An Empirical Approach to Colour in Glass

arts

Article An Empirical Approach to Colour in Glass

Heike Brachlow

School of Arts & Humanities, Royal College of Art, London SW7 2EU, UK; [email protected]

Received: 1 December 2018; Accepted: 15 January 2019; Published: 18 January 2019

Abstract: This paper focuses on the characteristics and use of transparent homogenous coloured glass for cast glass sculpture. It provides an overview of glass colouring agents and their characteristics, and establishes factors that inﬂuence the appearance of colour in glass. Methods to visually evaluate appropriate colour density for a given form are discussed, as well as essential characteristics that a form must possess to achieve results within a density threshold area, where coloured glass changes in value and/or hue between thick and thin sections.

Keywords: colour; volume colour; glass; glass casting; glass sculpture; glass art; optics; density threshold

1. Introduction Colour in glass is an extensive subject and one that I have been researching for many years, with a focus on colours for kiln cast solid glass sculpture. A given form’s appearance can change drastically depending on its colour, and on whether it is transparent, translucent or opaque. Colour decisions are of critical importance to glass artists—they can ‘make or break’ a work. Many glass artists’ approach to choosing colour may be loosely defined as ‘hit and miss’, a method which becomes more reliable as an artist’s experience increases. While glaze testing is an integral part of ceramics methodology, this approach is not always suitable for glass. Although glass artists do employ colour tests, especially when working with frits and powders or designs with multiple colours, to obtain effective test results on how a transparent colour works within a form, one would have to test at full scale, and essentially make a duplicate of the form, the first one being the test. This is labour-intensive and expensive—and if the colour is appropriate, the test becomes the work anyway. My PhD research (Brachlow 2012) explores the possibilities of colouring glass in a studio environment to achieve specific transparent colour results for given forms. The results, with regards to establishing a generic formula to calculate colour density at a given thickness, were somewhat disappointing: a different formula for each oxide or combination of oxides would be required. Furthermore, colouring larger amounts of glass in a studio environment would require a furnace, which means it is impractical and expensive. This paper draws from and builds on my previous research by exploring the relationship of colour, form and light in relation to commercially available casting glass. What are the factors that influence the appearance of transparent colour in glass? How can a studio glass artist make reliable colour judgements when using a single transparent homogenous colour? Rather than attempting to set up experiments with controlled lighting conditions, this paper attempts to establish general guidelines, using as example coloured glass made by the three companies best known for selling coloured glass for casting in the UK: Banas, Bullseye and Gaffer. While plenty of literature exists on the subject of surface colour, there is not much to be found about what Joseph Albers defines as “volume colour, which exists and is perceived in 3-dimensional fluid” (Albers 2006), and I have defined as “the colour of a transparent or translucent 3-dimensional object which is coloured throughout its mass” (Brachlow 2012). Sylvie Vandenhoucke and Max Stewart both explored colouring glass with oxides at casting temperatures in their MPhil and PhD research,

Arts 2019, 8, 15; doi:10.3390/arts8010015 www.mdpi.com/journal/arts Arts 2019, 8, 15 2 of 10 respectively, but this approach does not yield transparent glass (Vandenhoucke 2003; Stewart 2010). Scientiﬁc manuscripts about colour in glass do of course exist, for example W. A. Weyl’s seminal work Coloured Glasses (Weyl 1999), but are difﬁcult to understand by a layperson. An investigation into volume colour for artists requires a visual and empirical approach.

2. The Appearance of Colour in Glass My approach to understanding colour in glass is empirical and iterative: observation of glass objects leads to forming theories, which are then visually confirmed or refuted through observation and evaluation of glass objects. It is important to note that observations are conducted on glass objects rather than photographs thereof, as colours in photographs are seldom true, and specialist lighting is used, which is unsuitable for exhibition or other viewing environments. Volume colour is a curious subject. By definition, it is 3-dimensional, but it could be argued that the addition of light (and colour does not exist without light) lends a fourth dimension. Take a billet of coloured glass, for example a Bullseye billet, which is a solid cuboid of coloured glass measuring approximately 127 × 254 × 20 mm. How would you evaluate it? You are outside on an overcast day and your billet sits on a white background in front of you. It will appear an even light colour when viewed from the top, much darker along its long side and even darker along its short side. The same is true if you try this in artificial light, only there might be light reflections near the edges, and from certain viewing angles reflections from the uneven base of the billet. Pick it up and slowly lift it to ~20 centimetres above the white background. Your white background, when viewed through the moving billet, will show swirling shadows, caused by the billet’s uneven base, receding into nothing as you lift. The billet is now a much lighter even colour when viewed through the top surface, but shadows and background colours may interfere with an even appearance. The sides of the billet still appear much darker. Now hold it up to the light—the colour appearance will probably be even lighter and diaphanous, and the background will have an impact. The type of light also makes a difference, in the case of shift colours, that is colours that change hue in different types of light, such as Rhubarb, it can make a big difference: from greenish in fluorescent light to reddish in light with a continuous spectrum, such as daylight and incandescent light. Take it outside again and hold it up against the sky—the colour will be different. Again, the background colour has to be taken into account. If the colour appearance in a simple rectangular billet is this difficult to pin down or evaluate, how much harder is it to appraise a more complicated form? And how would one go about this? C. R. Bamford introduces his volume Colour Generation in Glass with the following paragraph:

The colour of glass is that of the light either reﬂected or transmitted by the glass. The two colours may be different but are interrelated. Three elements govern the colour of the light transmitted by glass, namely:

(i) the colour of the incident light; (ii) the interaction of the glass with that light; and (iii) the interaction of the transmitted light with the eye of the observer (Bamford 1977).

This is a good starting point, but how does the light interact with the glass? My PhD research identiﬁes a series of factors that inﬂuence the appearance of colour in glass objects, here slightly revised:

• colour of glass; • form of object; • form and size of object in relation to colour density of glass; • surface finish of glass; • intensity of light; • wavelength of light; Arts 2019, 8, 15 3 of 10 Arts 2019, 8, x FOR PEER REVIEW 3 of 10 •  refractiveincident angle index of of light; glass. and • refractive index of glass. Together, these factors (and possibly more that I have not yet taken into consideration) determine the appearanceTogether, these of a transparent factors (and object. possibly more that I have not yet taken into consideration) determine the appearance of a transparent object. 2.1. Glass Colouration 2.1. Glass Colouration A discussion about glass colour affecting an object’s appearance requires an overview of the two mainA discussion types of aboutglass colouration: glass colour ionic affecting colouration an object’s and appearance colloidal colouration. requires an In overview ionic colouration, of the two alsomain called types solution of glass colouration:colouration, ioniccolours colouration are achieved and colloidalthrough colouration.the addition Inof ionictransition colouration, metals, also for examplecalled solution cobalt colouration, (Co), chromium colours (Cr), are achieved copper through(Cu), manganese the addition (Mn) of transition and iron metals, (Fe). When for example these colourscobalt (Co), are chromiummelted in (Cr),an oxidised copper (Cu),atmosphere, manganese they (Mn) are andlargely iron unaffected (Fe). When by these repeated colours heat are treatments.melted in an Colloidal oxidised colouration atmosphere, is they achieved are largely through unaffected precipitation by repeated of very heat small treatments. metallic Colloidalparticles withincolouration the glass, is achieved such as throughthose of gold precipitation (Au), copper of very (Cu) small or silver metallic (Ag). particles These particles within thedevelop glass, when such glassas those of suitable of gold composition, (Au), copper which (Cu) oris melted silver (Ag).under Thesereducing particles conditions, develop is subjected when glass to a ofsecondary suitable orcomposition, extended heat which treatment, is melted which under causes reducing the conditions, metallic particles is subjected to grow to a and secondary produce or colour extended caused heat bytreatment, light absorption which causes of certain the metallic wavelengths particles toby grow the metal and produce nanoparticles. colour caused This secondary by light absorption process of generatingcertain wavelengths colour centres by the is metal called nanoparticles. ‘striking’. The This particle secondary size is process critical, of between generating 5 and colour 60 nm centres (Weyl is [1951]called ‘striking’.1999), as larger The particle crystals size give is critical, rise to between reflection 5 andand 60 scattering, nm (Weyl and 1999 can), as cause larger a crystals muddy give brown rise colourto reflection as well and as scattering,opacity. A and‘good can colour’ cause (red a muddy in the brown case of colour gold and as well copper) as opacity. is achieved A ‘good only colour’ when particles(red in the of case a smaller of gold size and than copper) one‐ isquarter achieved of a only wavelength when particles are obtained, of a smaller and sizethe colour than one-quarter is caused by of absorption,a wavelength not are reflection. obtained, andIf the the particles colour is grow caused considerably by absorption, larger not reflection.than the desirable If the particles range, grow for exampleconsiderably to 200–500 larger than nm, the they desirable scatter range,and reflect for example the light, to 200–500and may nm, cause they the scatter glass and to reflectappear the a considerablylight, and may different cause the hue glass in transmitted to appear a considerablyand reflected different light (Figure hue in 1). transmitted This can result and reflected in a dichroic light effect,(Figure also1). This referred can result to as in Lycurgus a dichroic effect effect, after also its referred most famous to as Lycurgus example, effect the afterLycurgus its most cup famous in the Britishexample, Museum, the Lycurgus which cup appears in the translucent British Museum, pink in which transmitted appears light translucent and opaque pink ingreenish transmitted‐yellow light in reflectedand opaque light greenish-yellow (Freestone et al. in 2007). reflected Stanislav light ( FreestoneLibenský and et al. Jaroslava 2007). Stanislav Brychtová Libensk haveý usedand Jaroslavaa similar typeBrychtov of dichroicá have usedcolour a similarin some type of their of dichroic sculptures: colour safirin in some appears of their blue sculptures: in transmitted safirin and appears brown blue in reflectedin transmitted light (Figure and brown 1). in reflected light (Figure1).

(a) (b)

Figure 1. Stanislav Libenský Libenský and Jaroslava Brychtová, Brychtová, Silhouettes of the Town III, 1989. Photographs by the author. ( a) In transmitted light. ( b) In reflected reﬂected light.

Various differentdifferent colour colour effects effects are labelledare labelled dichroic: dichroic: ﬁrstly, firstly, certain colloidalcertain colloidal colours ascolours described as describedabove. Secondly, above. Secondly, shift colours, shift also colours, called also polychromatic called polychromatic or photochromic or photochromic colours, which colours, change which in changedifferent in types different of light, types such of as light, Bullseye such Rhubarb, as Bullseye Light Rhubarb, Rhubarb, Neo-Lavender,Light Rhubarb, Lavender Neo‐Lavender, Green, LavenderGaffer Rhubarb, Green, Semillon Gaffer Rhubarb, and Sapphire Semillon Rose, and as well Sapphire as many Rose, of Banas’ as well glasses. as many Here, of Banas’ the hue glasses. change Here,is caused the byhue narrow change absorption is caused bandsby narrow produced absorption by lanthanides bands produced (rare earth by oxides)lanthanides interacting (rare earth with oxides)emission interacting spectra of with different emission light sources.spectra of This different phenomenon light sources. is described This phenomenon in detail by Charles is described Bellows in detail by Charles Bellows in his honour thesis (Bellows 2016) (see Supplementary Materials). In the case of neodymium oxide, the hue changes depending on the percentage of colouring agent in the

Arts 2019, 8, 15 4 of 10 inArts his 2019 honour, 8, x FOR thesis PEER (Bellows REVIEW 2016) (see Supplementary Materials). In the case of neodymium4 of oxide, 10 the hue changes depending on the percentage of colouring agent in the glass as well as depending on glass as well as depending on the illumination. Thirdly, glasses with metal deposits, mostly on the the illumination. Thirdly, glasses with metal deposits, mostly on the surface of the glass, which change surface of the glass, which change colour when viewed from different directions. This type is most colour when viewed from different directions. This type is most commonly described as dichroic glass. commonly described as dichroic glass. 2.2. Form of Object 2.2. Form of Object Form of object is intimately connected to light, to optics, and also to colour density of glass. What Form of object is intimately connected to light, to optics, and also to colour density of glass. paths will the light take through a given object? How does this affect its appearance? And, in reverse, What paths will the light take through a given object? How does this affect its appearance? And, in how can a form be constructed that shows the possibilities inherent in transparent coloured glass? reverse, how can a form be constructed that shows the possibilities inherent in transparent coloured Many forms created by glass artists are in fact lenses or prisms, at least in part. Rod lenses and spherical glass? Many forms created by glass artists are in fact lenses or prisms, at least in part. Rod lenses and lenses are used to direct light into a focal area or focal point. For an artist using cylindrical or round spherical lenses are used to direct light into a focal area or focal point. For an artist using cylindrical forms,or round this forms, means this that means when that lit from when the lit curved from the side, curved the light side, willthe light be refracted will be refracted towards towards the middle the of themiddle object, of leaving the object, the leaving circumference the circumference to appear darkerto appear (Figure darker2). (Figure 2).

(a) (b)

FigureFigure 2. 2.Appearance Appearance ofof roundedrounded forms.forms. ( (aa)) Heike Heike Brachlow, Brachlow, Mirrormovement, Mirrormovement, 2006, 2006, cast cast glass, glass, dimensionsdimensions 33.5 33.5× ×16.5 16.5× × 16.516.5; cm; 30.5 30.5 × 16.5× 16.5 × 16.5.× 16.5Photograph cm. Photograph by Sylvain by Deleu, Sylvain courtesy Deleu, courtesy of the artist. of the artist.(b) Bruno (b) Bruno Romanelli, Romanelli, Caliban, Caliban, 2018, 2018, cast glass, cast glass, 24 × 25 24 × 2525 cm.× 25 Photograph cm. Photograph by Andy by AndySmart Smartat AC at ACCooper, Cooper, courtesy courtesy of ofthe the artist. artist.

IfIf the the same same coloured coloured glass glass is usedis used for for a thick a thick and and a thin a thin object, object, the thickthe thick object object will appearwill appear darker. Fordarker. the purpose For the of purpose addressing of addressing colour appearance colour appearance in glass, Iin deﬁne glass, thickness I define thickness as the smallest as the dimensionsmallest ofdimension an object, theof dimensionan object, throughthe dimension which lightthrough penetrates which the light most. penetrates If there arethe variations most. If inthere thickness are withinvariations the same in thickness object, thinner within areasthe same will appearobject, thinner lighter, andareas sometimes will appear differ lighter, in hue. and Differences sometimes in huediffer happen in hue. in Differences some colloidal in hue glasses, happen such in some as Gaffer colloidal orange glasses, red, whichsuch as ranges Gaffer fromorange light red, yellow which at extremeranges thinnessfrom light to yellow deep red, at extreme and in some thinness solution to deep colours red, and containing in some neodymium solution colours oxide, containing such as the shiftneodymium colours described oxide, such above as the (Figure shift colours3). described above (Figure 3). IfIf the the object object has has angled angled planes, planes, like like a a prism, prism, and and thick thick thin thin variations, variations, a a colour colour fade fade and/or and/or shiftshift in huein ishue relatively is relatively easy easy to achieve to achieve unless unless a very a lightvery colourlight colour is used. is used. Often, Often, artists artists employ employ wedge wedge shapes toshapes achieve to a colourachieve fade a colour to almost fade nothing, to almost for nothing, example, for Czech example, artist PavelCzech Trnka artist inPavel his spectrum Trnka in series.his spectrum series. Voids or recesses can also be used to achieve highlighted areas, and are an Voids or recesses can also be used to achieve highlighted areas, and are an important part of Czech important part of Czech masters’ Stanislav Libensky and Jaroslava Brychtova’s vocabulary of form. masters’ Stanislav Libensky and Jaroslava Brychtova’s vocabulary of form. When making such forms, When making such forms, the outcome is difficult to predict, and experience does not always help, the outcome is difﬁcult to predict, and experience does not always help, especially when curves and especially when curves and angles are involved. Libensky and Brychtova’s Green Eye of the angles are involved. Libensky and Brychtova’s Green Eye of the Pyramid is a good example: Libensky Pyramid is a good example: Libensky used to develop forms through drawing, and Brychtova used to develop forms through drawing, and Brychtova would then use the drawing to render the would then use the drawing to render the shape in clay. Figure 4 shows that Libensky’s predictions shapewere in not clay. always Figure accurate:4 shows he that predicted Libensky’s the predictions thickest part were of the not sculpture always accurate: to be the he darkest predicted part, the thickestreaching part from of the the sculpture base all the to beway the to darkest the top part, of the reaching pyramid. from When the cast base in all glass, the way this to proved the top fairly of the pyramid.accurate When for the cast lower in glass,and upper this proved part of fairlythe sculpture, accurate but for the ‘eye,’ lower the and lens upper in the part middle, of the appears sculpture, butbrightly the ‘eye,’ lit, as the if lens from in within. the middle, appears brightly lit, as if from within.

ArtsArts2019 2019, ,8 8,, 15 x FOR PEER REVIEW 5 ofof 1010 Arts 2019, 8, x FOR PEER REVIEW 5 of 10

(a) (b) (a) (b) Figure 3. Bullseye 1844 Lavender Green shift glass showing a change in hue. (a) Heike Brachlow, FigureEquinox 3. I,Bullseye 2011, cast 1844 glass, Lavender stainless Green Green steel, shiftlead, shift glass dimensions showing 70 a× a 42change × 13 cm, in in hue. photograph ( (aa)) Heike Heike by Brachlow,Roger Brachlow, Lee, Equinoxcourtesy I, of 2011, the artist. cast glass,glass, (b) Ashraf stainlessstainless Hanna, steel,steel, lead, Lavenderlead, dimensions dimensions Green Form, 70 70× × 42 cast × 13 13glass, cm, cm, photographdimensions photograph by38 by ×Roger Roger 35 × 9Lee, Lee, cm, courtesyphotograph of the by artist. Ester (Segarra,b)) Ashraf courtesy Hanna,Hanna, of LavenderLavender the artist. GreenGreen Form, Form, cast cast glass, glass, dimensions dimensions 38 38× × 3535 ×× 99 cm, cm, photograph by Ester Segarra, courtesy of the artist.

(a) (b) (c) (a) (b) (c) FigureFigure 4. 4.Prediction Prediction and and reality. reality. Photographs Photographs by by the the author. author. (a) ( Stanislava) Stanislav Libensky, Libensky, design design drawing drawing for FigureGreenfor Green Eye 4. Prediction ofEye the of Pyramid. the and Pyramid. reality. (b) Stanislav Photographs(b) Stanislav Libensky byLibensky andthe author. Jaroslava and (Jaroslavaa) Brychtova, Stanislav Brychtova, Libensky, Green Eye Green design of the Eye Pyramid,drawing of the 1993–2009,forPyramid, Green 1993–2009,Eye cast of glass. the (castPyramid.c) Stanislav glass. ((bc) Libensky Stanislav and Libensky Jaroslava andand Brychtova, JaroslavaJaroslava Brychtova, sideBrychtova, view of sideGreen Green view Eye Eye of of ofGreen the Pyramid,Eye of the 1993–2009, Pyramid, cast1993–2009,cast glass.glass. (castc) Stanislav glass. Libensky and Jaroslava Brychtova, side view of Green Eye of the Pyramid, 1993–2009, cast glass. 2.3.2.3. SurfaceSurface Finish Finish of of Glass Glass 2.3. Surface Finish of Glass DifferentDifferent surface surface finishes finishes also also have have a considerable a considerable impact impact on the on appearance the appearance of an object.of an object. A matte A surfacematteDifferent surface diffuses surfacediffuses the light finishes the and light the also objectand have the appears objecta considerable translucent appears translucentimpact rather thanon therather transparent appearance than transparent (Figure of an5a). object. Rough(Figure A matte5a). Rough surfacessurface matte diffuses obscure surfaces thethe viewlight obscure intoand the thethe internal viewobject into appears space the moreinternal translucent than space fine rather mattemore surfaces,thanthan transparentfine and matte may surfaces,(Figure appear 5a).grainyand Rough may white, appear matte unless grainysurfaces treated white, obscure with unless a the sealant treatedview or into with mechanically the a internalsealant polished, orspace mechanically more for than example polished, fine brush-polishedmatte for surfaces, example withandbrush may pumice‐polished appear (Figure with grainy5 c).pumice Awhite, polished (Figure unless surface 5c). treated A polished allows with more a surface sealant light allows or to mechanically enter more the light object, polished,to andenter the the for inner object, example form and brushtothe become inner‐polished form visible, towith become but pumice the visible, polished (Figure but surface 5c). the A polished polished reflects surface light surface directly reflects allows rather lightmore directly than light diffusely, to ratherenter the than which object, diffusely, means and thereflectionswhich inner means form become reflectionsto become visible becomevisible, on the surfacebutvisible the onpolished (Figure the surface5b). surface When (Figure reflects one 5b). surface light When directly is polishedone surface rather and isthan the polished diffusely, opposite and whichthe opposite means surfacereflections matte become and the visible object on is the lit throughsurface (Figure the polished 5b). When surface one and surface viewed is polished on the matte and theside, opposite it will appear surface lighter matte andthan the the object same isobject lit through with all the matte polished surfaces. surface and viewed on the matte side, it will appear lighter than the same object with all matte surfaces.

Arts 2019, 8, 15 6 of 10 surface matte and the object is lit through the polished surface and viewed on the matte side, it will Arts 2019, 8, x FOR PEER REVIEW 6 of 10 appear lighter than the same object with all matte surfaces. Arts 2019, 8, x FOR PEER REVIEW 6 of 10

(a) (b) (c) Figure 5.( Surfacea) finish of glass. (a) Heike Brachlow,(b) Apophasis 2016, cast glass, fine( cmatte) surface, Figure30Figure × 31 5. ×5. Surface 27Surface cm, photograph finish finish of of glass. glass. by Ester ((aa)) HeikeHeike Segarra, Brachlow,Brachlow, courtesy Apophasis Apophasis of the artist. 2016, 2016, (b cast cast) Heike glass,glass, Brachlow, finefine matte matte Nimbus, surface,surface, 302018,30× × 3131cast ×× glass,27 cm,cm, one photographphotograph surface polished, byby EsterEster 36 Segarra,Segarra, × 38 × 31 courtesycourtesy cm, photograph ofof thethe artist.artist. by Ester(b)) HeikeHeike Segarra, Brachlow,Brachlow, courtesy Nimbus,Nimbus, of the 2018,artist.2018, cast (castc) Richard glass, glass, one one Whiteley, surface surface polished,Light polished, cone, 36 36 2013,× ×38 38 cast× × 31 glass, cm,cm, photographphotograph 33.5 × 33.5 × byby 13 EsterEster cm, Segarra,roughSegarra, sawn courtesycourtesy and sealed of of the the artist.surface.artist. ( (c c)Photograph) Richard Richard Whiteley, Whiteley, by Greg Light LightPiper, cone, cone, courtesy 2013, 2013, of cast cast the glass, artist.glass, 33.5 33.5× ×33.5 33.5× × 1313 cm,cm, roughrough sawn sawn and and sealed sealed surface.surface. Photograph Photograph by by Greg Greg Piper, Piper, courtesy courtesy of of the the artist. artist. 2.4. Light 2.4. Light 2.4. LightThe wavelength of ambient light has an impact on nearly all transparent colours, with differencesTheThe wavelength wavelength in hue ranging of of ambient ambient from light almost light has an hasimperceptible impact an impact on nearly in on the all nearly transparentcase ofall sometransparent colours, blues with and colours, differences greens with to inconsiderabledifferences hue ranging in in from huethe case almostranging of shift imperceptible from colours. almost It in imperceptibleshould the case be of noted some in thatthe blues thecase and angle of greens some and to intensityblues considerable and of greens ambient in the to caselightconsiderable ofcreate shift colours.different in the case It effects. should of shift beWhen colours. noted exhibiting that It should the anglean beobject, andnoted intensity lighting that the of optionsangle ambient and should light intensity create be takenof different ambient into effects.account,light create When however different exhibiting these effects. considerations an object, When lighting exhibitingare made options afteran object, shouldthe object lighting be is taken finished options into and account, should do not however normallybe taken these feedinto considerationsintoaccount, colour however decisions, are these made except considerations after in the the object case are of is polychromatic finishedmade after and the do glasses,object not normally is thatfinished is, feedglasses and into do that not colour change normally decisions, in huefeed exceptininto different colour in the typesdecisions, case of of light polychromatic except (Figure in the 6). caseIn glasses, order of polychromatic to that exhibit is, glasses this glasses,hue that change, changethat is,considering glasses in hue that in lighting different change options in types hue ofisin paramount. lightdifferent (Figure types If6 ).two of In light orderdifferent (Figure to exhibit light 6). sources,In this order hue tofor change, exhibit example this considering fluorescent hue change, lighting and considering incandescent, options lighting is paramount. illuminate options Ifthisis two paramount. glass, different both If lightcolour two sources, differentvariations for light exampleare sources,visually fluorescent forpresent example andat the incandescent,fluorescent same time. and However, illuminate incandescent, the this cause glass, illuminate of both the colourdifferentthis glass, variations hues both is colournot are necessarily visually variations present obvious are atvisually the to samethe present viewer. time. at However,I thehave same exhibited the time. cause However,polychromatic of the different the cause sculptures hues of the is notwithdifferent necessarily the lighting hues is obvious concealednot necessarily to the in viewer.the obvious plinth, I have switchingto the exhibited viewer. automatically polychromatic I have exhibited from sculptures fluorescent polychromatic with to incandescent the sculptures lighting concealedandwith back. the lightingViewers in the plinth, concealed were switching convinced in the automatically plinth,that the switching sculpture from automaticallywas fluorescent lit with to different from incandescent fluorescent coloured and lights.to back. incandescent The Viewers best werewayand toback. convinced convey Viewers the that colourwere the sculpture convinced behaviour was that of lit such the with sculpture glasses different is was with coloured lit both with lights.light different sources The coloured best visible, way lights. to and convey a The switch best the colourthatway can to behaviour conveybe operated the of colour such by viewers. glasses behaviour isHowever, with of bothsuch this lightglasses is more sources is within line visible, both with andlight science a sources switch demonstrations thatvisible, can and be operated thana switch art byexhibitions.that viewers. can be However,operated by this viewers. is more However, in line with this science is more demonstrations in line with science than art demonstrations exhibitions. than art exhibitions.

Figure 6.6. Heike Brachlow, Theme and Variations I,I, 2009.2009. Line blend, Bullseye glass doped with neodymium,Figure 6. Heike cerium, Brachlow, and titanium Theme oxides and Variations in the ratio I, 2:1:1. 2009. Photographed Line blend, Bullseye in incandescent glass doped (top) andwith ﬂuorescentfluorescentneodymium, light cerium, by Ester and Segarra, titanium courtesycourtesy oxides ofinof thethe artist.artist.ratio 2:1:1. Photographed in incandescent (top) and fluorescent light by Ester Segarra, courtesy of the artist.

Arts 2019, 8, 15 7 of 10

Arts 2019, 8, x FOR PEER REVIEW 7 of 10 2.5. Optics 2.5.A studyOptics of optics in order to gain an understanding of light paths through cubes, lenses, prisms, etc. mayA be study beneﬁcial of optics for in artists order wanting to gain an to understanding explore transparent of light colour paths through in glass, cubes, or indeed lenses, form prisms, in clear glass.etc. There may be are beneficial many manuscripts for artists wanting on basic to explore optics, fortransparent example colour Leno in Pedrotti’s glass, or Basicindeed Geometrical form in Opticsclear (Pedrotti glass. There and Roychoudhuri are many manuscripts 2008) (see on Supplementary basic optics, for Materials). example MyLeno own Pedrotti’s interest Basic in optics beganGeometrical with the desireOptics to (Pedrotti solve colour and Roychoudhuri mysteries such [2000] as 2008) the one (see posed Supplementary by Frantisec Materials). Vizner’s My Bowl own with point,interest on permanent in optics began exhibition with the in thedesire glass to solve collection colour of mysteries the Victoria such and as the Albert one posed Museum. by Frantisec Why is the pointVizner’s dark, evenBowl thoughwith point, it is on quite permanent thin? The exhibition study ofin the optics glass provides collection the of solution:the Victoria due and to Albert its shape, Museum. Why is the point dark, even though is is quite thin? The study of optics provides the all light entering the point is refracted down into the base (Figure7). While I do not actually trace light solution: due to its shape, all light entering the point is refracted down into the base (Figure 7). While paths when creating a new form, this developing understanding of optics undoubtedly feeds into my I do not actually trace light paths when creating a new form, this developing understanding of optics designundoubtedly process. feeds into my design process.

FigureFigure 7. Frantisec 7. Frantisec Vizner, Vizner, Bowl, cutBowl, and cut sandblasted. and sandblasted. 1984, with 1984, approximated with approximated light paths light superimposed. paths Photographssuperimposed. by the Photographs author. by the author.

3. Working3. Working with with Colour Colour and and Form Form ThereThere are are two two basic basic options options when when working working with coloured coloured glass: glass: choosing choosing colour colour for fora specific a specific formform or making or making a form a form for afor predetermined a predetermined colour. colour. The The starting starting point point for for most most artists artists is is form, form, and and once the formonce the is completed, form is completed, a colour a is colour chosen. is Glasschosen. blowers Glass blowers may start may with start colour with (orcolour at least, (or at colour least, and colour and shape may be more intertwined), as when working with rod, the physical starting point shape may be more intertwined), as when working with rod, the physical starting point is the colour. is the colour. But most likely, they have a shape in mind when choosing their colour. In kiln casting, But most likely, they have a shape in mind when choosing their colour. In kiln casting, the necessity to the necessity to choose colour occurs later in the process, after determining form (making or choosechoosing colour a model) occurs and later making in the a process, mould. after determining form (making or choosing a model) and making aTo mould. understand how to approach the question of achieving a desired colour result, one needs to defineTo understand the desired how result. to approachAt the very the beginning question of of my achieving research into a desired volume colour colour, result, my primary one needs to definefocus was the desiredon obtaining result. coloured At the glass very that beginning is light enough of my to research achieve intoan inner volume glow colour,in artificial my light, primary focusspecifically was on obtaining exhibition coloured lighting, glassfor a thatcylindrical is light form enough with to a achievediameter an of inner15 cm. glow Here, in the artificial desired light, specificallyresult can exhibition be defined lighting, as “very forlight, a cylindricalproducing an form inner with glow”. a diameter When I first of 15came cm. across Here, this the issue, desired resultBullseye can be Glass defined did not as “very have many light, casting producing colours an that inner were glow”. suitable When for Isuch first thick came forms, across but this they issue, Bullseyebecame Glass aware did of not this haveissue manyand have casting since coloursexpanded that their were range suitable to include for lighter such thick colours. forms, While but all they becameglass aware manufacturers of this issue provide and colour have sincecharts, expanded in form of their catalogues, range toposters include and lighter on their colours. websites, While the all difficulties of accurately reproducing colour, especially transparent colour, are well known. Such glass manufacturers provide colour charts, in form of catalogues, posters and on their websites, colour charts can give a general idea, but to make visual colour judgements prior to casting a form, the difficulties of accurately reproducing colour, especially transparent colour, are well known. fairly large samples of coloured glass are needed. Both Banas and Bullseye billets are of a good size Suchand colour shape, charts but Gaffer can give billets a general are less idea,useful but due to to make their compact visual colour and rounded judgements shape, prior which to makes casting a form,them fairly harder large to samples cut and of harder coloured to evaluate, glass are as needed. curved surfaces Both Banas enter and the Bullseye equation. billets Using are Bullseye of a good sizebillets and shape, as an example, but Gaffer each billets billet are measures less useful approximately due to their 127 compact × 254 × and19 mm. rounded To work shape, out if which a colour makes themis hardersuitable, to a cut billet and can harder be cut to so evaluate, one of its as dimensions curved surfaces matches enter the thethickness equation. of the Using proposed Bullseye billetssculpture, as an example, and the light each on billet two or measures four sides approximately of the sample can 127 be× blocked254 × 19 out mm. (depending To work on outthe if a colour is suitable, a billet can be cut so one of its dimensions matches the thickness of the proposed sculpture, and the light on two or four sides of the sample can be blocked out (depending on the Arts 2019, 8, 15 8 of 10

Arts 2019, 8, x FOR PEER REVIEW 8 of 10 dimensions of the sample in relation to the dimensions of the proposed sculpture), leaving a channel dimensions of the sample in relation to the dimensions of the proposed sculpture), leaving a channel of the desired length to emulate a larger piece of glass. Now a visual colour density judgement can of the desired length to emulate a larger piece of glass. Now a visual colour density judgement can be made by looking through the channel, ideally in different light conditions. This could be daylight be made by looking through the channel, ideally in different light conditions. This could be daylight and various different types and intensities of electric light. If the sculpture is thicker than a billet’s and various different types and intensities of electric light. If the sculpture is thicker than a billet’s length, two billets could be glued together or attached with museum gel or similar, in order to reach length, two billets could be glued together or attached with museum gel or similar, in order to reach the desired length. Superimposing—stacking up—chunks of glass does not work in the same way, the desired length. Superimposing—stacking up—chunks of glass does not work in the same way, as light is reﬂected from each surface, making the overall appearance lighter. Glue, museum gel or as light is reflected from each surface, making the overall appearance lighter. Glue, museum gel or even water, all of which have a refractive index greater than air, can, when forming an unbroken layer even water, all of which have a refractive index greater than air, can, when forming an unbroken in between pieces of glass, help simulate one larger piece. Generally, colour samples produced by layer in between pieces of glass, help simulate one larger piece. Generally, colour samples produced glass manufacturers are of limited use, because they are too small to give an idea of colour appearance by glass manufacturers are of limited use, because they are too small to give an idea of colour for larger sculptures. With Banas or Bullseye glass, a billet could be cut diagonally to make a wedge, appearance for larger sculptures. With Banas or Bullseye glass, a billet could be cut diagonally to for recurrent use as a colour sample. For each colour judgement, the area of roughly the required make a wedge, for recurrent use as a colour sample. For each colour judgement, the area of roughly length could be masked off; otherwise it is difﬁcult to focus in when looking at a sliding scale, which is the required length could be masked off; otherwise it is difficult to focus in when looking at a sliding what a wedge sample represents. scale, which is what a wedge sample represents. When evaluating the colour samples I created as part of my PhD research (Brachlow 2012), the area When evaluating the colour samples I created as part of my PhD research (Brachlow 2012), the which holds the most interest for me is where the difference in value between thick and thin sections area which holds the most interest for me is where the difference in value between thick and thin is the greatest, where the colour result hovers around a tipping point or density threshold (Figure8, sections is the greatest, where the colour result hovers around a tipping point or density threshold highlighted areas). (Figure 8, highlighted areas).

(a) (b)

FigureFigure 8. 8.Heike Heike Brachlow, Brachlow Line, Line blends, blends, cast cast glass, glass, with with tipping tipping point point highlighted. highlighted. Photographs Photographs by by EsterEster Segarra, Segarra, courtesy courtesy of the of artist. the artist. (a) Cobalt (a) oxide:Cobalt 0.00025–0.01%; oxide: 0.00025 copper–0.01%; oxide: copper 0.01–2%; oxide: chromium 0.01–2%;

oxide:chromium 0.01–0.5%. oxide: (b ) 0.01 Nickel–0.5 oxide:%. (b) 0.0025–0.1%;Nickel oxide: red 0.0025 iron oxide–0.1%; Fe 2 redO3 : iron0.1–3%; oxide and Fe neodymium2O3: 0.1–3% oxide; and (3neodymium cm): 0.25–15%. oxide (3 cm): 0.25–15%.

TheThe tipping tipping point point can can only only be be reached reached in in glasses glasses with with certain certain colour colour saturation. saturation. If If the the colour colour is is tootoo light, light, it very it very gradually gradually darkens darkens as the as thickness the thickness increases. increases. Witness Witness Roni Horn’s Roni Horn’s very large, very lightly large, colouredlightly coloured glass cylinders glass cylinders and cubes—glass and cubes with—glass such with little such colour little density colour will density never will reach never a tipping reach a point.tipping Also, point. certain Also, colours certain do not colours exhibit do a defined not exhibit density a threshold, defined fordensity example threshold, Neodymium for examp purplele (FigureNeodymium8b). During purple the last (Figure decade, 8b). my During desired the results last decade, have been my centred desired around results this have tipping been point centred or densityaround threshold, this tipping and point could or be density defined threshold,as ‘exhibiting and considerable could be defined shifts in as colour ‘exhibiting value and/orconsiderable hue betweenshifts in thickcolour and value thin and/or areas’. hue When between working thick within and thin this areas’. territory, When objects working will havewithin light this and territory, dark areas,objects as ifwill the have artist light had employed and dark differentareas, as coloured if the artist glasses had ranging employed from different dark to lightcoloured within glasses the sameranging hue, from or differing dark to light in hue within as well the as same in value, hue, or in differing case of shift in hue colours as well and as somein value, reds in and case greens. of shift Tocolours explore and this some tipping reds point, and greens. a specific To type explore of form this istipping needed, point, a form a specific with variations type of form in the is thickness needed, a ofform the glass.with variations Depending in onthethe thickness type of of coloured the glass. glass, Depending these variations on the type might of coloured be as little glass, as a these few millimetresvariations tomight much be larger.as little Much as a few of Libensky millimetres and to Brychtova’s much larger. work Much is hovering of Libensky around and this Brychtova’s tipping point,work which is hovering is likely around part of this the secrettipping to point their, success. which is Good likely contemporary part of the secret examples to their are ceramicsuccess. artistGood Ashrafcontemporary Hanna’s glassexamples sculptures. are ceramic When artist he decided Ashraf to Hanna’s make works glass in sculptures. glass, his forms When were he decided designed to tomake explore works colour in glass, density his in forms various we differentre designed ways to withinexplore the colour same density work: the in basicvarious form different is a wedge, ways taperingwithin the from same thick work: to thin. the Out basic of the form wedge is a protrudeswedge, tapering the anecdotal from thick body to of thin. a vessel, Out in of the the middle wedge protrudes the anecdotal body of a vessel, in the middle of which, through exchange of positive and negative shape, a hemispherical cavity tapers from fairly thick around the sides to almost nothing in

Arts 2019, 8, 15 9 of 10 of which,Arts 2019 through, 8, x FOR exchange PEER REVIEW of positive and negative shape, a hemispherical cavity tapers from9 of 10 fairly thick around the sides to almost nothing in the middle. The back surface is polished, which means Artsthe 2019 middle., 8, x FOR The PEER back REVIEW surface is polished, which means more light will enter the form, especially9 of 10 morewhen light lit will from enter behind the form,(Figure especially 9b). when lit from behind (Figure9b). the middle. The back surface is polished, which means more light will enter the form, especially when lit from behind (Figure 9b).

(a) (b)

FigureFigure 9. Ashraf 9. Ashraf Hanna. Hanna. PhotographsPhotographs by by Ester Ester Segarra, Segarra, courtesy courtesy of the of artist. the ( artist.a) Ripple (a Series,) Ripple Blue Series, (a) (b) BlueBowl Bowl Form, Form, cast cast glass, glass, dimensions dimensions 11 × 33 11 × 31× cm.33 (×b) 31Blue cm. Vessel (b) Form, Blue Vessel2014, cast Form, glass, 2014, dimensions cast glass, dimensionsFigure32 × 48 9. × 32 Ashraf10× cm.48 Hanna.× 10 cm. Photographs by Ester Segarra, courtesy of the artist. (a) Ripple Series, Blue Bowl Form, cast glass, dimensions 11 × 33 × 31 cm. (b) Blue Vessel Form, 2014, cast glass, dimensions With32With little × 48 ×little experience 10 cm. experience in evaluating in evaluating volume volume colour, colour, having having worked worked with with glass glass for for only only aa shortshort time, Hannatime, both Hanna sought both advice sought and advice applied and applied the insights the insights gained gained from from his previous his previous sculptures, sculptures, the the Ripple Ripple Series (Figure 9a), low bowl shapes sitting on a surface, which prevents the light from Series (FigureWith little9a), lowexperience bowl shapes in evaluating sitting volume on a surface, colour, whichhaving preventsworked with the lightglass fromfor only transmitting a short transmitting through the 11 centimetre thickness of the glass. The 33 and 31 centimetre length and throughtime, theHanna 11 centimetre both sought thickness advice and of applied the glass. the The insights 33 and gained 31 centimetre from his previous length andsculptures, width the are too Ripplewidth Seriesare too (Figure thick in 9a), relation low tobowl the shapescolour densitysitting onof thea surface, glass for which much preventslight to transmit. the light These from thick in relation to the colour density of the glass for much light to transmit. These sculptures appear transmittingsculptures appear through much the 11lighter centimetre in an upright thickness position, of the glass.but their The design 33 and does 31 centimetre not allow forlength such and a much lighter in an upright position, but their design does not allow for such a display, therefore Hanna widthdisplay, are therefore too thick Hanna in relation changed to the the colourorientation density in his of subsequentthe glass for forms. much light to transmit. These changedsculptures theWhen orientation appear choosing much coloured in hislighter subsequent glass in an for upright such forms. a form,position, the samebut their method design as for does uniformly not allow thick for shapes such a display,Whenapplies: choosingtherefore one or more Hanna coloured billet(s) changed glass can thebe for cutorientation such into a form,several in his the pieces, subsequent same with method each forms. one as for matching, uniformly in one thick of its shapes applies:dimensions,When one or choosing one more of the billet(s)coloured various canglass thicknesses be for cut such into of a theform, several proposed the same pieces, sculpture, method with asthen each for the uniformly one light matching, on thicktwo or shapes in four one of its dimensions,applies:sides of one each or one sample more of the billet(s)can various be blocked can thicknessesbe outcut tointo emulate several of the a largerpieces, proposed piece with sculpture,of each glass one and matching, then visual the colour in light one density on of twoits or fourdimensions,judgements sides of each onecan sample ofbe themade, various can taking be thicknesses blockedinto account out of thethe to rounded emulateproposed shape asculpture, larger of the piece protrudingthen ofthe glass light vessel andon twobody. visual or This four colour method is not possible with strikers, that is, glass which develops its colour during the firing. With densitysides judgements of each sample can can be be made, blocked taking out to into emulate account a larger the roundedpiece of glass shape and of visual the protruding colour density vessel Bullseye red strikers, the billet needs to be pre‐fired to develop its colour before such an experiment body.judgements This method can be is made, not possible taking into with account strikers, the thatrounded is, glass shape which of the developsprotruding its vessel colour body. during This the is possible. With some other strikers, for example opalines, pre‐evaluating the colour is not possible method is not possible with strikers, that is, glass which develops its colour during the firing. With firing.at Withall as Bullseyethe final appearance red strikers, depends the billet on the needs exact to firing be pre-fired cycle. Red to strikers develop are its often colour very before dark and such an Bullseye red strikers, the billet needs to be pre‐fired to develop its colour before such an experiment experimentintense colours, is possible. which With works some very other well for strikers, small and for thin example forms opalines,and can be pre-evaluatingdramatically effective the colour for is is possible. With some other strikers, for example opalines, pre‐evaluating the colour is not possible not possibleforms with at all considerable as the final thick appearance thin variations depends (Figure on the10). exact firing cycle. Red strikers are often very at all as the final appearance depends on the exact firing cycle. Red strikers are often very dark and dark and intense colours, which works very well for small and thin forms and can be dramatically intense colours, which works very well for small and thin forms and can be dramatically effective for effectiveforms for with forms considerable with considerable thick thin variations thick thin (Figure variations 10). (Figure 10).

(a) (b)

Figure 10. Bullseye 1931‐65 Ruby Pink Striker. Photographs by Ester Segarra, courtesy of the artist. (a) Heike Brachlow, Perihelion 2018, cast glass, 39 × 55 × 43 cm. (b) Heike Brachlow, Samoon 2018, cast (a) (b) glass, 34.5 × 35 × 32 cm. Figure Figure 10. 10.Bullseye Bullseye 1931-65 1931‐65 Ruby Ruby PinkPink Striker. Striker. Photographs Photographs by byEster Ester Segarra, Segarra, courtesy courtesy of the ofartist. the ( artist.a) (a) HeikeHeike Brachlow,Brachlow, Perihelion Perihelion 2018, 2018, cast cast glass, glass, 39 × 39 55× × 5543 ×cm.43 (b cm.) Heike (b) HeikeBrachlow, Brachlow, Samoon Samoon 2018, cast 2018, castglass, glass, 34.5 34.5 × ×35 35× 32× cm.32 cm.

Arts 2019, 8, 15 10 of 10

4. Concluding Remarks Evaluating and using coloured glass in such a fashion can compensate for an artist’s lack of experience in the use of volume colour, and ensure better control over colour in transparent solid objects. While the methods described above may give a rough idea of colour value, there are many considerations that determine the appearance of a given colour in a given shape. What path does the light take through the shape? Is the shape rounded, will it appear darker in its circumference? What is the proposed surface ﬁnish? Thinking about glass sculpture in terms of glass colouration, form of object, surface ﬁnish, impact of light, optical principles, etc. may promote a better understanding of the relationship between colour, form and light and open up new and exciting creative possibilities. Working in the density threshold or tipping point area in coloured glass remains my personal challenge. How does one design a form which takes full advantage of the possibilities offered by the density threshold? Ashraf Hanna’s work is particularly successful in using this phenomenon, utilising spherical protrusions which trap the light, set within larger wedge shapes. Libenský and Brychtová’s forms use sharp angles, geometric apertures and very thin sections set within a larger, thicker shape. In my own current work, curves taper from thick to thin, dark to light, in a more subtle approach. There are many possibilities within the extensive range of variables provided by form, texture, colour and light still to be explored.

Supplementary Materials: The supplementary materials are available to download here: http://spie.org/ publications/fundamentals-of-photonics-modules, https://aura.alfred.edu/handle/10829/7232. Funding: This research received no external funding. Conﬂicts of Interest: The author declares no conﬂict of interest.

References

Albers, Josef. 2006. Interaction of Color. New Haven and London: Yale University Press, pp. 45–46. ISBN 978-0-300-11595-6. First published 1963. Bamford, C. R. 1977. Colour Control and Generation in Glass. Amsterdam: Elsevier Scientiﬁc Publishing Company, vol. 2, pp. 1–2. ISBN 0-444-41614-5. Bellows, Charles H. 2016. Photochromism in Rare Earth Oxide Glasses. Bachelor’s Thesis, Alfred University, Alfred, NY, USA; pp. 12–18. Brachlow, Heike. 2012. Shaping Colour: Density, Light and Form in Solid Glass Sculpture. Ph.D. Thesis, Royal College of Art, London, UK. Freestone, Ian, Nigel Meeks, Margaret Sax, and Catherine Higgitt. 2007. The Lycurgus Cup—A Roman Nanotechnology. Gold Bulletin. The Journal of Gold Technology, Science and Applications. 40: 270–77. [CrossRef] Pedrotti, Leno S., and Chandrasekhar Roychoudhuri, eds. 2008. Basic Geometrical Optics. In Fundamentals of Photonics. Connecticut: University of Connecticut, Chapter 3. ISBN 9781628412710. First published 2000. Stewart, Max. 2010. The Sense of My Screaming Skin. An investigation into the colouring process of Amalric Walter (1870–1959) using metallic salts in pâtes-de-verre. Ph.D. Thesis, University of Edinburgh, Edinburgh, UK. Vandenhoucke, Sylvie. 2003. Glass Towards and Inner Space: On Introducing Metallic Oxides in pate de verre. M. Phil Thesis, Royal College of Art, London, UK. Weyl, Woldemar A. 1999. Coloured Glasses. Shefﬁeld: Society of Glass Technology, ISBN 0-900682-06-X. First published 1951.

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