Signage

I like just about everything about this. The black and white checkers along the top look like part of a film strip, like it's a small clip from a road movie. The dots on the arrow and letters are 'cats eyes' so they reflect the headlights. I can imagine the driver of an Austin 7 squinting through his rain soaked windscreen at the faint shine of the numbers.  I don't often wish this, but I hope it's listed.

You can hide a house behind a tree

Appropriate signage

I 

Has this been recognised yet for its historic value?
Brougham Castle, Penrith.

Graffiti No. 1

This isn't about gang tags or fashionable slogans but rather looking at historic graffiti.  Things that might be hidden under modern decorations and might be a single mark or might be a whole fleet of aircraft.  I mention this because one of the most emotive pieces of graffiti I have ever seen is the silhouettes of German bombers penciled onto dado paneling of an Oxfordshire church tower.  It is always exciting to find this sort of graffiti.  It raises so many question and for me at least, becomes a direct link to an individual, a time or event.  But it also presents the problem of whether and how to preserve it.  After all, it may well have been done clandestinely and quickly covered up.  It was almost certainly never meant to last.

This curvy figure was hidden under some nasty, inappropriate and badly done matchboard.  Before that it had been partly obscured with blackboard paint.  It's not the usual builders scrawl, the chalk lines have depth and texture and the proportion and shading show some skill.  But it's not part of any design within the building and in chalk, it is probably not possible to consolidate and preserve in any way.  Does this matter?  Here, I think not.  But I would like to think that whatever happens next might allow it to survive for someone else to find.

Invisible Green

Invisible green is a green so dark that it renders itself invisible and appears black.  So you could call it black but that's not very poetic is it.  It's origination is attributed to Humphrey Repton for use on railings in gardens.  It was apparently used on the door to No.10 Downing Street.  Coincidently, invisible green is also used at Hughenden Manor, the home of Benjamin Disraeli, Prime Minister and resident of No.10 in 1868 and later from 1874 to 1886.

You can't hide a house behind a tree

In front of every building is a space to stand and look at the building.

A short glossary from the information sheets

See earlier posts for the information sheets.

Ashlar; Stone that is dressed with sharp square arisses (corners).
Bed/bedding; Line of mortar between courses.
Brick trowel; Large diamond shaped trowel used for placing mortar (not pointing).
Bucket trowel; Medium sized trowel, slightly tapered sides, flat end. For working out of a bucket. Good general purpose trowel.
Burning; Heating limestone to remove carbon dioxide and produce lump lime.
Carbonation; The process of absorbing carbon dioxide (in a mortar) that causes calcium hydroxide to revert to calcium carbonate.
Coarse stuff; Lime with sharp sand.
Course; A line of bedded building material (brick or stone).
Fat lime; A matured slaked lime.
Fine stuff; Lime with soft sand.
Finger trowel; Thin trowel for pointing. Width varies from 6- 24mm in 6mm increments. Also known as a tuck pointer or Frenchman.
Float; Rectangular trowel, usually metal, and used for floating plaster onto wall.
Free lime; Any uncarbonated lime (calcium hydroxide) within a set mortar.
Gauged mortar; Lime mortar with pozzolana added.
Haired mortar; Lime mortar with hair for use on laths.
Hot lime; Lump lime slaked with sand and used immediately.
Hydrated lime; Lump lime that has been partially slaked, dried, ground and bagged. Used as a plasticiser in cement mortar.
Hydraulic lime; Lime that has naturally occurring pozzolana.
Hydraulic set; The ability to set without carbon dioxide.
Lime cycle; Limestone (calcium carbonate)+heat→Lumplime (calcium oxide)+water→Lime putty (calcium hydroxide)+carbon dioxide→Limestone.
Lime mortar; A mix of aggregate and lime.
Lime putty; Slaked lumplime (calcium hydroxide).
Lumplime; Burnt limestone (calcium oxide). Also known as quick lime.
NHL; Natural Hydraulic Lime.
OPC; Ordinary Portland Cement.
Plasterers small tool; Double ended tool with leaf shape trowel at one end and rectangular trowel at other. Useful for pointing very fine joints.
Pointing trowel; A small diamond shaped trowel, best with slightly curved edges.
Pozzolana; Any material that will impart an hydraulic set in a lime mortar. Originally a fine volcanic ash from Pozzuoli near Vesuvius. Can be almost any burnt material but most commonly clay.
Putty joint; A thin bedding joint in ashlar stone or brick.
Sharp sand; Sand with broken particles.
Soft sand; Sand with rounded particles.

Compton Abbas, Dorset

The SPAB working party at the old church, Compton Abbas, was a strong grounding in conservation philosophy and practice. Many long hours were spent mixing mortars, pointing and rebuilding the boundary walls and discussing every aspect of what we were doing.  Later events were at Greatham in Hampshire and are now at Ratcliffe-on-Soar, Nottinghamshire.

Carbon footprint-

-describes the amount of pollution generated during the manufacture of a product. Carbon refers specifically to carbon dioxide (co²). This occurs naturally in the atmosphere as a gas in small quantity, approximately 0.04%, but it is also a by product of combustion. It absorbs light in the infra red range and in doing so it increases the temperature of the atmosphere, for this reason it is described as a greenhouse gas.

Stone repairs

Many years ago, 1989 I think, I drew up and specified repairs to window mullions in an Oxfordshire church. They were split where the ferrementa (the iron framing used to support larger leaded windows) had corroded and had previously been repaired with iron straps, which had also corroded and caused further damage. The internal sections of the mullions had several hundred years of grafitti and were to be retained and consolidated. The new mullions were in Taynton limestone and the ferrementas were repaired with stainless ends to prevent future corrosion. Richard Noviss did the work.

Lime training III

One of the training events was held at Low Luckens Organic Resource Centre. Great location; lots of old stonework and pointing and a tatty bit of wall on which we could practice pointing and limewashing.

Lime training II

Also did this one on limewash.

APPLICATION OF LIMEWASH

There are many different ideas about limewash mixes and whilst some people advocate a mix as thin as skimmed milk and in many coats (as much as twenty) I have seen mixes like thick cream, applied in a single coat that still work well. It seems that preparation and aftercare are more important. Though it is unlikely that this was often achieved traditionally it will minimise the risk of flaking or dusting.

If pigments are to be added these should be kept to a single colour as it is very hard to maintain consistent colour between batches.

The surface should be wetted and allowed to soak in so that the surface appears damp but there is no standing water. Apply the limewash with a natural bristle brush (use a cheap one because the limewash rots them very quickly) working it well into the surface to fill irregularities and cracks. A fresh coat will be translucent but as it carbonates and dries it will become more opaque. Limewash may dry at different rates because the water content varies within the substrate. Limewash sets by carbonation through absorbing carbon dioxide. If it dries too quickly it cannot carbonate and becomes dusty, however if it is too wet it will remain translucent. If it dries too rapidly it should be lightly wetted or covered with dampened hessian.

Limewashing should be avoided in very hot or cold weather. I suggest that the best time is at the end of summer and early autumn when the weather is cooler and damper.

Assuming that the existing surface is old limewash this should be brushed down to remove any loose areas and dust.

Old render that has never previously been limewashed should be inspected to check the cement content. If it absorbs water readily then it may be a relatively soft mix or even contain some lime. Limewash will not bond to a hard cement mix. This will require thorough cleaning to remove moss and lichen, especially if it is roughcast or harling. If one applies a thin limewash it will kill moss and lichen and it is possible to leave this for a few weeks, months or even a year before brushing it down and limewashing properly.

If there is modern masonry paint this should be scraped off or removed with a pressure washer or the Stonehealth DOFF system. However great care is needed as all pressure washers, including the DOFF can cause great damage if used carelessly. Limewash can be applied over small patches of paint. It will be patchy but this will help seal edges of the paint and as it is very alkaline it will accelerate its decay.

If further coats are required it is important that the previous coat is allowed to carbonate thoroughly before applying the next, otherwise the brush will lift the previous coat. The surface should be wetted again before applying the next coat.

Lime training

Recently did a couple of lime training events for Cumbria Green Build. I like to do this sort of thing spontaneously, partly because I seldom get time to prepare properly, but at the last moment I wrote up some handouts.

LIME MORTARS

Lime mortar is a mix of aggregate and lime putty. There are many mixes that are gauged with materials such as cement to give an hydraulic set. This adversely affects the plasticity and porosity and in my opinion should be avoided.

A typical mix will be one part of lime putty and 2.5 to 3.5 parts of aggregate. 1:3 is always a good starting point but you may require a stronger mix (1:2.5) for a sharp sand and weaker (1:3.5) for a soft sand. When mixing, there should be just enough lime to cover each particle of aggregate. If there is any less the mortar looses plasticity, if there is too much the risk of shrinkage is increased. Lime mortars must be mixed thoroughly and when properly mixed they become very plastic and will stick to a trowel even when held upside down. Proper mixing is essential to ensure the optimum vapour exchange. Lime mortars set by carbonation through absorbing carbon dioxide. If mortar dries too quickly it cannot carbonate and becomes friable, however if it remains wet it will not set at all. If it dries too rapidly it should lightly wetted or covered with dampened hessian.

Traditional mortars appear to use any available sand and I have occasionally seen mortars with no obvious aggregate, just fine silt. It is not unusual to find mixes that have large lumps of unslaked lime. In theory these mixes will always fail though they clearly work in the long term.

Avoiding failure in modern lime work is a difficult issue, especially as the specifier may have professional or contractual liabilities. A cement mortar or render can be thrown onto the wall and will usually look good for many years, despite the problems developing underneath. However a lime mix may show dampness or spall and crack in the first winter. This is not failure, it is the result of using a softer and more breathable material, which in the long term is beneficial for the building. The specifier should resist the temptation to design a gauged mix or a complicated application procedure as this may give an inappropriate finish. It is very important to educate the client so they understand the limitations of lime.

If one makes a mix with a sharp sand, with a maximum particle size of about 5mm then this can be used as a general purpose mix for pointing, bedding, render base coat and haired plaster base coat. If the pointing is to be in thin joints of ashlar stone then the particle size should be 2mm max. Hair is used in plaster base coat to reduce cracking when applied onto laths and if cracks do appear then the hair will help retain strength. For a plaster top coat you will need a fine and soft sand, for example silver sand and a strong mix, possibly as much as 1:1.

Natural hydraulic lime (NHL) is becoming more popular in the UK. This is available as a dry powder much like cement and can mixed in any site mixer. It does require 15to 20 minutes to give a thorough mix, this will ensure the optimum vapour exchange. NHL is usually available in three grades of strength, 5, 3.5 and 2. This describes the rate at which set occurs and eventual hardness. NHL 5 is rather hard and should be reserved for structural work only.

Vapour exchange is the rate at which moisture evaporates through the mortar and is measured in g air/m²/hour/mmHg. Soft mixes perform better and hard mixes, like 1:3 OPC, are in effect waterproof.

Matrix Ratio VEx
Lime putty 1:2.5 0.71 approximately
NHL 3.5 1:2.5 0.68
OPC/hydrated lime 1:1:6 0.23
OPC 1:3 0.15 approximately