CHEMISTRY rtUD INDUSTRY
. 21 for Intlis. and tlic first. coiit of pliister. l~suniplcs tire Thistle 13onrd, which is nindc from Phstcr of Paris, Insulite, nnd TcnTcsT. T h y arc nintlc from n w r y wide v:iricty.of mat,crials : i d t,licir propert ies :is n bnsc for pliistcring differ consitlcmbly. Tliistlc 13o:ird is coniwliicli is run :L Inycr of gypsiini pliistcr. A mixture of Pl;lstcr of Piiris, siiwliist :ind rctnrtlcr arc st irrcd up wit 11 wiit-cr in n miser of the t.ypc one sccs used for concrct c. It. is t,licn run on onc sheet of pitper resting on :I conveyor belt, :ind tlic ot.licr slicct is placcd on tbc top. Tlic ni:iss piisscs t,liroiigli two prcssiirc rollers wlicn it is cut. illto tlic norniiil sized bonrtls. Tlic pliistcr is tlicii iillo\vctl to sct nntl tlic bo:irds tlrir.91 oiit in n tunnel drier. Tlic iidhcsion of tlic pnpcr to the boiirtl is cstrcniely good Only n gypsum plastcr of tlic Iicnii1iydr:itc type ciiii I J ~ used upon somc of tlicsc, wliilc with others any kind of gypsiini plnstcr is siiitiiblc. Thc nrcliitcct sh~iiltl consult t,lic m:iniifiict.urcr of thc piirticulnr board lie is 1i:Lving used hcfore specifying tlic gypsuni pliister to 1)c used upon it. Tlic tlifferciice is due to tlic cspnnsioii of tlie gypsiini plnstcr during sctt ing. Rctnrdctl Iicniiliytlriit c pliistcrs s e ~ coniplct cly and suddenly, cspindiiig :ilJoiit 0. t"/b during tliis proccss. It cnnblrs the plnstcr to bit(! into tlic recesses in t,lic surfiicc of tlic boirrd and grip firnily. Tlierc is no subscqiicnt cspiinsion. Ccrtiiin otlicr pliistcrs, uiilcss t h y dry out in tlic mcmtinic, set slo\vly and continuously iind continue to espnnd during tliis period. This results in the tlcst,ruction of tlic bonii niid tlic plnstcr pnrts froin tlic bo:irtl. This occiirs with Tliistlc J3oitrt1, wliicli iitllicrcs ~vr.c!ll to liciiiiliytlrritc pliistcrs :ind h l l y to otlicr t,ypcs. ~1ecliiiiiic:tl strength dcpcntls Iiirgcly on tlic t,liick-ncss