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San Giovanni protettore dei bestemmiatori freschisti

San Giovanni protettore dei bestemmiatori freschisti

Il Carbonato di calcio, per chi si intende di chimica è il sale di calcio dell’acido carbonico e il maggior componente del calcare, nei trattati d’arte del XV secolo definito come “Bianco di San Giovanni”: <…Questo bianco si tria con acqua, e vuole essere bene macinato. E’ buono da lavorare in fresco, cioè in muro,, senza tempera; e senza questo non puoi fare niente, come l’incarnazione ed altri mescolamenti degli altri colori che si fa in muro, cioè in fresco; e mai non vuole tempera nessuna.>

Noi abbiamo avuto modo di sperimentarlo, applicarlo sul fresco e alterarlo con altri pigmenti, ma consideriamo molto sconveniente il suo uso, nella pittura in generale e soprattutto nella tecnica a fresco.
E’ un pigmento problematico e dispotico, e crediamo che San Giovanni oltre ad essere uno pseudonimo affascinante, sia anche il santo designato a proteggere tutti coloro che dopo “giornate” estenuanti, scoprono l’opera completamente mutata nella post-carbonatazione, causata da tale pigmento.

Dobbiamo anche dire che inizialmente gode di una buona stabilità, è abbastanza luminoso e non tossico e/o velenoso. Le sorprese però spuntano  il giorno seguente alla sua applicazione. Nella tecnica a fresco entra in carbonatazione modificando molto rapidamente il suo candore. La disfatta più totale si compie quando si scopre che intorbidisce gli altri pigmenti, ingrigendoli di molti gradi tonali

Come una donna seducente, che una volta entrata in sintonia con gli altri pigmenti, si trasforma in una Lamia pronta a divorarli, nutrendosi della loro brillantezza.

… e pensare che è che alcuni manuali storici e molti scritti contemporanei, lo reputano un ottimo bianco da affresco!!!

Per fortuna abbiamo nuove vie da percorrere per evitare “giornate” estenuanti a combattere contro la carbonatazione e la modificazione cromatica.
Il Bianco di Piombo è la prima di queste alternative, già conosciuto nei tempi antichi ha un ottimo potere coprente, brillante, indicato nella tecnica ad olio e anche nell’affresco. Ha due note sfavorevoli che non lo rende il nostro bianco preferito e non fa parte della nostra selezione di pigmenti. La prima debolezza è che reagisce molto male agli agenti atmosferici, che tendono a renderlo scuro. Scordatevi quindi di utilizzarlo in opere alla luce del sole, è destinato a restare all’ombra delle tenebre.  La seconda carenza è data dal fatto che per natura è molto velenoso. Tenetelo lontano dai bambini e dai dilettanti, nonché lontano dai solfuri e dai pigmenti al cadmio. Proprio per quest’ultimo motivo, ci è anche noto che alcuni pittori veneziani del passato, tendevano a smorzarlo e mescolarlo con creta bianca.
Un’altra buona alternativa è l’ossido di zinco, ancora in sperimentazione presso i nostri studi, avendo anch’esso alcuni punti d’ombra nella sua applicazione, che spesso non vengono considerati nei manuali passati e contemporanei, sconsigliato comunque nell’affresco.
C’è poi un’ulteriore soluzione, di recentissima scoperta, di cui per ora vogliamo poco rivelare, sia per  avere la tranquillità di studiare in modo più approfondito, sia perché in questo diario diremo quello che è utile dire.

Ad oggi è il nostro pigmento bianco preferito, lo avessero avuto Michelangelo Buonarroti e co. oggi ne sapremmo di più sulla sua modifica cromatica nei secoli.

In futuro se decideremo di pubblicare il frutto delle nostre esperienze, approfondiremo il problema del bianco in pittura, espandendo anche la lista dei pigmenti bianchi, adatti alle differenti  tecniche pittoriche (esempi: solfogeno, b. di Meudon, b. di Firenze ecc).

 

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