WHY USE LIME STUCCO INSTEAD OF CEMENT STUCCO?
one shovel ---- White Portland Cement*
2 bags ---- Type-S Lime
65 shovels ---- Masonary Sand
16 cups ---- Ferrous Sulfate Powder (water soluable) as coloring**
*A small amount of Portland Cement was added to increase drying/curing time. Lime plaster takes a minimum of two to three weeks to cure otherwise. Since we were under time constraints and had to shoot the walls within a two day weekend, adding approximately 20%cement (one to five ratio of cement to lime) produced sufficient drying/curing that the second finish coat could be applied within 24 hours. Only a small amount of cement was added to the final coat. Adding more than 20% cement is not recommended as it significantly decreases the breathability of the stucco which is so crucial for straw bale walls.
White cement was used since coloring added later to the finished walls will take and cover better than over the grey of the more common form of Portland cement.
**Ferrous Sulfate comes in 50 pound bags (for about $16) and is available from most stores which sell fertilizer -- get the water soluable kind in powdered form. It is the main ingredient in "Ironite" (as ferrous sulfite), used frequently by gardeners to increase the iron levels in their soil, but this comes in a granular form which is not readily soluable in water. On the bag are the words "Caution - stains concrete", and as it turns out it makes a lovely stain for both concrete and lime stucco after it has been applied. But don't panic when it turns DARK GREEN for awhile when it is first brushed on the plaster surface.
Ferrous Sulfate also adds a light tan color to the otherwise white/grey plaster when added directly to the stucco mix. In the picture to the right the left side of the bale has been plastered with Ferrous Sulfate added directly to the stucco mix; the right side has been stained with a Ferrous Sulfate solution after the plaster was applied and had started to cure (but was not completely dry).
Here Mindy is measuring out some Ferrous Sulfate to add to a sample formula before applying it to a test bale at the left. Keep lots of notes and keep testing till you get what you like...
THINGS TO LOOK OUT FOR WHEN SHOOTING LIME
Somewhat more water than is used for traditional cement stucco was needed in order to keep the pumper/mixer and supply hose from clogging up. Picture below -- grumbling stucco crew getting their pumper unclogged.
We had some cracking as the stucco started to cure. This was easily repaired with a wet float sponge before the plaster was dry. A warm (but not hot) and humid environment is best to allow the stucco to cure slowly and not dry out too quickly.
Lime is very alkaline and caustic -- producing a terrific case of "dishpan hands" in just a few minutes. Don't breath the dust as you shovel it or get it in your eyes. Wearing gloves, a dust mask, and eye protection is highly recommended. Also the ferrous sulfate not only stains plaster, it stains your hands so wear gloves.
Serious Straw Bale by Paul Lacinski and Michel Bergeron, A Real Goods Solar Living Book, 2000 - pages 244-261.
The Straw Bale House by Athena and Bill Steen, A Real Goods Solar Living Book, 1994 - pages 207-210.
The Last Straw Journal, Issue 29, The Green Prairie Foudation for Sustainability, 2000 - an entire issue devoted to lime. www.thelaststraw.org
"Lime-Based Renders" by Bob Bennett, artical on the Lime Centre web site: www.thelimecentre.co.uk/Library/renders.pdf
"Staining With Ferrous Sulfate" by John Swearington, artical on the Skillful Means web site: www.skillful-means.com/strawbale/papers/staining.htm