The Chemical Makeup of Ash
Ash is the non-aqueous, non-gaseous mineral residue that remains after combustion. While many people primarily think of wood ash, the chemical composition of ash varies dramatically based on what was burned. Understanding the origin of the ash is crucial to knowing its mineral content, including how much calcium it contains and in what form.
Wood Ash: A Rich Source of Calcium
Ash from burnt wood is well-known for its high mineral content, particularly calcium. The burning process concentrates the minerals that the tree absorbed from the soil during its lifetime. Calcium is the most abundant element, and its exact concentration can vary based on the wood species and burning temperature. For instance, hardwood trees generally produce more ash with a higher mineral density than softwoods.
During combustion, the calcium in the wood is primarily converted into calcium carbonate ($CaCO_3$). At higher temperatures, some calcium carbonate can further decompose into calcium oxide ($CaO$), also known as quicklime. This alkaline nature is why wood ash is often used by gardeners and farmers as a liming agent to raise the pH of acidic soils.
Other notable components of wood ash include:
- Potassium: 5–15%
- Magnesium: 1–4%
- Phosphorus: 1–3%
- Trace elements: Iron, manganese, zinc, copper, and boron
Cremation Ash: Pulverized Bone Minerals
What is commonly referred to as human "cremation ashes" is not the powdery ash from a campfire. Instead, it consists of pulverized bone fragments that remain after the body's organic materials have been vaporized in a high-temperature cremation chamber. The primary mineral compound in these remains is calcium phosphate, reflecting the makeup of the skeletal system.
Chemical analysis shows that cremation ashes are composed of:
- Phosphate: Approximately 47.5%
- Calcium: Approximately 25.3%
- Sulfate: 11.0%
- Potassium: 3.69%
- Sodium: 1.12%
- Chloride: 1.0%
Because of the high concentration of both calcium phosphate and salts, untreated cremation ashes can have a very high pH (11–12.5), making them toxic to plants and soil ecosystems when scattered directly.
Comparing Different Types of Ash
While both wood ash and cremation ash contain calcium, they are chemically distinct and have different applications.
| Feature | Wood Ash | Cremation Ash |
|---|---|---|
| Source Material | Burnt plant matter (wood) | Pulverized bone fragments (human/animal) |
| Primary Calcium Compound | Calcium carbonate ($CaCO_3$) and calcium oxide ($CaO$) | Calcium phosphate ($Ca_3(PO_4)_2$) |
| Approx. Calcium Content | 25–50% calcium compounds | ~25% calcium by total weight |
| Main Use | Soil amendment/liming agent to increase pH | Memorialization; not recommended for direct use as a soil amendment due to high pH and salt content |
| pH Level | High, alkaline (9–12) | Very high, alkaline (11–12.5) |
| Other Significant Elements | Potassium, magnesium, phosphorus | Phosphate, sulfate, potassium, sodium |
The Role of Calcium in Ash
The presence of calcium in ash is significant for several reasons. In the case of wood ash, it makes for an effective and natural soil amendment, counteracting soil acidity and providing a beneficial nutrient for plant growth. For cremation ash, the calcium phosphate from the bones is what gives the remains their coarse, sand-like texture and significant weight. In industrial applications, such as the production of cement or certain ceramics, the calcium in specific types of fly ash or bone ash can be a valuable raw material.
The Importance of Ash Source
It is critical to distinguish between ash from different sources. Ash from treated or painted wood can contain toxic heavy metals and should never be used in a garden. Likewise, ash from coal combustion can vary in composition and may contain heavy metals, making it unsuitable for general use. The specific composition of ash is a direct reflection of its source, and a material's mineral content can dictate its appropriate, safe use.
In conclusion, whether from a backyard bonfire or a cremation chamber, ashes indeed contain calcium. The form and amount differ dramatically, influencing how the material can be safely handled or repurposed. Wood ash is an excellent source of calcium carbonate for gardening, while cremation ashes, which are high in calcium phosphate, require special handling due to their high salt and pH levels. Understanding the origins and chemical composition of ash is essential for its responsible use.
For more information on using wood ash in your garden, consult resources from trusted agricultural extension programs, such as this guide from the University of New Hampshire: https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/Resource004042_Rep5718.pdf.
Conclusion
Yes, all types of ash contain calcium, but the chemical composition and concentration depend heavily on the source material. Wood ash is rich in calcium carbonate and is a useful soil amendment for balancing pH. In contrast, cremation ashes consist primarily of calcium phosphate from bone fragments. While both contain calcium, their distinct properties mean they are not interchangeable and require different handling and application methods. Knowing the source is paramount to understanding ash's properties.
