In-depth stories about Marmorino Veneziano

07Dec 2014

Venetian’s Chimneys

Venetian’s Chimneys

The history of Venetian chimneys is intimately linked with the creation of the city.
The primary material that had made possible the daunting task of establishing such a lagoon-based city had been, without doubt, wood.
Wood formed the foundation of the city in the literal sense: all construction work is necessarily built on top of long wooden stakes driven into the mud. This holds for all buildings as well as the sea defences. Wood was also used as the structural frame and the system of roof beams. Then there was the Venetian fleet, the key factor insuring Venice’s protection from invasion for almost 1,000 years.
Add to this the wood required for domestic fuel and the operation of typical industries and the quantity mounts.
As these activities grew it became apparent the risks involved due to burning wind-blown embers floating in the very heart of the city. Indeed, Murano was soon allocated as the island where all glass furnaces were relocated and Arsenale, the island set apart for shipbuilding. Even foundries were allotted their own space.
So by now the major fire risks are some distance from the main residential zones yet the smaller workshops, the bakers and others pus the domestic chimneys are all still interspersed throughout the city. These chimneys continued to spew their hot cinders which, together with the wind, whirled around the city like dragon’s breath.
Resolving this problem became vitally important. Around 1300 building in bricks and mortar was on the increase but wooden constructions still abounded and it was at this juncture that fireplaces and chimneys come under a great deal of architectural attention.

THE INVENTION OF THE UPSIDE-DOWN BELL
In order to visually clarify the situation at the time a few photos are useful to show that the city was starting to concoct something rather wonderful and so functional to merit decoration.

In the above painting “Miracolo della croce al ponte di Rialto” (Miracle at Rialto Bridge) by Vittore Carpaccio (1465-1526) one may see how chimney decoration had become something of a competitive art, seemingly simulating the most beautiful glass goblets of Murano. Painters such as Giorgione and Tiziano more than once attempted frescos and decorations for these smoking towers. It is said that those who hadn’t taken the care to have a chimney like this constructed, if they had a fully functioning workshop, would be reprimanded; face a fine or closure of their activity.

THE CONSTRUCTION TECHNIQUE
The location of the chimney flue in a building is onto the load-bearing walls or the central wall (interior). The flue that juts out from the wall starts on the same floor as the fireplace and is supported externally by two brackets of Istrian stone and arch that joins them. On the inside, in order to have pavement base for the hearth, the central beam that would otherwise continue underneath the hearth needs to be truncated and supported by its neighbouring parallel beams by way of a short perpendicular cross-beam (in Venetian ciavarol). Filling the space previously occupied by the cut beam a segmental brick arch is built. This arch rests on the remaining parallel beams which have been trimmed along the length to give the desired base angle. The chimney’s flue runs vertically, half outside, half inside. The bricks are laid according to regular bond yet usually on their sides (shiner) externally. This is not only to lighten the projecting portion but also to gain a little more space internally.

In this photo (on the left) one can see how the fine technical work of the bricklayer also serves the plasterer’s finishing work which will be covered by cocciopesto (ground-up roof-tiles and plaster mix explained in more detail on earlier pages) and then marmorino onto the protruding brickwork. Another technical solution, as can be seen, is the Istrian stone guttering around the chimney stack to give it the right support to the chimney’s body upon the load-bearing wall and its irregularities; the strong sides (astride the main wall) indeed help support the sideway placed (shiner) bricks.
 
canna fumaria di palazzoThe protruding diagonal line of brick steps are to help direct rainwater away otherwise infiltration can be almost guaranteed over time.
 
Basamento torretta camino veneziano

Camini 1
This base footing on the left is a cube-type chimney (in Venetian dialect ‘col dado’-  with cube or ‘con la camicia’ – with jacket). However, the footing of a cylindrical chimney with the conic upside-down bell form on top is completely different; the circular shaft is inserted into the footing like any cylinder that is lodged into a cube with the casing readied so as to block it in place.

Let’s look at a few examples of a chimney with cube atop and the inside of an octagonal bell-type chimney:

Camini a campana rovesciata ottagonale

Camini a campana rovesciata ottagonale

A detailed technical explanation of construction will start becoming rather complicated. So as not to commit errors it would best to dwell over every phrase. Anyway above are a couple of photos that illustrate quite a lot, the main advice I offer is not to underestimate the magnitude of such a project. It is a question of constructing something that requires the full expertise of a mason plus an acute powers of observation. For instance, the assembly of the octagonal bell-form chimney that gradually opens out little-by-little… well it’s not a piece of cake! The initial setting off from the support (the brackets) is fundamental. When to commence the flaring is another consideration.

Camino veneziano con dado

Camini di Venezia

The chimney on the left had be restored a number of years ago. Replacing the worn-out brickwork and wrapping “the jacket” with fine carbon fibre, finishing off with marmorino.
At the end the top is roofed over with a marble slab. Above the roofing is completed with local roof-tiles (these can either form a circular cone or a normal roof apex). The tiles are positioned so the rainwater is always directed to flow between the supporting brackets, where the bell’s base meets the straight shaft . The final crowning top ring completes the job.

MATERIALS TO BE USED
Very strongly advised to avoid cement for this work, with cement the chimney would crumble in many places. I would rather offer my own recipe for the job at hand. It’s best to use a “magra di marmorino” mix – a “thin” (proportionally less than otherwise) mixture of slaked lime with powdered marble and then combined with old ground-up terracotta roof-tiles (1.2 mm granules). The dosage is as follows:
• 1 gallon slaked lime
• 3 gallons white marble powder sieved to 0.7 mm granules
• 1 quart impalpable marble powder
• 1 quart ground roof-tiles
Mix and allow at least a day to settle.
With such a blend the chimney should last for at least a century before any touching-up need be considered. This work should be carried out with traditional old bricks and tiles, quality material can be bought from Antica Fornace Carraro di Arzerello in Piove di Sacco, Padova, Italy –  tel. +39 049 9775015). Here it might be possible to pick-up some semicircular half-bricks for the upside-down bell chimney. I would also recommend the book ‘Camini e Campanili‘, published by “Filippi editore”, Venezia which contains illustrations of a wide variety of Venetian chimneys such as the  “fork” and “trident” forms.

camino con campana a piramide rovesciata

Camini di Venezia

WHY THE BELL FORM ON CHIMNEYS
The protecting bell nders much more functional the air flow and as an aid in fire prevention. Smoke passes through the turret still hot yet inside the bell a sort of circular vortex is created where external air enters below, in the gaps between the supporting brackets (modiglioni) and rises. As far as the glowing floating embers are concerned they would tend to exit more-or-less horizontally from a normal chimney stack, shooting out burning embers all around. However, these embers, once out of the flue and inside the bell, they start to rebound back-and-forth against the walls, like a pin-ball’s “flipper effect”, and continue dancing and ricocheting for some time until they reach the exit, effectively extinguishing any cinder and any danger.
Furthermore, this type of chimney copes just as well with air flow on days with low atmospheric pressure and, importantly, wasp nest infestation is a very rare occurrence.

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