Many of the wonderful properties of earth floors, including their strength and water resistance, derive from the oil used to harden and bind them – I find it useful to think of earth floors as linoleum made with earth, instead of cork and sawdust. Linoleum was originally hardened with boiled linseed oil, but linseed isn’t the only oil that works for earth floors. The main types of oil that have been tried for earth floors are boiled linseed, raw linseed, hemp, and tung (note that boiled linseed oil actually contains drying agents, and you should source oil that is free of heavy metal dryers). Sometimes resins are also included in oil blends used for earth floors. While each type of oil has its adherents for earth floors, it seems that there is little data that may be used to compare them – there isn’t even that much anecdotal evidence, other than that they can all work to some extent.
Because it haven’t seen it done elsewhere, I tested hardness of several oil and solvent combinations using an improvised Brinell hardness test. Brinell hardness is a simple test performed by pressing a 10mm ball bearing into a sample at a set pressure (usually 220 lbs) and time, and measuring the width of the resulting indentation. I used an improvised setup (inspired by this discussion), and the sample size is small – but despite a lack of scientific rigor some very striking trends can be seen. I’ll summarize the results first, but the actual measurements are presented in the table below.
- Tung oil has very high initial and later strength, but has terrible penetration, and must be cut with thinner 25-50% in order to be usable, or it can blended with other oils.
- Boiled linseed oil had relatively low 10-day strength, but gained a lot of strength with curing and was second only to tung oil in terms of hardness after 3 months. Mixing it with thinner reduced initial and probably late strength (the sample cracked and failed during the 3 month test). However it was clear that more oil-thinner mix could be applied to the sample, so the lesson is that when thinner is used, make sure to use extra coats of oil to ensure good saturation. Boiled linseed oil had moderate to poor penetration, and needs to be either heated (safely?!) or usually cut with thinner.
- Raw linseed oil had slightly better initial strength than boiled linseed, but gained virtually no strength over the next 90 days, making it one of the weakest samples after 90 days. Raw linseed oil has moderate penetration.
- Hemp oil has excellent penetration, and reasonable 10-day strength, but does not appear to gain strength very much over time, certainly not when compared to boiled linseed oil.
- Blending oils can capture the benefits of each. A blend of 1 hemp: 1 tung: 1 boiled linseed, diluted 10% with citrus solvent had very good strength (though not quite as good as tung oil and thinner alone), as well as intermediate penetration. The resulting hardness of 1.81 is better than that reported for Swedish pine flooring (1.6) but much lower than most hardwoods (e.g. black cherry is 3.6, whereas Swedish birch flooring is 2.6).
- Note that the boiled linseed oil I used was old, and may have suffered from some oxidation. Even so it appears to be one of the best oils for earth floors, though blending even a little tung oil into it will improve the resulting hardness. The raw linseed oil wasn’t fresh either.
|Oil||Comments||Penetration (1=best, 8=worst)||Hardness 10 days||Hardness 90 days|
|100% Boiled Linseed||strong odour, intermediate penetration||5||0.94||1.38|
|100% Hemp||mild odour, excellent penetration||3||1.06||Sample broke (weak)|
|75% Tung / 25% Thinner||Poor penetration. Smell is nutty and smoky, reminiscent of chipotle||8||1.71||2.05|
|100% Raw Linseed||intermediate odour, excellent penetration||2||1.03||1.04|
|75% Boiled Linseed / 25% Thinner||best penetration||1||0.85||Sample broke (weak)|
|75% Boiled Linseed / 25% Tung||Poor-Intermediate penetration||7||1.01||1.61|
|75% Hemp / 25% Tung||intermediate penetration||6||1.24||1.49|
|30% Boiled Linseed / 30% Tung/ 30% Hemp / 10% thinner||intermediate penetration||4||1.37||1.81|
|White pine||Trim grade||0.97|
|Pine Flooring||Value from internet||1.6|
|Black Cherry Flooring||Value from internet||3.6|
Drying oils are very reactive, and oily rags are prone to catching fire when left bunched up (sponges are even worse, and should be avoided). Immediately after oiling a floor all rags, rollers etc should be spread out outdoors – otherwise the photo you take of your freshly oiled floor might be the last photo of your house before it burned down.
A few tips
- Get as many coat on as you can over about two days (usually 4-6 coats depending on how heavily you apply each one).
- Wait until one coat is completely penetrated before applying the next one (initially the coats can be applied almost immediately, later coats may take hours to finish penetrating).
- Wipe off any excess oil after a few hours, don’t leave it overnight or it will gel.
- Your freshly oiled floor is incredibly fragile – use bare feet or socks and roll up pant legs. Use soft rollers or brushes. Try to wait until oil has penetrated before crossing it.
- Read Sukita’s book.