Vpliv pH na stabilnost izbranih barvil in pigmentov

Povh, Matija (avtor), Kralj Cigić, Irena (mentor)

Razkislinjenje je pomemben in pogosto uporabljen konservatorski postopek za upočasnitev razgradnje papirja. Bistvo postopka je zvišanje pH vrednosti papirja iz kislega v šibko alkalno območje in s tem upočasnitev razgradnje papirja zaradi kislinske hidrolize celuloze. To lahko dosežemo z uporabo različnih sredstev za razkislinjenje. V restavratorskih delavnicah se pogosto uporabljajo vodne raztopine/suspenzije hidroksidov, karbonatov in hidrogenkarbonatov zemeljsko alkalijskih kovin. Če so na papirju prisotna vodotopna črnila, pigmenti ali barvila, se za zvišanje pH vrednosti papirja uporabljajo brezvodna sredstva za razkislinjenje. Pri postopku razkislinjenja moramo biti pozorni na stabilnost prisotnih barvil in pigmentov, ki lahko ob prehodu iz kislega v alkalno območje spremenijo barvo. Namen moje diplomske naloge je bilo ugotoviti, kakšen je vpliv različnih sredstev za razkislinjenje na stabilnost nekaterih barvil in pigmentov. Najprej sem preveril barvne spremembe v odvisnosti od pH različnih raztopin/suspenzij barvil in pigmentov in iz nadaljnjega dela izločil tista barvila in pigmente, ki niso spremenili barve v alkalnem območju. Nato sem pripravil vzorce barvil in pigmentov na papirju, jih obdelal z različnimi vodnimi in brezvodnimi sredstvi za razkislinjenje ter izmeril spremembo barve na spektrofotometru. Po analizi rezultatov sem ugotovil, da je do največjih sprememb v barvi prišlo pri obdelavi z vodno raztopino kalcijevega hidroksida, sledila je vodna raztopina kalcijevega hidrogenkarbonata, pri obdelavi s suspenzijo Bookkeeper pa do sprememb skoraj ni prišlo. Najverjetnejši vzrok za to je vnos alkalije v nepolarnem mediju, saj alkalija (MgO) težje prodre v papir z barvilom oziroma pigmentom. Razkislinjenje sem ponovil in vzorcem ponovno izmeril barvo po termično pospešeni razgradnji pri 80 °C in 65 % relativni vlažnosti za en teden. Ugotovil sem, da je po termično pospešeni razgradnji večji delež MgO v papirju, saj sem izmeril večje spremembe barve pri obdelavi s suspenzijo Bookkeeper. Ugotovil sem, da so organska barvila in pigmenti manj stabilna od anorganskih, čeprav tudi nekatera anorganska spremenijo barvo po razkislinjenju. Za najmanj stabilno barvilo se je izkazal hematoksilin, sledi kurkuma ter gamboško rumeno, medtem ko pri lakmusu, alizarinu, zelenemu kromovem oksidu in kadmijevi rumeni do barvne spremembe sploh ni prišlo. Med anorganskimi barvili, ki sem jih testiral, je do spremembe v barvi prišlo pri prusko modrem, do spremembe barve pri Van Dyck rjavem pa je verjetno prišlo zgolj zaradi spiranja barve med vodnimi postopki. Vodna raztopina kalcijevega hidroksida se je izkazala kot najbolj agresivno sredstvo za razkislinjenje in posledično najmanj primerno za razkislinjenje raznih zapisov, ročno obarvanih del, risb in podobnih gradiv. Bolj primerni sta vodna raztopina kalcijevega hidrogenkarbonata in suspenzija Bookkeeper, pri čemer je prednost slednje tudi to, da ne spira barve s površine papirja. Vendar pa moramo biti pri uporabi suspenzije Bookkeeper pozorni na to, da je sam postopek razkislinjenja manj učinkovit v primerjavi z vodnimi, saj alkalija bistveno počasneje prehaja v notranjost papirja.

• Jezik: Slovenski jezik
• Ključne besede: pH, razkislinjenje, sprememba barve, barvila, pigmenti.
• Vrsta gradiva: Diplomsko delo/naloga
• Tipologija: 2.11 - Diplomsko delo
• Organizacija: FKKT - Fakulteta za kemijo in kemijsko tehnologijo
• Leto izida: 2020
• PID: 20.500.12556/RUL-118207
• COBISS.SI-ID: 26534403
• Datum objave v RUL: 26.08.2020

Sekundarni jezik

• Jezik: Angleški jezik
• Naslov: Influence of pH on the stability of selected dyes and pigments

Izvleček

Deacidification is an important and frequently used conservation procedure to decrease paper degradation. The aim of deacidification is to increase the pH value of paper from acidic to mildly alkaline values in order to slow down the degradation of paper caused by acidic hydrolysis. This can be achieved by the use of various deacidification agents. In conservation workshops, aqueous solutions/suspensions of hydroxides, carbonates and hydrogen carbonates of alkaline earth metals are frequently used. In the case if inks, dyes and pigments in paper are soluble in water, non-aqueous suspensions can be used instead. When increasing pH value, the stability of dyes and pigments is important, as they can be susceptible to colour changes during deacidification. The aim of my work was to assess the effect of various deacidification agents on the colour stability of some dyes and pigments. I have tested the colour changes of some dyes and pigments in solutions/suspensions first and excluded all dyes and pigments where colour changes were not noticed due to increase of pH value from future experiments. Afterwards I have prepared colour samples, treated them with various deacidification agents and measured colour changes using a spectrophotometer. After analysis of results I have found out that biggest colour changes were observed after treatment with water solution of calcium hydroxide, smaller changes were observed when using water solution of calcium hydrogen-carbonate while after treatment with the Bookkeeper suspension hardly any changes were noticed. The most probable cause for that is that the alkali is presented in nonpolar media. The alkali (MgO) cannot penetrate into the paper and therefore deacidification is incomplete. I have repeated the deacidification procedure and measured colour changes once again after accelerated aging conditions at 80 °C and 65 % relative humidity for one week. I have found out that after the accelerated aging conditions most probably more MgO has been in the paper, as the colour changes after treatment with the Bookkeeper suspension were higher in comparison to unaged samples. Organic dyes and pigment are less stable compared to inorganic dyes and pigments, although some inorganic dyes and pigments change colour after deacidification as well. Logwood extract proved to be the least stable dye, followed by turmeric and gamboge, while no colour changes were observed in litmus, alizarin crimson light, chromium oxide brilliant green and cadmium yellow. Water solution of calcium hydroxide was the most aggressive deacidification agent and is therefore the least appropriate for deacidification of different manuscripts, coloured works of art, drawings and similar materials. Water solution of calcium hydrogen carbonate and the Bookkeeper suspension induced less colour changes. Another advantage of the Bookkeeper suspension is that it does not wash colours from the surface of the paper. However, when using the Bookkeeper suspension (or any other non-aqueous deacidification agents), we have to take into account that it is less effective deacidification agent in comparison to aqueous ones.

• Ključne besede: pH, deacidification, colour change, dyes, pigments.