3 Different Types of Modified Woods: 

A Deep Dive into Furfurylation, Acetylation, and Heat & Steam Treatment

Wood modification is a process that enhances the properties of wood, making it more suitable for various applications. Over the years, several methods have been developed to modify wood, with furfurylation, acetylation, and heat & steam treatment being among the most popular. 

In this blog post, we’ll explore these three methods, discussing the benefits and concerns of each method and the one we like best.

1. Furfurylation

What is Furfurylation?

Furfurylation is a wood modification process wherein wood is impregnated with a mixture primarily consisting of furfuryl alcohol, which is a liquid product derived from agricultural waste like sugarcane bagasse and corn cobs. Under the influence of heat, the furfuryl alcohol polymerizes, filling the wood cell walls. The result? A more durable and stabilized wood product.

Benefits of Furfurylated Wood

  1. Enhanced Durability: Furfurylated wood exhibits an increased resistance to biological decay. This makes it ideal for applications where wood is exposed to moisture or conditions that might otherwise result in decay.
  2. Improved Stability: The wood’s dimensional stability is improved, meaning it is less susceptible to swelling and shrinking due to changes in moisture content.
  3. Reduced Maintenance: Given its enhanced durability and stability, furfurylated wood requires less maintenance over time, which can translate to cost savings.
  4. Aesthetic Appeal: The furfurylation process can give the wood a darker, rich color, which is often sought after in various interior and exterior applications.

Concerns with Furfurylated Wood

  1. Potential Health Impacts: Furfuryl alcohol is classified as a possible human carcinogen by the International Agency for Research on Cancer (IARC). Although the risk is associated with its vapor form and not the polymerized form present in treated wood, it’s essential to handle the raw material with care during the treatment process.
  2. Release of VOCs: During the furfurylation process, volatile organic compounds (VOCs) can be released. Proper ventilation and precautionary measures should be in place during the treatment process to ensure the safety of workers as well as the environment.
  3. Cost: The furfurylation process is one of the more expensive compared to some other wood treatment methods. However, when considering the lifespan and reduced maintenance costs, it might offer long-term value.
  4. Limited Awareness: Being a relatively new treatment in comparison to traditional methods, there might be limited awareness and understanding of furfurylated wood’s long term potential issues.
  5. Fire Risk: Fire risk when fabricating, cutting or modifying products. Sawdust can spontaneously combust as it contains remnants of Furfuryl alcohol used in the preservation.
  6. Checking: Many installations have experienced significant surface checking as well as end checking after a few months 
  7. Metal Reaction: Certain metals can react and experience accelerated corrosion such as zinc, copper and galvanized steel to name a few. Only stainless steel fasteners should be used with this wood. You need to take precautions as many building components contain metals such as railing, flashings, gutters and more that are not always available in stainless but could react with this wood.
  8. Color Variation: Installers have had color variation issues with different batches where the colors are noticeably different and hard to match. This can be an issue on larger jobs that require multiple batches of wood.
  9. Water Reaction: There’s been some issues with water staining from sprinkler over spray etc.
  10. Off Gassing: Some workers while cutting and installing have reported issues with burning eyes, nose and throat from the furfuryl alcohol treatment as well as the heat treating process. Many complain about the strong burnt odor it gives off even after several years of being exposed to the elements.
  11. Deceptive Eco Marketing: This wood commonly used is softwood Radiata Pine shipped from New Zealand to a chemical processing plant in Norway or Belgium. Then the wood is treated with furfuryl alcohol and carbon intensive heat treating. The distance from Auckland New Zealand port to Oslo Norway where they process the wood is 11,617 NM (13,368 miles). Next it goes from Oslo to Los Angeles (the target market place for the material) is an additional 8,014 NM (9,222 miles). That’s a total of 22,590 miles. The circumference of the earth is 24,902 miles, the furfurylated wood is about the farthest thing from the job site you can possibly source, with the largest possible carbon footprint for transportation.

Pictures of Furfurylated Wood Issues

Furfurylated wood showing excessive checking and black oxidation around some of the fastener holes.
Furfurylated wood siding showing water stains, black oxidation on all the corner and ends where the fasteners were installed
Furfurylated siding color variation from different batches

2. Acetylation

Acetylated wood is a modified wood product that has undergone a chemical process called acetylation. This process alters the wood’s cell structure, making it more resistant to decay, water absorption, and swelling. Here are the benefits and concerns associated with acetylated wood:

Benefits of Acetylated Wood:

  1. Durability: Acetylated wood has enhanced resistance to rot, fungi, and pests, which can extend the lifespan of the wood.
  2. Dimensional Stability: The wood is less prone to swelling and shrinking due to changes in moisture content, which can reduce warping, twisting, and cupping.
  3. Reduced Maintenance: Due to its increased resistance to environmental factors, acetylated wood often requires less maintenance compared to untreated wood.
  4. Environmental Benefits: Acetylation can be done using sustainably sourced wood, and the process doesn’t introduce toxic substances. The increased lifespan also means reduced frequency of replacement, which can be environmentally beneficial.
  5. Improved Paint Retention: Acetylated wood can hold onto paint longer than untreated wood, leading to longer intervals between repainting.

Concerns of Acetylated Wood:

  1. Cost: Acetylated wood products can be more expensive than their untreated counterparts or other traditional treated wood options.
  2. Availability: As a relatively newer product in the market, acetylated wood might not be as readily available everywhere. Many installers have complained about long lead times slowing down crucial parts of a build.
  3. Brittleness: Some reports suggest that acetylated wood can be more brittle than untreated wood.
  4. Limited Track Record: While initial results are promising, acetylated wood hasn’t been in use as long as other traditional wood products, so its long-term performance in various applications is still being observed.
  5. Special Fasteners: Due to its altered properties, acetylated wood requires stainless steel or other specific fasteners to prevent corrosion or staining.
  6. Odor: It gives off a strong vinegar smell that can tend to irritate some people’s eyes, nose and throat.
  7. Environmental Impact of Production: While the end product is non-toxic, the production process requires acetic anhydride, and there might be environmental concerns associated with its production and use.

3. Heat & Steam Treatment

Thermally modified wood using heat & steam is wood that has been treated at high temperatures (typically between 180°C and 230°C) in the presence of steam to reduce the wood’s moisture content and chemically change its structure.

The cellular composition of the wood is altered in this high-heat, oxygen-deprived environment which converts the natural acids and sugars so as to no longer be a food source for mold, rot or fungal decay. This change also renders the wood ‘hydrophobic’, meaning it loses much of the natural tendency to absorb water going forward.

thermally modified ash decking
Thermally modified ash decking

Here are the benefits and concerns associated with thermally modified wood:

Benefits of Thermally Modified Wood:

  1. Durability: The thermal modification process increases the wood’s resistance to decay, fungi, and pests, which can extend its lifespan.
  2. Dimensional Stability: Thermally modified wood is less prone to swelling and shrinking due to changes in moisture content, which can reduce warping, twisting, and cupping.
  3. Reduced Moisture Absorption: The wood’s ability to absorb water is significantly reduced, making it more suitable for exterior applications such as decking, siding and fencing.
  4. Aesthetics: The process gives the wood a darker, richer color, which many find aesthetically pleasing.
  5. Environmental Benefits: The process uses heat and steam without the need for chemicals, making it an environmentally friendly wood treatment method. Additionally, the increased lifespan means reduced frequency of replacement.
  6. Non-toxic: Since no chemicals are used in the process, thermally modified wood is safe for various applications, including indoor use, children’s play equipment.
  7. Made in USA: We sell a thermally modified ash wood that is entirely sourced and manufactured in the USA. Making this one of the lowest carbon footprint options of the modified woods.

Concerns of Thermally Modified Wood:

  1. Cost: Thermally modified wood can be more expensive than untreated wood. However it’s usually the most cost effective of these 3 modified wood options.
  2. Brittleness: The thermal modification process can make the wood more brittle, which might reduce its impact resistance and you should pre-drill all your screw holes to reduce splitting.
  3. Color Changes: Over time, the rich color of thermally modified wood can fade when exposed to UV light, similar to other woods.
  4. Limited Track Record: While the process has been around for a few decades, it hasn’t been in use as long as other traditional wood products, so its long-term performance in various applications is still being observed.
  5. Heat Sensitivity: There are some suggestions that thermally modified wood can be more sensitive to high temperatures after the treatment, which might limit its use in specific applications.
  6. Energy Intensive Process: The thermal modification process requires significant amounts of energy, which can have environmental implications depending on the energy source.
  7. Odor: It does have a slight burnt smell from the high heat treatment applied to the wood. It’s the least irritating of the 3 modified wood options as the other two methods can cause burning eyes and throat.


Of the three modified woods we discussed, thermal modification is one of the most natural, chemical-free ways to extend the life of wood products. As a result thermally modified wood that is treated only with heat and steam is our preferred modified wood option.

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