If the Oven Is Set Correctly, Why Do the LOD Results Keep Changing? 

Loss on Drying is one of the most commercially significant tests in a mining laboratory. The moisture percentage it produces determines the dry weight of the final product, and that dry weight is used to calculate the value of every shipment leaving the site. 

Yet LOD is frequently treated as a routine step. An oven temperature is set, a sample is loaded, and a result is reported. 

That assumption is where commercial risk begins. 

When LOD Inconsistency Becomes a Commercial Problem 

Most laboratories do not realise their moisture determination has a problem until a buyer does. 

The signs are familiar before the dispute becomes explicit: 

• The smelter queries the moisture result because it relies directly on the mine laboratory’s figure to calculate dry weight and payment. 

• QA requests for temperature logs or calibration records that are missing, incomplete, or out of date. 

• Moisture results vary between shifts on similar material, without an obvious explanation for the difference. 

• Dry weight disagreements between buyer and seller that delays payment until they are resolved. 

By the time these appear, the dispute is already in motion. The opportunity to prevent it has already passed. 

Why LOD Errors Are Rarely About the Method

The ISO method for moisture determination is clear: dry at 105 degrees Celsius to constant weight. It is not a complex procedure. Most laboratories follow it. 

The variability enters through execution, not intent. 

Economy Oven Temperature Drift

Economy ovens are set using a dial. A dial does not hold a fixed temperature. It holds an approximation. 

In practice, an economy oven with the dial set to 105 degrees may run at 101 or 109 degrees, depending on load, ambient temperature, and wear on the heating element. Neither the operator nor the laboratory manager will know this without a calibrated probe check. 

A digital oven is set to an exact temperature and holds it. When a buyer’s contract specifies ISO moisture determination at 105 degrees Celsius, a digital oven provides evidence that the method was followed. An economy oven without a temperature log does not. 

“If a buyer finds your oven is at 108 degrees, they can challenge your results. You have to prove you meet the ISO requirement.” 

— Dave Harvey, Technical Director, Effective Laboratory Supplies 

Overloaded Sample Trays 

Final concentrate samples are not small, and the drying tray size should be selected to match the sample weight, typically in the 20 to 30 kilogram range for commercial work. 

When tray loading exceeds the oven’s realistic capacity, airflow is restricted, heat distribution becomes uneven, and drying slows in the denser portions of the sample. The moisture result is pulled down relative to the actual figure because some of the samples have not fully dried. 

If sample volumes have grown over time, the answer is a second oven. Not more time in one. 

Drying to Time, Not to Constant Weight 

The ISO method requires drying to constant weight: weigh, return to the oven, reweigh, until no further mass loss is recorded. This is how the method confirms that all available moisture has been removed. 

Under shift pressure, this step is shortened. 

ELS has observed cases where technicians follow the SOP to the letter, drying the sample for the specified time, and still report a result from a sample that is still wet. 

One case involved a nickel mine in South Africa, where a shipment sample arrived with visible free water in the bag. The technician divided the sample into two portions for duplicate testing, as required. He also poured the free water from the bag into one of the drying trays. Both trays were placed in the oven for 24 hours, as required by the SOP. The tray with the added free water was still wet when it came out. The result was reported. 

The SOP said to dry for the required time. The discipline requires drying to constant weight. Those are not the same instructions. 

Sample Reabsorption During Cooling 

A bone-dry sample removed from the oven is at its most vulnerable. Fine-grained concentrate materials are hygroscopic. They draw moisture from the ambient air the moment they are exposed. 

If samples sit on the bench before weighing, or if the cooling environment is uncontrolled, the samples gain mass before the final weight is recorded. The reported moisture result is then understated. 

This is an underappreciated control point. It does not require complex infrastructure. It requires discipline. 

No Temperature Logging 

Without temperature records or calibration checks, a laboratory cannot prove control. In an audit or a commercial dispute, the absence of documentation is treated as evidence of non-compliance, regardless of the actual oven temperature. 

The Commercial Stakes Are Larger Than Most Laboratories Realise 

Concentrate pricing is based on dry weight plus grade. Both the seller and the buyer test moisture independently. When values differ, the discrepancy does not get resolved by averaging.  

It needs to be worked through directly between the two parties, often through renewed sampling or recalculation, which takes time and puts pressure on the commercial relationship while it remains unresolved. 

An ISO standard cited in a sales contract carries legal weight. A buyer who can demonstrate that the oven was out of specification, that the temperature was undocumented, or that the result cannot be reproduced has grounds to challenge a shipment value. 

On a 60-ton platinum concentrate truck with 10% moisture, a 1% moisture error shifts the calculated dry weight by 600 kilograms. On high-value concentrate, there is significant commercial exposure on every single shipment. 

“Accurate results in sampling preserve your commercial relationships. This is all about trust between buyer and seller.” 

— Gareth Arnold, Managing Director, and Dave Harvey, Technical Director, Effective Laboratory Supplies 

What the Right Setup Looks Like 

Effective Laboratory Supplies designed the concentrate sample preparation laboratory at Two Rivers Platinum, where moisture determination was built in from the start as a commercial discipline. The laboratory was designed to handle LOD work on every platinum concentrate shipment before it departs for the smelter approximately 20 kilometres away, with oven specification, weighing accuracy, workflow, and documentation all designed to produce results that are defensible, traceable, and auditable. 

That is the standard required for commercial LOD work. For most laboratories, reaching it does not require a full redesign. It requires a clear view of where variability is entering the process and the right equipment to remove it. 

The practical controls are specific: 

• Replace economy ovens with digital models that hold the set temperature precisely and provide loggable, auditable evidence of temperature control. 

• Run regular temperature verification using a calibrated probe and record the results. Monthly at minimum; more frequently where buyer contracts specify tighter intervals. 

• Lock oven setpoints on digital models to prevent staff from adjusting temperature between shifts. 

• Load trays within the oven’s realistic capacity. If sample volumes have increased, add oven capacity rather than overloading a single oven. 

• Train teams on the principle of constant-weight drying, not just the time-based instruction in the SOP. Dry until no further mass loss, not until the timer runs out. 

• Manage the cooling phase, particularly with fine, hygroscopic concentrates. Protect the sample from ambient humidity between removal from the oven and final weighing. 

LOD Is a System, Not a Step 

Loss on drying is not finished when the timer stops. It is finished when moisture has been fully removed, the sample is protected during cooling, the balance is verified, and the result is documented in a way that can be defended. 

Each of those stages is a control point. When any one of them slips, the result slips with it. 

The laboratory that treats LOD as a system, from sample receipt through drying, cooling, weighing, and documentation, is the laboratory that does not receive the call from the buyer. 

Speak to ELS about improving LOD consistency and moisture results. 
011 434 1450   |   Sales@effectivelab.co.za   |   effectivelab.co.za