restoration) . OziO 1971+ 282OO-2O9.

Ttre author starts by pointing out thevery real d.ifficulties of knoving vhat hashappened to any glass, even during the presentcentury. She then cl-iscusses four extremeJ-yinteresting case histories.

(i) The Angel parel at Kremsmllnster, (Figs.762-f6\) " Mter removal- iluring the var it wasre-installed the wrong rray aroundt so that thepainted side was exposed to the veather. In30 years the painted linework has sufferecl nofurther: danage (the author suggests thatbadly-fired linework had previously been lostand that the renaining ve1l-fired linework didnot deteriorate in 'uhe 30 years ) but the half-tones have veathered away almost completelyand. the highligitts (unpainted. glass) havef,ormed an opaque crust. Tlrus there is nownear-Ly a rfnegative imagett. Thus 30 years I

exposure to the veather vas enougb to fom acrust on this glass. (nOU - but we must takecare not to accept the corollaryr that thefirst 30 years of life in the l-5th centuryvoul-d have prod.uced. l-ess veathering becan:sethe air vas cleaner! There are the possibili-ties that (a) :OO years of exposure to theinterior of the church had pred.isposeci theinner surface to become prematurely crusted"vhen it vas reversea or (n) the ',rar-timestorage conditions haci influenced both surfacesof the glass in such a vay that severe ileter-ioration and crusting cou.l-d" then occur in 30

(ii) nre Presentation, Magdalen Church,Judenburg (fies. 155-170). Ttianks to thereaciy cooperation of the parish priestn and ofthe curator, it vas possible to remove this1\tn c. vindov in 1973 to compare its con-d"ition vith that in 1963 (see p.360 of ttreLi.tsge CongrBs of the AIIIV). Briefly, the sixphotographs shov that some parts hacl becomemore opaque in the l-0 years a.ncl some partsless so, and the key to this vas the behaviourof an abnonnally (?) ffaf<y crust. fhus someparts of the crust had disappearecl in the 10-year period whereas other parts of the glassshoveci reneved- crusting. Thus, for this glassat 1east, the outside shows a cyclical crust-ing and ttde-crustingtt process vhich can beobserved d-riring these 10 years ' The vel1-fired. black paint has, hovever, been particul-arJ-y resistant to veathering ancl the authorherself points out (p.20\ co1.2) that thepolluted air of Jutlenburg can have affectetlthe outside surface only althouglr the internal-atmosphere of the church must also have beenpo11uted. (ttselbst venn die allgemeineLuftquaJ-itet sehr schlecht ist (dies trifftftlr Jucienburg zu) uncl d.ie l,uft in Kirchenin-neren dieselbe Zussanensetzwrg hat r.rie dieAussenluft (reine Filtenrng).tr RGN - perhapsthere woul-d not be so much sulphur d.ioxideinside the church because it woulcl react viihthe furnishings! )

(iii ) Ttre Founders Winctorr fron the chancel ofSt Walpurgis (r'igs. 171-185). This vinclow vasre-inserted in 1950n after rrar-time storage,but in recent years the clarkening of the pa^ne1shad. become increasingly noticeabte and theouter face is regarcletL as an exarnpJ.e ofparticularly acivanced veathering. It has beencl-eaned and external protective glazing hasnov been insta11ed., use being nade of the nevsystem of hexagonat leading to give a betterappearance to the outsid.e of the vinclov thanvould have been provided. by flat sheets.

During the cleaning the restorer remarked. thatthe crust consisted of unifo:rn layers vhichcoulcl be removeci onJ-y by repeating the oper-ation several tines. After each cleaning a

*nerr vhite film vas present vithin a fev clays.'Ihe vindov haci been photographed in 1950(figs. 180-182) but no-one had thought of alsorecording the extent of weathering on theoutsicle and hence there must be some specul-ation about the course of events, especially asthere are unusual-ly large variations in thestate of veathering, from al-nost intact glassto heavily cratered surfaces (fig. ffg), evenon the same piece of glass (Fie. ftl); herethere is much ciiscussion of the possibleeffects of glass couposition, extent of expos-ure to the veather, effects of the presence ofpaint or not, but without any firn conclusionsabout the relationship vith veatheringbehaviour. Nevertheless the discussions aboutthe presence of painting, ed of its type,have a profound- bearing on arry possible tech-nique of restoration. The Austrian Ministryconcerned- vith historical monuments has, infact, ru1ed. against restoration techniquesvhich are clepend-ent upon subjective judgnents,even though they niglrt be jr:stified on con-servational grouncLs. Prolonged contact withvater, in the corners of the leacls, seems toencourage weathering.

In 1950 some of the cracked. or splinteredpieces of glass vere platecl vith an interlalrerand the conalition vas quite "";h"rc"eE$iqeven though the cover glasses had been flatand not eoul-ded to fit the nectieval- glass asvould be d.one nov. The effectiveness of thissystem has also been confirneal froe the panelsat Waidhofen on the Ybbs which vere ptated- in19\7, no ctust or moisture having penetrated.fhe lrork by Taralon and Bettenbourg in Parishas demonstrated" that silicone adhesives are1ike1y to give even better resul-ts.

(iv) rtre protective glazing at Burg Kreuzen-stein, on the Da.nube. As part of the dis-cr:ssion on the effects vhich heating bysunshine may have on the air in the interspace,the case of Burg Kreuzenstein is quoted..Stsnderd. vinclov panes vere installed not laterthan 191\ antl the nedievaJ- glass we.s movetlinvard.s to fora sliding winctovs which meet inthe nid.dl-e and vould- have been unlikely everto have for"mecl a sea^I. Nevertheless these13th c. pe^nels are at risk because the l-ead.shave distorted. ( ? expanded under the i-nfluenceof the heat tr- infolge cler v6rmebedingtenAusdehnrng ctes Bleis ...tt ?) and bave becomeso bent that they have come out of the franesand no longer adhere to the individ.ual piecesof g1ass.

If the frames had not been sunk into theva11 grooves ve should nov be J-eft with nothingbut a piJ.e of fragnents, and adequate ventil-ation of the interspace between the twoglazings is necessary for external- protectiveglazing to function properly. ltre efficacy ofthis external glazing can be seen by comparingthe mecli.eval glass here r,rith other unprotectedvind.ovs at Kreuzenstein, vhere vind. erosion isinportant.

10

Fi-nally, the author reninds us that almostevery conservation technique used. in thedistant past, or even quite recentl-yr has hadsome d-isadvantage vhich its initiators oradvocates d-id not (or even could not) knor'rabout; therefore it is particularJ-y inportantto share erperiences vhich have been backecl upby systematj.e experiments.

tB\. NEI{roft, R.G. 1197\e) I'Recent impetusin tHe stuAy of tfre conservation of medievalsta-ined- glass vindovsrt. G1ass, 197)+ 51 405-)+09.

This article, timecl to co-incide vith theappeal for funds for the restoration ofCa.nterbury Cathed.ral, d.escribes the reasonsfor the upsurge in technical activities sinceSeptember 19?1 and outlines the stages i-n thehi.story of the Technicaf Sub-Connittee of theBritish Committee of the CVMA. Brief referenceis made to the various scientific advancesmad"e in the last three Years.

185. NEwrON. R.G. (1975) I'conservation

or r"ffivurlffi?J-(-i "otherua.l gl-azins )

t' .

Preprint of his paper for the IfC-NKF JointInternational Congress, Stoekholmr l-7 June19?5r,.le,.Pages of tYPeseriPt'

This account of the advantages and dis-adventa4es of isothermal glazing is essentiaJlyan expandecl version of that given in N.L. No.7tpages l+-11.

-o-o-o-o-o-o-

NOTE: Will readers of these Nevs LetterslF*. clrav my attention to any papers vhichshould be abstracted- here. It vould beparticularly helpful if photocopies of thepapers could be suppliect. My adclress is!, Hardvick Crescent, Sheffielcl, S11 BWB'

EngJ-and.

NOTES

11 Reproduced by the University of York Printing Unit