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Contents Contents ................................................................................................................................... 2 Disclaimer ................................................................................................................................ 2 Introduction ............................................................................................................................. 3 Lath and Plaster ....................................................................................................................... 3 A brief history of lime........................................................................................................... 3 Lime Cycle and use in mortar .............................................................................................. 4 Lath and Plaster ................................................................................................................... 5 The Model................................................................................................................................. 7 Materials Used: .................................................................................................................... 7 Restrictions .............................................................................................................................. 8 Model Strategy ......................................................................................................................... 8 The construction process: ........................................................................................................ 9 Plaster Coats: ...................................................................................................................... 10 The Models .............................................................................................................................. 11 Model 1................................................................................................................................. 11 Model 2 ................................................................................................................................ 11 Model 3 ................................................................................................................................13 Reflections ...............................................................................................................................14 Production: ..........................................................................................................................14 Curing: .................................................................................................................................15 The Models: .........................................................................................................................16 Further development .............................................................................................................. 17 Conclusion .............................................................................................................................. 18 Terms used ..............................................................................................................................19 Works Cited ............................................................................................................................ 20 Appendix A: Lime Cycle Diagram ......................................................................................... 22
Disclaimer This project was undertaken during the Covid-19 pandemic and associated national Stay at Home order. The affects this had on the project are many. These limitations have been recorded and reflected upon throughout the report, but they also include the lack of practical teaching experience and sufficient workspace. With this said, I would like to extend my gratitude to Dr Dimitris Theodossopoulos for providing us with the best education within his means, and Audrey Dakin for her guidance. All images are my own.
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Introduction Lath and plaster are a commonly used historic internal finish, comprising of timber frame, lath substrate, and a lime plaster finish applied in layers. It is used both as a ceiling and wall finish and can be altered to produce a simple surface or decorative elements. There are many pathologies related to construction with lime plaster. The combination of materials and the preparation of its substrate must all be done with precision. The aim of this project was to produce a model of a lath and plaster model to investigate some of these delicate relationships. Due to the restrictions imposed under the Scottish Stay at home order, this model was constructed in my living room, and as a result several alterations had to be made which will be addressed.
Lath and Plaster A brief history of lime
Limestone (CaCO3) is a sedimentary rock. It is commonly found throughout the UK, particularly in the Peak District and eastern coasts. Lime takes its form in construction as a mortar, or more accurately it is a binder. Mortars are made of a binder – in lime construction this would typically be a Natural Hydraulic Lime (NHL), however it could also be a cement, clay, or earth - an aggregate such as sand, and water. 1 The earliest records of mortar dates back to 6500-4000BCE where muds, clays and sands would be used to create mortars where these binders were more readily available. There are examples of the Ancient Egyptians using a lime-based plaster on the walls of the pyramids. Lime was commonly used by the Romans, who introduced pozzolanic additives, and it continued in popularity until the introduction of a cementitious replacement at the turn of the 20th century. Gypsum (CaSO4 2H2O) is alternate material found in sedimentary rocks used in plaster of Paris. 2 Due to the fast-setting nature of Gypsum, it is used for decorative mould formed plasterwork. 3 Lime is a preferred binder as it is permeable, or breathable. Early 19th century saw the introduction of Portland cement, a mixture of limestone and clay or shale that would see the use of lime phased out over the following decades, as it was stronger and set quicker than Lime. 4 However, these renders would trap the moisture in the masonry behind, causing damp problems. Cement is also less flexible than lime which can even repair itself when it (rarely) cracks. Following the failure of cementitious mortars, lime has returned and become the preferred specification for historic fabric. Lime mortar can be used internally or externally as a finish to masonry as it is hardy against harsh environments. As an external render it will protect the masonry beneath and it is easily repairable or replaceable, making it a sacrificial finish. Internally, it is flexible and can be applied directly to masonry or applied to stud walls in a system known as Lath and Plaster. It is important to understand the process of preparing lime for construction use.
(The Engine Shed, n.d.) (Dana, 1985) 3 (Ratcliffe, 1997) 4 (Britannica, 2019) 1
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Lime Cycle and use in mortar
As part of what is known as the Lime Cycle, the chemical composition of the lime changes through the introduction of heat, water, and carbon dioxide. 5
After calcining or burning the Limestone, quicklime is produced which is the basis for all construction lime. This quicklime is mixed with water, or slaked, as quickly as possible to stop it reacting with the air, known as air slaking. 6 Three method options are used, each creating a different form. Lime putty is made by adding excess water to quicklime, and is the preference of many plasterers. 7 Hydrated lime is the powdered version, made with a small amount of water, it is the most stable. The third is Hot Mix, which is slaked with damp sand and Most conservators prefer to work with lime putty as it will be more matured and will not have begun to react with the air. There are two types of lime; Hydraulic and non-Hydraulic. Hydraulic Lime is quicklime that sets in connection with water, due to a clay content. Successful hydraulic lime requires a clay content greater than 12% (any less is considered non-hydraulic). The higher the clay content, the stronger and less permeable the mortars become. 8 A hydraulic lime would be well suited to an external, masonry substructure. Non-Hydraulic lime sets by a reaction with carbon dioxide in the air which is absorbed, reacting with the quicklime and hardening. This means they set very slowly, and require controlled conditions, however it is also more flexible and breathable. A non-hydraulic would be more suitable for a timber structure. In cases where the mortar, lime (hydraulic or non-hydraulic) or cementitious, is required to set rapidly pozzolanic additives can be added to the mortar mix. An example area would be
(British Lime Association, n.d.) (Tŷ-Mawr, 2016) 7 (Lime Green, 2017) 8 (Taylor, 2000) 5
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an external, vulnerable settings where a freeze-thaw cycle would threaten the structure of the lime. These additives, such as ash and brick dust, increase durability and create smaller chemical reactions which accelerate the setting process. 9 Pozzolans are not often required for internal plastering. A Pigments are added to lime washes to add colour. Traditionally, local earth was used which resulted in regional variation in colours. Today, a wider selection of pigments is available regardless of location, so this differentiation has been lost. Today the most common procurement of lime is as an NHL (Natural Hydraulic Lime) in increasing strengths of NHL2, NHL3.5 and NHL5.
Lath and Plaster
Earliest records of lime mortar used were directly onto masonry, however an alternate, internal application was introduced into Scotland in the 16th century. Lath and plaster is an internal finish to walls and ceilings, by which plaster is applied to timber laths fixed to a stud frame. These plasters can be made using any binders, but lime is the most common traditional method. The lime plaster used is the same for internal and external use, but differing ratios and aggregates, and additives to adapt it to new conditions. These additives can be used to improve performance, such as animal hair to provide flexibility. Modern alternatives to timber laths include expanded metal lath (EML) fixed to timber studs or blockwork. The plaster is built up over a number of coats, each of which serve a sperate function (see future Plaster Coats section). These layers can be applied by hand using tools including a trowel and hawk and levelled with a float, or mechanically with a plastering machine (producing projection plastering). From the introduction of plaster on lath, began the development of decorative enrichments and mouldings. There were made with course lime plaster producing a core on detailed timber substructure, or gypsum pre-poured elements. Since the 1880s, plasterboard has replaced lime mortar as the favoured level internal lining.
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(Gibbons, 1997)
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Typical Construction: Wood laths are fixed to a timber frame or wall studs. The same construction is also used for lath and plaster ceilings, where the laths would be fixed to ceiling joists. The laths are typically 25x6x1250mm long, made of riven or split oak or beech (or later red baltic fir). They should be fixed at each end and mid points to vertical studwork with nails, and are spread with gaps of 810mm between each.
Facing view of model
Rear view of model
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Three plaster mixes are applied individually; the scratch, float and finish laters. The ratios of Lime:Aggregate for these vary, with decreasing aggregate size and increasing lime proportions from base to surface layers. Drying time between these layers varies based on water content and environment.
Facing view of model, with plaster faded to show lath staggering behind.
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The Model Materials Used:
Laths (25mm x 6mm). Used for the 1:1 model, taken from pallets.
Laths (5mm-1mm). Used for a 1:5 model, thinner laths were used, taken from a blind.
Hydrated Lime or ‘Bag Lime’. Hydrated lime is safer to work with at home, as it is more stable. It is also non-hydraulic, meaning it will set naturally in the air and be more flexible and suitable for internal plastering onto a timber frame.
Sharp sand. The sand was then seived to provide some grading, and seperated into fine and course
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Sheep’s wool. Goats or horse is traditional and would have been preferred, but was not available.
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Restrictions Due to the national restrictions in place, the following amendments were made • •
• •
The model sizes were reduced to 500x250mm, due to smaller working space. Hydrated lime was used, as this was the safest to use at home. Lime putty would be preferred, as it allows the lime to continue maturing in the water, and not begin the recarobonisation reaction with the air. With Bag Lime, it is unknown how much of the dry lime would have already reacted with the air before arriving to site 10. Sheep’s wool was used instead of the traditional goats or horsehair. Each of the layers would ideally have sufficient time to dry naturally, which is days or even weeks. However, as I only had one bag of lime to use, I was conscious that the lime begins to react with the air as soon as it is opened, and is recommended useless after 3 days, so I reduced the drying time to 24 hours between layers and left the models in a well-ventilated area to dry as quickly as possible.
Model Strategy The strategy to make the two models was as follows: Frame Option A (1:1)
Frame
Frame
•1:1 laths at 30mm centres nailed to frame
•1:5 laths at 6mm centres stapled to frame
Scratch Coat •1:3 lime to course sand •With Sheeps wool •8-10mm
Scratch Coat •1:3 lime to fine sand •No sheeps wool •8-10mm
Floating Coat
Floating Coat
•1:3 lime to fine sand •12-16mm
• 1:2 lime to fine sand • 12-16mm
Finishing Coat •3:1 lime to fine sand •Aprox 2mm
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Frame Option B (1:5)
Finishing Coat • 3:1.5 lime to fine sand • Approx 2mm
(Taylor, 2000)
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The construction process: 1. A timber frame is constructed, and laths are affixed, with laths spaced 8-10mm apart. 2. The laths will require dampening before application, as they have a high level of suction. If they are not saturated, they will draw the moisture out of the plaster, causing it to set too quickly and to separate from the laths. 3. The sand and lime are dry mixed together, in varying ratios depending on the coat it will be used for. The scratch coat includes courser aggregates and a lower level of lime, and the final coats include more lime and finer aggregate.
4. In the scratch coat, a fibers such as animal hair or sythnetic fibers are used to strengthen the layer. These should be evenly distributed, spaced only a few millimeters appart. As sheeps wool is being used (which is weaker than traditionally used goats or horse hair) more will be used to
5. The mixture is cobined, thoroughly mixing the dry materials first, and then adding water slowly to allow the lime to react with the water and make sure that no more than necessary is added.
6. The plaster mixes are then applied to substrate in coats, each of which is left to dry before the next is applied. The timing of this will vary depending upon water content of the mixture, environment in which it was applied (low temperature and high moisture content in the air will slow drying).
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Plaster Coats:
1 2 3
The strategy used here is three-coat work. 1. Scratch / first / render coat. Ratios vary from 1:2 to 1:5 and the layer should be 8-10mm thick, applied diagonally across laths. The scratch coat serves two purposes. It provides a solid conncetion between the plaster and its substrate; the animal hair provides flexible reinforcement and the plaster squeezes through the laths to provide a key. The course aggregate (3.5mm down) also provides a rough substrate to which the the later layers can stick. This layer should also be scored, to provide improve surface friction for improved adhesion to the next layer. 2. Floating / second / brown / straightening coat. The purpose of this is to level out the rough-ness of the previous layer, or straighten. The aggregate is finer and hair can be used but is not required. This layer should be 12-16mm thick. 3. Finishing / setting /white coat. This layer provides the finished coat, which should be thin and level. Radios vary from 3:1 to 1:1 with fine silica sand 0.8 to 0.075mm. The layer should be approx 2mm.
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The Models Model 1
The first model was a 1:1 model on 25mm x 6mm laths. The layers, as identified above, included a very course scratch coat, a fine floating coat, and a high lime finishing coat.
Model 2
The second model was built to a 1:5 scale. Although the lime particles cannot be scaled, only fine sand was used for the scratch coat as an attempt the scale the mixture. Unfortunately, quite quickly cracking and bowing began to show, so I investigated by testing the plaster. Helen BERESFORD – S2137191
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Cracking and collapse:
1. A crack appeared in the top right corner of the scratch coat.
2. With a small amount of force, chunks quickly fell away from the frame.
3. It became clear that the scratch coat had not connected to the laths; the keys were undeveloped.
4. The coats began to delaminate, bowing away from the laths.
5. As they fell away from the substrate, it also became clear that the scratch coat and floating coat had not binded together.
6. With a little encouragement, all the plaster pealed away from the lath structure. 7. Once the plaster had been removed from the laths, I brushed down the surfaces and removed the laths from the structure. I added an additional centrally placed vertical post to provide more support and refixed the laths at slightly wider intervals to allow for more secure keys to form. I then began again, making Model 3.
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Model 3
For the third model (1:5 scale), the frame of the second model was reused. To address some of the previous issues the following changes were made: • • • •
An additional vertical stud was introduced to reduce the span of each lath. The laths were removed and reattached at larger vertical intervals, spread further apart to allow for better keys to form. The aggregate of the scratch coat was graded, including both fine and courser materials, so improve the strength of the base layer. Sheep’s wool was also introduced to the scratch coat to improve flexibility.
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Reflections Following the production of these models, I reflected upon the process
Production
Ideally, the mixture would be mixed mechanically for 20+ minutes (10 minutes dry and 10 minutes wet) until the mixture has reached the right consistency. By mixing this by hand, I was unable to provide the same thoroughness. Additionally, recognising the appropriate consistency appears to be a skill that is learned by practice and apprentice, so the consistencies I reached were less reliable. The tool used for the application was a paint scraper, however a professional would have more appropriate tools available to them. Laying on, angle and gauging trowels are used for application. A floating rule is used to create a level straightened finish, and drags are used to create good adhesion. The application itself proved difficult. There are many tools that help, and I did not have them Also, the aggregate used in this mix was not graded well enough, but was mainly large
On the 1:5 model, the laths were unable to support the weight of the lime and bent or snapped. Decreasing the spans by bringing the vertical posts closer together may have helped this. Additionally, the laths were too close here. Being this close together meant the lime could not squeeze through and sufficient keys were not made.
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Laths that were not fixed securely or could not support the weight of the lime collapsed. Where this happened, the plaster was unable to bridge this gap. The addition of fibres in the mix might have helped bridge this gap, by increasing the
Curing
In the first model I used a coarse aggregate, however, I think I should have used a more graded aggregate combining both the fine, mid and coarse aggregate. I was also conscious that sheep’s hair is not as strong as the typical goats or horse, so I used more than normal which resulted in clumping. I addressed this in model 3, using less. Cracks were visible throughout the layers. Lime is known as a self-healing material; in that it can often repairs cracks when it gets wet. I suspect that the cracks are caused by too much water in the mixture, diluting the strength of the lime, and by uncontrolled drying conditions, but I cannot be sure. Where the scratch coat of model 2 fell away from the substrate, I believe this is due to several factors. There appeared to be too much water in the mixture, the laths were too close together so good keys were not achieved, and only fine aggregates were used making the mixture overall very weak.
A lot of mixture squeezed through to make the keys. Although this creates a good connection to the laths, making it very secure, it is much more than I have seen in examples by others. This leads me to think the keyways were too large, or the mortar was too malleable, suggesting too much water in the mix.
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The Models Model 1
The first model proved the most successful, which I attribute to the fact it was a 1:1 scale. The scratch coat included coarse aggregate, but this should have been better graded. There was also too much sheep’s wool added. Where this was done to compensate for its weakness in comparison to traditional hairs used, it ended up clumping and not spreading evenly. The finishing coat, however, came out the smoothest and whitest, upon which a lime wash could be easily applied.
Model 2
The second model failed by all accounts. The laths were too close, resulting in keyways to small for keys to develop. They were also spanning too far, so bent under the weight of the plaster. It was difficult and perhaps unachievable to scale the plaster materials down.
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Model 3
The final model was a success. Each of the layers dried well, with successful keying at the back, and the laths were not bowing as they did in the second model. The smoothness of the final layer, or lack thereof, is a result of my poor application skills. The sheep’s wool added was spread finer and cut shorter to avoid the clumping evident it the scratch coat of the first model which worked well.
Further development Plastering can truly be said to be a ‘craft’ rather than a ‘trade’. A plasterer has nothing more than a heap of sand, or a bag of plaster, from which to produce his finished work. The quality of the finished product is dependant, entirely, on the skill of the operative. 11 It was difficult to tell when each mixture was at the right consistency. Most of the literature suggests that it is almost instinctual, recognised by expert plasters and a practical skill taught to apprentices through years of training. If I were to re-do the experiment, I would seek professional training and advice first.
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(Baker, 1990, p. ix)
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The first 1:5 model failed in part to the laths struggling under the weight of the plaster. Producing several 1:1 models would have allowed me to compare the plaster mixtures better, with less variables to consider.
I thought it was interesting how much the colours changed between the wet and dry product. This would be particularly interesting when using pigments, whose set colour could vary hugely from that when wet.
The temptation to add extra water to make the product more malleable and easier to apply was strong, but is ultimately damaging to the curing process.
Conclusion The process of making this model was extremely helpful for me to understand the process of working with lime, despite most of these lessons being learned by mistakes. Having previously specified lime mortars, it was useful for me to practically see the reasoning behind each of the requirements and understand the process.
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Terms used Aggregate
A mass of fragments or particles loosely compacted together, such as sand.
Calcining
Process of burning Limestone at 900-1000°c to make quicklime.
Coat
Each layer of application of a plaster mix. Also known as screed.
Hydrated Lime
Slaked quicklime, mixed with a small amount of water to produce a powder.
Hydraulic Lime
Hydrated quicklime (calcium hydroxide) that sets in contact with water.
Finished coat
The final coat of plaster. Also known as setting or white coat.
Floating coat
The coat of plaster under the finished coat. Also known as second / brown / straightening coat.
Non-Hydraulic Lime Quicklime with a low clay content, that sets by reaction to the air. Key
Plaster pushed through laths that mushrooms to the rear and sets, wrapping around and griping onto the laths.
Keyway
Gaps between laths through which plaster is pushed to form keys.
Lath
Narrow strips of wood fixed perpendicular to timber studwork to form a base upon which plaster is added.
Lime Putty
Slaked quicklime, mixed with an excess amount of water.
Mortar
A mixture of binder, aggregate and water that sets to form a thick, rigid, semi-porous mass.
Non-hydraulic Lime Hydrated quicklime (calcium hydroxide) with a low clay content that sets in contact with air. Plaster
A material used for creating a smooth finish on a wall or ceiling, applied wet in layers/coats, and allowed to harden.
Quicklime
Limestone that has been burned/calcined.
Slaking
The curing of quicklime, generally by adding water but can happen in air.
Scratch coat
The first coat of plaster applied to substrate. Also known as Scratch / first / render coat.
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Works Cited Baker, M., 1990. The Skills of Plastering. London: MacMillan Education Ltd.. Bennett, B., 2002. Lime Plaster and Render Reinforcement. The Building Conservation Directory. BigWalleze, 2014. Plaster Wall Repair Basics - Anatomy of a lath and plaster wall. [Online] Available at: https://www.youtube.com/watch?v=03FpvpwAa3c&ab_channel=BigWalleze [Accessed 20 04 2021]. Britannica, T. E. o. E., 2019. Portland Cement. [Online] Available at: https://www.britannica.com/technology/portland-cement British Lime Association, n.d. Lime cycle. [Online] Available at: https://britishlime.org/education/lime_cycle.php#:~:text=The%20lime%20cycle%20is%20 one%20of%20nature's%20best%20known%20examples%20of%20chemistry.&text=When% 20limestone%2Fchalk%20(calcium%20carbonate,hydrated%20lime%20(calcium%20hydro xide). [Accessed 17 04 2021]. Dana, J. D., 1985. Manual of mineralogy. New York: Wiley. Gibbons, P., 1997. Pozzolans for Lime Mortars. Conservation and Repair of Ecclesiastical Buildings. Holmes, S. a. W. M., 2002. uilding With Lime: A Practical Introduction. 2nd ed. London: Intermediate Technology Publications. Lime Green, 2017. Hydraulic Or Hydrated Lime. [Online] Available at: https://www.lime-green.co.uk/knowledgebase/hydraulic_or_hydrated_lime [Accessed 17 04 2021]. Lime Stuff, n.d. Mixing Hydraulic Lime. [Online] Available at: https://www.limestuff.co.uk/blog/how-to-mix-hydraulic-lime [Accessed 11 04 2021]. Limebase, 2020. The History of Lime Mortar. [Online] Available at: https://www.limebase.co.uk/guides/the-history-of-lime-mortar [Accessed 17 04 2021]. Lime-Plastering, I., 2006. Tim Ratcliffe. The Building Conservation Directory. Lynch, G., 2007. The Myth in the Mix, The 1:3 ratio of lime to sand. The Building Conservation Directory. Ratcliffe, T., 1997. The Use of Gypsum Plaster. The Building Conservation Directory. Ribble Valley, 2016. LBS 3/2016/1070 Specification for Lime Plaster Repairs. [Online] Available at: https://www.ribblevalley.gov.uk/planx_downloads/17_0098_Spec_for_Lime_Plaster_Rep airs.pdf [Accessed 20 04 2021]. Helen BERESFORD – S2137191
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Schofield, J., n.d. Lime in Building: A Practical Guide. s.l.:s.n. SPAB, n.d. Lime. [Online] Available at: https://www.spab.org.uk/advice/lime [Accessed 11 04 2021]. Taylor, J., 2000. Lime: The Basics. Historic Churches. The Engine Shed, n.d. Lime Mortar Components. [Online] Available at: https://www.engineshed.scot/building-advice/building-materials/lime/#limemortar-components [Accessed 11 04 2021]. Tŷ-Mawr, 2016. Lime and its burning. [Online] Available at: https://www.lime.org.uk/community/the-lime-cycle/lime-and-itsproduction.html [Accessed 20 04 2021].
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Appendix A: Lime Cycle Diagram
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